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This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. The ghostly object, catalogued as NGC 1052-DF2, doesn’t have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either, as its velocity dispersion is all wrong. Even its globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies. All of these oddities pale in comparison to the weirdest aspect of this galaxy: NGC 1052-DF2 is very controversial because of its apparent lack of dark matter. This could solve an enormous cosmic puzzle. (NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))Has the mystery really been solved? Doubtful. The real science goes much deeper.

For perhaps the last year or so, a small galaxy located not too far away has captivated the attention of astronomers. The galaxy NGC 1052-DF2, a satellite of the larger NGC 1052, appears to be the first galaxy ever discovered that shows no evidence of dark matter. Paradoxically, that has been reported as indisputable evidence that dark matter must exist! Now, a new team has come out with a result that claims this galaxy cannot be devoid of dark matter, and Yann Guidon wants to know what’s really going on, asking:

I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What’s really going on here, Ethan?

We have to be extremely careful here, and dissect the findings of the different teams with all the implications correctly synthesized. Let’s get started.

The full Dragonfly field, approximately 11 square degrees, centred on NGC 1052. The zoom-in shows the immediate surroundings of NGC 1052, with NGC1052–DF2 highlighted in the inset. This is Extended Data Figure 1 from the publication announcing the discovery of DF2. (P. VAN DOKKUM ET AL., NATURE VOLUME 555, PAGES 629–632 (29 MARCH 2018))

Whenever you have a galaxy in the Universe and you want to know how much mass is inside, you have two ways of approaching the problem. The first way is to rely on astronomy to give you the answer.

Astronomically, there are a slew of observations we can make to teach us about the matter content of a galaxy. We can look in a myriad of wavelengths of light to determine the total amount of starlight that’s present, and infer the amount of mass that’s present in stars. We can similarly make additional observations of gas, dust, and the absorption and emission of radiation in order to infer the total amount of normal matter that’s present. We’ve done this for enough galaxies for long enough that simply measuring some basic properties can lead us to infer the total baryonic (made of protons, neutrons, and electrons) matter within a galaxy.

The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. The dashed curve would correspond to a galaxy without dark matter, which represents less than 1% of galaxies. While initial observations of the velocity dispersion, via globular clusters, indicated that NGC 1052-DF2 was one of them, newer observations throw that conclusion into doubt. (WIKIMEDIA COMMONS USER STEFANIA.DELUCA)

On the other hand, there are additional gravitational measurements we can make that will teach us about the total amount of mass that’s present within a galaxy, irrespective of the type of matter (normal, baryonic matter or dark matter) that we see. By measuring the motions of the stars inside, either through direct line-broadening at different radii or through the velocity dispersion of the entire galaxy, we can get a specific value for the total mass. In addition, we can look at the velocity dispersion of the globular clusters orbiting a galaxy to obtain a second, complementary, independent measurement of total mass.

In most galaxies, the two values for the measured/inferred matter content differ by about a factor of 5-to-6, indicating the presence of substantial amounts of dark matter. But some galaxies are special.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

From a theoretical perspective, we know how galaxies should form. We know that the Universe ought to start out governed by General Relativity, our law of gravity. It should have approximately a 5-to-1 mix of dark matter to normal matter, and should begin almost perfectly uniform, with underdense and overdense regions appearing at about the 1-part-in-30,000 level. Give the Universe time, and let it evolve, and you’ll form structures where the overdense regions were on small, medium and large scales, with vast cosmic voids forming between them, in the initially underdense regions.

In large galaxies, comparable to the Milky Way’s size or larger, very little is going to be capable of changing that dark matter to normal matter ratio. The total amount of gravity is generally going to be too great for any type of matter to escape, unless it speeds rapidly through a gas-rich medium capable of stripping the normal matter away.

A Hubble (visible light) and Chandra (X-ray) composite of galaxy ESO 137–001 as it speeds through the intergalactic medium in a rich galaxy cluster, becoming stripped of stars and gas, while its dark matter remains intact. (NASA, ESA, CXC)

But for smaller galaxies, there are interesting processes that can occur that are vitally important to this ratio of normal matter (which determines the astronomical properties) to dark matter (which, combined with the normal matter, determines the gravitational properties).

When most small, low-mass galaxies form, the act of forming stars is an act of violence against all the other matter inside. Ultraviolet radiation, stellar cataclysms (like supernovae), and stellar winds all heat up the normal matter. If the heating is severe enough and the mass of the galaxy is low enough, enormous quantities of normal matter (in the form of gas and plasma) can get ejected from the galaxy. As a result, many low-mass galaxies will exhibit dark matter to normal matter ratios far in excess of 5-to-1, with some of the lowest-mass galaxies achieving ratios of hundreds-to-1.

Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it. The dark matter-to-normal matter ratio of nearly 1000-to-1 is the greatest ratio ever seen in the dark matter-favoring direction. (MARLA GEHA AND KECK OBSERVATORIES)

But there’s another process that can arise, on rare occasion, to produce galaxies with either very small or even, in theory, no amounts of dark matter. When larger galaxies merge together, they can produce an extreme phenomenon known as a starburst: where the entire galaxy becomes an enormous star-forming region.

The merger process, coupled with this star-formation, can impart enormous tidal forces and velocities to some of the normal matter that’s present. In theory, this could be powerful enough to rip substantial quantities of normal matter out of the main, merging galaxies, forming smaller galaxies that will have far less dark matter than the typical 5-to-1 dark matter-to-normal matter ratio. In some extreme cases, this might even create galaxies made of normal matter alone. Around large, dark matter-dominated galaxies, there might be smaller ones that are entirely dark matter-free.

A decade ago, there were a small number of scientists who claimed that the observed lack of these dark matter-free galaxies was a clear falsification of the dark matter paradigm. The overwhelming majority of scientists countered with claims that these galaxies should be rare, faint, and that it was no surprise we hadn’t observed them yet. With more data, better observations, and superior instrumentation and techniques, small galaxies with either small amounts of dark matter, or even none at all, ought to emerge.

Last year, a team of Yale researchers announced the discovery of the galaxy NGC 1052-DF2 (DF2 for short), a satellite galaxy of the large galaxy NGC 1052, that appeared to have no dark matter at all. When the scientists looked at the globular clusters orbiting DF2, they found the velocity dispersion was extremely small: at least a factor of 3 below the predicted speeds of ±30 km/s, which would have corresponded to this typical 5-to-1 ratio.

The KCWI spectrum of the galaxy DF2 (in black), as taken directly from the new paper at arXiv:1901.03711, with the earlier results from a competing team using MUSE superimposed in red. You can clearly see that the MUSE data is lower resolution, smeared out, and artificially inflated compared to the KCWI data. The result is an artificially large velocity dispersion inferred by the prior researchers. (SHANY DANIELI (PRIVATE COMMUNICATION))

About 8 months later, another team, using a different instrument (rather than the unique Dragonfly instrument used by the Yale team), argued that the stars, rather than the globular clusters, should be used to determine the galaxy’s mass. Using their new data, they found an equivalent velocity dispersion of ±17 km/s, about twice as great as the Yale team had measured.

Undaunted, the Yale team made an even more precise measurement of the stars in DF2 using the upgraded KCWI instrument, and went back and measured the motions of the globular clusters orbiting it once again. With a superior instrument, they got a result with much smaller error bars, and both techniques agreed. From the stellar velocity dispersion, they got a value of ±8.4 km/s, with the globulars giving ±7.8 km/s. For the first time, it looked like we truly had found a dark matter-free galaxy.

The predictions (vertical bars) for what the velocity dispersions ought to be if the galaxy contained a typical amount of dark matter (right) versus no dark matter at all (left). The Emsellem et al. result was taken with the insufficient MUSE instrument; the latest data from Danieli et al. was taken with the KCWI instrument, and provides the best evidence yet that this really is a galaxy with no dark matter at all. (DANIELI ET AL. (2019), ARXIV:1901.03711)

But perhaps something was flawed. When scientists are truly engaging in good science, they’ll try to take any hypothesis, novel result, or unexpected find and poke holes in it. They’ll try to knock it down, discredit it, or find a fatal flaw with the result whenever possible. Only the most robust, well-scrutinized results will stand up and become accepted; controversies are at their hottest when a new result threatens to decide the issue once and for all.

The latest attempt to knock the DF2 results down come from a group at the Instituto de Astrofísica de Canarias (IAC) led by Ignacio Trujillo. Using a new measurement of DF2, his team claims that the galaxy is actually closer than previously thought: 42 million light-years instead of 64 million. This would mean it isn’t a satellite of NGC 1052 after all, but rather a galaxy some 22 million light-years closer, in the cosmic foreground.

The ultra-diffuse galaxy KKS2000]04 (NGC1052-DF2), towards the constellation of Cetus, was considered to be a galaxy completely devoid of dark matter. The results of Trujillo et al. dispute that, claiming that the galaxy is much closer, and therefore has a different mass-to-luminosity ratio (and a different velocity dispersion) than was previously thought. This is extremely controversial. (TRUJILLO ET AL. (2019))

This could change the story dramatically. The distance to a galaxy is extremely important to the intrinsic brightness you infer, which in turn tells you how much matter must be present in the form of stars. If the galaxy is much closer than previously thought, then there’s actually more mass present, and the inferred velocity dispersion will be higher, indicating the need for dark matter, after all.

Case closed, right?

Not even close. First off, DF2 isn’t the only galaxy that exhibits this effect anymore; there’s another satellite of NGC 1052 (known as DF4) that exhibits the same dark matter-free nature, so both would have to have their distances mis-estimated. Second, even if they are at the closer distance preferred by the Trujillo et al. team, that still renders DF2 and DF4 both extremely low-dark matter galaxies, which still necessitates a mechanism to separate normal matter from dark matter. And third, the Yale team had previously (in August) published a calibration-free distance measurement to the galaxy, from surface-brightness-fluctuations, inconsistent at 3.5 sigma with Trujillo’s results.

The galaxy NGC 1052-DF2 was imaged in great detail by the KCWI spectrograph instrument aboard the W.M. Keck telescope on Mauna Kea, enabling scientists to detect the motions of stars and globular clusters inside the galaxy to unprecedented precisions. (DANIELI ET AL. (2019), ARXIV:1901.03711)

In other words, even if the distance estimates by Trujillo et al. are correct, which they probably aren’t, these galaxies are extremely low in dark matter, with DF4 possibly still even being dark matter-free. Neither team has yet observed this galaxy with the Hubble Space Telescope, but that will provide the most unambiguous distance estimate at all. Subsequent observations of DF4 with Hubble are slated for later in 2019, which should help clarify this ambiguity.

A short distance for these galaxies does not actually resolve the central issue: that they have much less dark matter, no matter how you massage it, than a naive, conventional dark matter-to-normal matter ratio would indicate. Only if dark matter is real, and experiences different physics in star-forming and collisional environments than normal matter, can galaxies like DF2 or DF4 exist at all.

Many nearby galaxies, including all the galaxies of the local group (mostly clustered at the extreme left), display a relationship between their mass and velocity dispersion that indicates the presence of dark matter. NGC 1052-DF2 is the first known galaxy that appears to be made of normal matter alone, and was later joined by DF4 earlier in 2019. (DANIELI ET AL. (2019), ARXIV:1901.03711)

The one takeaway, if you learn nothing else, is this: this new result resolves nothing. Stay tuned, because more and better data is coming. These galaxies are likely extremely low in dark matter, and possibly entirely free of dark matter. If the Yale team’s initial results hold up, these galaxies must be fundamentally different in composition from all the other galaxies we’ve ever found.

If all galaxies follow the same underlying rules, only their compositions can differ. The discovery of a dark matter-free galaxy, if that result holds up, is an extremely strong piece of evidence for a dark matter-rich Universe. Keep your eyes open for more news on DF2 and DF4, because this story is far from over.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Ask Ethan: What’s The Real Story Behind This Dark Matter-Free Galaxy? was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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This visualization of a planetary system around a red dwarf star is commonly thought to have every planet be uninhabitable. But this may not be the case at all, and we won’t know unless we look. (JPL-Caltech/NASA)If we only look for life on worlds like our own, we might miss the most commonly inhabited planets of all.

With all the planets out there in the galaxy and Universe, it’s only a matter of time and data until we find another one with life on it. (Probably.) But while most of the searches have focused on finding the next Earth, sometimes called Earth 2.0, that’s very likely an overly restrictive way to look for life. Biosignatures, or more conservatively, bio-hints, might not only be plentiful on worlds very different from our own, but around Solar Systems other than our own. Earth-like worlds, in fact, might not even be the most ubiquitous places for life to arise in the Universe.

I’m happy to welcome scientist Adrian Lenardic onto the Starts With A Bang podcast, and explore what just might be out there if we look for life beyond our idea of Earth 2.0!

The Starts With A Bang podcast is make possible through the support of our patrons on Patreon, which you can join today!

Starts With A Bang Podcast #45: Beyond Earth 2.0 was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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In a hypertorus model of the Universe, motion in a straight line will return you to your original location. If time is like a torus, it may be cyclical in nature, rather than having always existed or coming into existence a finite amount of time ago. We do not, even today, know the origin of time. (ESO AND DEVIANTART USER INTHESTARLIGHTGARDEN)When we think about the birth of the Universe, was time already in place?

When we look at the Universe today, we know with an extraordinary amount of scientific certainty that it wasn’t simply created as-is, but evolved to its present configuration over billions of years of cosmic history. We can use what we see today, both nearby and at great distances, to extrapolate what the Universe was like a long time ago, and to understand how it came to be the way it is now.

When we think about our cosmic origins, then, it’s only human to ask the most fundamental of all possible questions: where did this all come from? It’s been more than half a century since the first robust and unique predictions of the Big Bang were confirmed, leading to our modern picture of a Universe that began from a hot, dense state some 13.8 billion years ago. But in our quest for the beginning, we know already that time couldn’t have started with the Big Bang. In fact, it might not have had a beginning at all.

After the Big Bang, the Universe was almost perfectly uniform, and full of matter, energy and radiation in a rapidly expanding state. As time goes on, the Universe not only forms elements, atoms, and clumps and clusters together that lead to stars and galaxies, but expands and cools the entire time. No alternative can match it, but it doesn’t teach us everything, including (and especially) about the very beginning itself.(NASA / GSFC)

Whenever we think about anything, we apply our very human logic to it. If we want to know where the Big Bang came from, we describe it in the best terms we can, and then theorize about what could have caused it and set it up. We look for evidence to help us understand the Big Bang’s beginnings. After all, that’s where everything comes from: from the process that gave it its start.

But this assumes something that may not be true about our Universe: that it actually had a beginning. For a long time, scientifically, we didn’t know whether this was true or not. Did the Universe have a beginning, or a time before which nothing existed? Or did the Universe exist for an eternity, like an infinite line extending in both directions? Or, quite possibly, is our Universe cyclic like the circumference of a circle, where it repeats over and over indefinitely?

The three major possibilities for how time behaves in our Universe are that time has always existed and will always exist, that time only existed for a finite duration if we extrapolate backwards, or that time is cyclical, and will repeat, with no beginning and no end. The Big Bang looked like it provided an answer for a time, but has since been superseded, plunging our origins back into uncertainty. (E. SIEGEL)

For a time, there were multiple competing ideas which were all consistent with the observations we had.

  1. An expanding Universe could have originated from a singular point — an event in spacetime — where all of space and time emerged from a singularity.
  2. The Universe could be expanding today because it was contracting in the past, and will contract again in the future, presenting an oscillating solution.
  3. Finally, the expanding Universe could have been an eternal state, where space is expanding now and always had been and always would be, where new matter is continuously created to keep the density constant.

These three examples represent the three major options: the Universe had a singular beginning, the Universe is cyclical in nature, or the Universe has always existed. In the 1960s, however, a low-level of microwave radiation was found everywhere across the sky, changing the story forever.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation. The temperature and spectrum of this radiation has now been measured, and the agreement with the Big Bang’s predictions are extraordinary. (NASA / WMAP SCIENCE TEAM)

This radiation wasn’t just the same magnitude everywhere, but also the same in all directions. At just a few degrees above absolute zero, it was consistent with the Universe emerging from an earlier, hot dense state, and cooling as it expanded.

As improved technology and novel techniques led to better data, we learned that the spectrum of this radiation had a particular shape: that of a near-perfect blackbody. A blackbody is what you get if you have a perfect absorber of radiation heated up to a certain specific temperature. If the Universe expands and cools without changing its entropy (i.e., adiabatically), something that starts off with a blackbody spectrum will remain a blackbody, even as it cools. This radiation was not only consistent with being the leftover glow from the Big Bang, but was inconsistent with alternatives like tired light or reflected starlight.

The unique prediction of the Big Bang model is that there would be a leftover glow of radiation permeating the entire Universe in all directions. The radiation would be just a few degrees above absolute zero, would be the same magnitude everywhere, and would obey a perfect blackbody spectrum. These predictions were borne out spectacularly well, eliminating alternatives like the Steady State theory from viability. (NASA / GODDARD SPACE FLIGHT CENTER / COBE (MAIN); PRINCETON GROUP, 1966 (INSET))

According to the Big Bang, the Universe was hotter, denser, more uniform and smaller in the past. It only has the properties we see today because it’s been expanding, cooling, and experiencing the influence of gravitation for so long. Because the wavelength of radiation stretches as the Universe expands, a smaller Universe should have had radiation with shorter wavelengths, meaning it had higher energies and greater temperatures.

Billions of years ago, it was once so hot that even neutral atoms couldn’t form without being blasted apart. Even earlier than that, today’s microwave radiation were so energetic that they dominated over matter as far as the Universe’s energy content was concerned. At even earlier times, atomic nuclei were instantly blasted apart, and at still earlier ones, we couldn’t even create stable protons and neutrons.

A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. (NASA / CXC / M. WEISS)

If we extrapolate all the way back, to arbitrarily hot temperatures, small distances, and high densities, you’d intuit that this would truly equate to the beginning. If you were willing to run the clock backwards as far as you could, all of the space that makes up our visible Universe today would be compressed down to a single point.

Now, it’s true that if you went to these extreme conditions, compressing all the matter and energy present in today’s Universe into a tiny enough volume of space, the laws of physics would break down. You could try to calculate various properties, but you’d only get nonsense for answers. This is what we describe as a singularity: a set of conditions where time and space have no meaning. At first glance, if you do the math, it appears that a singularity is inevitable, regardless of what dominates the Universe’s energy content.

The scale of the Universe, on the y-axis, is plotted as a function of time, on the x-axis. Whether the Universe is made of matter (red), radiation (blue), or energy inherent to space itself (yellow), it decreases towards a size/scale of 0 as you extrapolate backwards in time. (E. SIEGEL)

Singularities are where the law of gravitation governing the Universe — Einstein’s General Relativity — yields nonsense for predictions. Relativity, remember, is the theory that describes space and time. But at singularities, both spatial and temporal dimensions cease to exist. Asking questions like “what came before this event where time began” is as nonsensical as asking “where am I” if space no longer exists.

Indeed, this is the argument that many make, including Paul Davies, when they claim that there can be no discussion of what occurred before the Big Bang. This is a tautology, of course, if you assert that the Big Bang is where time began. But as interesting as this argument is, we know that the Big Bang isn’t where time began anymore. Ever since we’ve made modern, detailed measurements of the cosmos, we’ve learned that this extrapolation to a singularity must be wrong.

The leftover glow from the Big Bang, the CMB, isn’t uniform, but has tiny imperfections and temperature fluctuations on the scale of a few hundred microkelvin. While this plays a big role at late times, after gravitational growth, it’s important to remember that the early Universe, and the large-scale Universe today, is only non-uniform at a level that’s less than 0.01%. Planck has detected and measured these fluctuations to better precision than ever before, and can even reveal the effects of cosmic neutrinos on this signal. The properties of these fluctuations strongly support an inflationary origin to our observable Universe. (ESA AND THE PLANCK COLLABORATION)

In particular, the patterns and magnitudes of the fluctuations that we’ve discovered in the modern radiation left over from that early, hot, dense state teach us a number of important properties about our Universe. They teach us how much matter was present in dark matter as well as normal matter: protons, neutrons and electrons. They give us a measurement of the Universe’s spatial curvature, as well as the presence of dark energy and the effects of neutrinos.

But they also tell us something vitally important that’s often overlooked: they tell us whether there was a maximum temperature for the Universe back in its earliest stages. According to the data from WMAP and Planck, the Universe never achieved a temperature greater than about 1029 K. This number is enormous, but it’s over 1,000 times smaller that the temperatures we’d need to equate to a singularity.

Our entire cosmic history is theoretically well-understood, but only qualitatively. It’s by observationally confirming and revealing various stages in our Universe’s past that must have occurred, like when the first stars and galaxies formed, and how the Universe expanded over time, that we can truly come to understand our cosmos. The relic signatures imprinted on our Universe from an inflationary state before the hot Big Bang give us a unique way to test our cosmic history.(NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

The particular properties of the Universe that are imprinted upon it from the earliest stages provide a window into the physical processes that took place at those times. Not only do they tell us that we cannot extrapolate the Big Bang all the way back to a singularity, but they tell us about the state that existed prior to (and set up) the hot Big Bang: a period of cosmic inflation.

During inflation, there was a tremendous amount of energy inherent to space itself, causing the Universe to expand both rapidly and relentlessly: at an exponential rate. This period of inflation occurred prior to the hot Big Bang, set up the initial conditions that our Universe began with, and left a series of unique imprints that we searched for and discovered after the theory had already predicted them. By any metric, inflation is a tremendous success.

The quantum fluctuations that occur during inflation get stretched across the Universe, and when inflation ends, they become density fluctuations. This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. These new predictions are essential for demonstrating the validity of a fine-tuning mechanism, and have validated inflation as our new, leading theory of how our Big Bang got its start. (E. SIEGEL, WITH IMAGES DERIVED FROM ESA/PLANCK AND THE DOE/NASA/ NSF INTERAGENCY TASK FORCE ON CMB RESEARCH)

But this severely alters our conceptions of how the Universe began. Earlier, I presented you a graph of how the size (or scale) of the Universe evolved with time. The graph displayed the differences between how the Universe would expand if it were dominated by matter (in red), radiation (in blue), or space itself (such as during inflation, in yellow) at early times. However, I wasn’t completely honest with you in displaying that graph.

You see, I omitted something in the earlier graph, because I truncated it at a positive, finite time. In other words, I stopped the graph before we reached a size of zero. If I were to continue to extrapolate backwards, the matter and radiation curves do indeed reach a singularity at a specific time: t = 0. That would have been where the original idea of the Big Bang occurred. But in an inflationary Universe, you only asymptote to a size of zero; you never reach it. Not at a specific time of t=0, and not at any early time, no matter how far back you go.

Blue and red lines represent a “traditional” Big Bang scenario, where everything starts at time t=0, including spacetime itself. But in an inflationary scenario (yellow), we never reach a singularity, where space goes to a singular state; instead, it can only get arbitrarily small in the past, while time continues to go backwards forever. The Hawking-Hartle no-boundary condition challenges the longevity of this state, as does the Borde-Guth-Vilenkin theorem, but neither one is a sure thing. (E. SIEGEL)

Like many great discoveries in science, this leads to a slew of delightful new questions, including:

  1. Was the inflationary state a constant one? We do not know whether the Universe inflated at the same rate everywhere, or whether it inflated for long periods of time. If the Universe inflated in ways that changed very quickly from one moment to the next, varying from location-to-location, it might still have the properties we observe it to have today.
  2. Did the inflationary state last forever, going backwards in time? Inflation certainly has the potential to be an eternal state; we believe in the regions where it doesn’t end in a hot Big Bang, it continues eternally into the future. But could it have also been eternal to the past? With nothing forbidding it, we must consider the possibility.
  3. Is inflation connected to dark energy, which is also a form of exponential expansion? Although they’re different in scale and magnitude, the early-stage cosmic inflation and the late-stage dark energy both give the same mathematical form for the Universe’s expansion. Are these two stages related, and will our future expansion increase in strength and rejuvenate our Universe, like some sort of cosmic cycle?
The different ways dark energy could evolve into the future. Remaining constant or increasing in strength (into a Big Rip) could potentially rejuvenate the Universe, while reversing sign could lead to a Big Crunch. Under either of those two scenarios, time may be cyclical, while if neither comes true, time could either be finite or infinite in duration to the past. (NASA/CXC/M.WEISS)

Observationally, we don’t know the answer to any of these questions. The Universe, as far as we can observe it, only contains information from the final 10–33 seconds or so of inflation. Anything that occurred prior to that — which includes anything that would tell us how-or-if inflation began and what its duration was — gets wiped out, as far as what’s observable to us, by the nature of inflation itself.

Theoretically, we don’t fare much better. The Borde-Guth-Vilenkin theorem tells us that all points in the Universe, if you extrapolate back far enough, will merge together, and that inflation cannot describe a complete spacetime. But that doesn’t necessarily mean an inflating state couldn’t have lasted forever; it could just as easily imply that our current rules of physics are incapable of describing these earliest stages accurately.

The three major possibilities for how time behaves in our Universe are that time has always existed and will always exist, that time only existed for a finite duration if we extrapolate backwards, or that time is cyclical, and will repeat, with no beginning and no end. We do not have enough information in our Universe, today, to know which of these possibilities is accurate. (E. SIEGEL)

Even though we can trace our cosmic history all the way back to the earliest stages of the hot Big Bang, that isn’t enough to answer the question of how (or if) time began. Going even earlier, to the end-stages of cosmic inflation, we can learn how the Big Bang was set up and began, but we have no observable information about what occurred prior to that. The final fraction-of-a-second of inflation is where our knowledge ends.

Thousands of years after we laid out the three major possibilities for how time began — as having always existed, as having begun a finite duration ago in the past, or as being a cyclical entity — we are no closer to a definitive answer. Whether time is finite, infinite, or cyclical is not a question that we have enough information within our observable Universe to answer. Unless we figure out a new way to gain information about this deep, existential question, the answer may forever be beyond the limits of what is knowable.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Did Time Have A Beginning? was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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This image shows a composite of optical, X-ray, Microwave and radio data of the regions between the colliding galaxy clusters Abell 399 and Abell 401. The X-rays are concentrated near where the clusters are, but there’s a clear radio bridge between them (in blue). (M. MURGIA / INAF, BASED ON F. GOVONI ET AL., 2019, SCIENCE)Dark matter’s naysayers latched onto one tiny puzzle. This new find may have solved it completely.

Imagine the largest cosmic smashup you can. Take the largest gravitationally bound structures we know of — enormous galaxy clusters that might contain thousands of Milky Way-sized galaxies — and allow them to attract and merge. With individual galaxies, stars, gas, dust, black holes, dark matter and more inside, there are bound to not only be fireworks, but novel astrophysical phenomena that might not show up elsewhere in the Universe.

The gas within these clusters can heat up, interact, and develop shocks, causing the emission of spectacularly energetic radiation. Dark matter can pass through everything else, separating its gravitational effects from the majority of the normal matter. And, in theory, charged particles can accelerate tremendously, creating coherent magnetic fields that could span millions of light-years. For the first time, such an intergalactic bridge between two colliding clusters has been discovered, with tremendous implications for our Universe.

This Chandra image shows a large-scale view of the galaxy cluster MACSJ0717, where the white box shows the field-of-view of an available Chandra/HST composite image. The green line shows the approximate position of the large-scale filament leading into the cluster, suggesting a connection between the great cosmic web and the galaxy clusters that populate our Universe. (NASA/CXC/IFA/C. MA ET AL.)

In our cosmos, astronomical structures aren’t all created equal. Planets are dwarfed by stars, which themselves are far smaller in scale than Solar Systems. Collections of many hundreds of billions of these systems are required to make up a large galaxy like the Milky Way, while galactic groups and clusters might contain thousands of Milky Way-sized galaxies. On the largest scales of all, these enormous galaxy clusters can collide and merge.

Back in 2004, two sets of observations came in concerning a pair of galaxy clusters in close proximity: 1E 0657–558, more commonly known as the Bullet Cluster. From an optical image alone, two dense collections of galaxies — the two independent clusters — can clearly be identified.

The Bullet cluster, the first classic example of two colliding galaxy clusters where the key effect was observed. In the optical, the presence of two nearby clusters (left and right) can be clearly discerned.(NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

There are then two additional things you can do to tease out additional information about what’s going on in this system. One physically interesting measurement you can make is to look at the light from all the galaxies you can see in the image, and identify which ones are behind (background galaxies) the clusters versus which ones are in front (foreground galaxies) of them.

When you look at the foreground galaxies, their orientations should be random: they should be circular or elliptical or disk-like with no average distortion skewed to favor any particular direction. But if there’s a large mass in front of the light, there should be gravitational lensing effects that distort the background images. The statistical differences in shape between the background and foreground galaxies can tell you information about how much mass is located at various positions in space, at least from our point of view.

Any configuration of background points of light, whether they be stars, galaxies or galaxy clusters, will be distorted due to the effects of foreground mass via weak gravitational lensing. Even with random shape noise, the signature is unmistakable. By examining the difference between foreground (undistorted) and background (distorted) galaxies, we can reconstruct the mass distribution of massive extended objects, like galaxy clusters, in our Universe. (WIKIMEDIA COMMONS USER TALLJIMBO)

The second thing you can do is to observe the exact same region of the sky in X-rays, using an advanced X-ray observatory in space. Observations that were conducted with NASA’s Chandra X-ray observatory were sufficient to do exactly that. What Chandra discovered was fascinating: two enormous clumps of gas were spotted, each one moving along with its home galaxy cluster. As expected, there’s an enormous amount of gas not only associated with each galaxy, but with the overall cluster as a whole.

But what was unexpected was the finding that the gas, making up about 13–15% the overall cluster’s mass, was actually separated from the gravitational effects! Somehow, the normal matter and the gravitational effects were separated, as though the overall mass had simply passed straight through. This result was taken as overwhelming astrophysical evidence for the existence of dark matter.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable. (X-RAY: NASA/CXC/CFA/M.MARKEVITCH ET AL.; LENSING MAP: NASA/STSCI; ESO WFI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.; OPTICAL: NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

Since that time, more than a dozen other galaxy groups and clusters have been spotted colliding with one another, with each one demonstrating the same effect. Before a collision, if a cluster emits X-rays, those X-rays are associated with the cluster itself, and any gravitational distortion is found coincident with the location of the galaxies and the gas.

But after a collision, the X-ray emitting gas is offset from the matter, implying that the same physics is at play. When the clusters collide:

  • the galaxies take up only a small volume inside each cluster, and pass straight through,
  • the intracluster gas interacts and heats up, emitting X-rays and slowing down,
  • while the dark matter, expected to occupy an enormous halo surrounding each cluster, passes through as well, affected only by gravitation.

In every colliding group and cluster we’ve observed, the same separation of X-ray gas and overall matter is seen.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Although some of the simulations we perform indicate that a few clusters may be moving faster than expected, the simulations include gravitation alone, and other effects may also be important for the gas.(X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

You might think that this empirical proof of dark matter, seen in so many independent systems, would sway any reasonable skeptic. Alternative theories of gravity were concocted to try to explain the misalignment between the gravitational lensing signal and the presence of matter, postulating a non-local effect that resulted in a gravitational force that was offset from the matter. But any theory that worked for one particular alignment of colliding clusters failed to explain clusters in a pre-collisional state. 15 years later, alternatives still fail to explain both configurations.

But a Universe with dark matter has a very high burden of proof: it has to explain every single observed property of these clusters. While many of these colliding groups and clusters have speeds that are predicted by a dark matter-rich Universe, the Bullet cluster — the original example — moves extremely quickly.

The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact. Despite the indirect evidence for dark matter, we’d love to be able to detect it directly, which is something that can only happen if there’s a non-zero cross-section between normal matter and dark matter. The structures that arise, however, including galaxy clusters and larger-scale filaments, are undisputed. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

When you know the ingredients of your Universe and the laws of physics that govern what’s in it, you can run simulations to predict what types of large-scale structure emerge. When we include simulations with gravitation alone, the fastest colliding clusters we predict move slower than the Bullet cluster does; the likelihood of having a single example like it in our Universe is less than 1-in-a-million.

When we buck the cosmic odds like this, we demand an explanation. While it’s always possible that our Universe is simply a lottery-winner in terms of what’s present within it, this observation poses a legitimate problem. Either the observations were wrong, or something else — some physical mechanism — is causing this normal matter to accelerate beyond what the gravitational effects alone would indicate.

The galaxy Centaurus A is the closest example of an active galaxy to Earth, with its high-energy jets caused by electromagnetic acceleration around the central black hole. If large-scale electromagnetic fields can exist between two colliding galaxy clusters, they could potentially be responsible for generating larger particle velocities than gravity alone would appear to permit. (NASA/CXC/CFA/R.KRAFT ET AL.)

One possibility for this would be a large-scale electric or magnetic field. When charged particles (like protons and electrons, which help make up the normal matter in the Universe) encounter an electromagnetic field, they accelerate. While galaxy clusters typically form at the intersection of cosmic filaments and are driven by dark matter, there’s normal matter present as well, much of which is in the form of an ionized plasma.

Charged particles in motion must generate magnetic fields, and when objects fall into a galaxy cluster, this generates both magnetic fields and relativistic, fast-moving particles, such as electrons. When electrons move quickly in the presence of a magnetic field, they exhibit a special type of radiation known as synchrotron radiation, which could be revealed if scientists looked in the right wavelengths of light.

The full-scale image of the colliding galaxy clusters Abell 399 and Abell 401 shows X-ray data (red), Planck microwave data (yellow), and LOFAR radio data (blue) all together. The individual galaxy clusters are clearly identifiable, but the radio bridge of relativistic electrons connected by a magnetic field 10 million light-years long is incredibly illuminating. (M. MURGIA / INAF, BASED ON F. GOVONI ET AL., 2019, SCIENCE)

In a new study out in the June 7, 2019 issue of Science, scientists used the LOFAR radio telescope to find exactly this effect, for the first time, in a pair of colliding galaxy clusters. Federica Govoni and her colleagues used LOFAR to observe the region between the galaxy clusters Abell 0399 and Abell 0401, and detected a ridge of low-frequency radio emissions extending between them.

The emission indicates the presence of both a magnetic field connecting the two clusters and a population of relativistic electrons spanning the cosmic filament that ties them together. These two galaxy clusters are separated in space by a distance of approximately 10 million light-years, which would make this magnetic field and the electrons lining it one of the largest known such structures in the Universe.

As imaged by the Planck satellite (in yellow), the bridge of hot gas connecting Abell 399 and Abell 401 was discovered back in 2012. It was the first conclusive detection of a bridge of hot gas connecting a pair of galaxy clusters across intergalactic space. It is now thought to play an important role in Bullet-like clusters and the formation of galaxies and galaxy clusters overall. (ESA/PLANCK COLLABORATION / STSCI/DSS)

This radio ridge is also larger than most naive simulations predict, but that’s an extremely good thing for dark matter theories. The big puzzle for some of the colliding clusters we’ve observed is to explain how these particles can accelerate to such large speeds. Meanwhile, this enormous magnetic field and electron bridge between the two clusters suggests a mechanism to re-accelerate the particles present in the intergalactic gas: shock waves generated in the merger.

Govoni and her colleagues performed exactly this type of simulation. Her team showed that the electrons located between the galaxy clusters, already moving at speeds close to the speed of light, could be re-accelerated owing to these shock waves. If we apply this finding to the Bullet cluster, it stands to reason that we’d expect to find shock waves there, too, if we look at the X-ray emitting gas.

The X-ray observations of the Bullet Cluster, as taken by the Chandra X-ray observatory. Note the white portions of the image, which show gas that’s heated sufficiently that it requires a shock wave to explain.(NASA/CXC/CFA/M.MARKEVITCH ET AL., FROM MAXIM MARKEVITCH (SAO))

Lo and behold, these shocks are some of the first things you notice if you look at the Chandra images of the Bullet cluster on their own! The fact that we’ve identified relativistic charged particles in the presence of a large-scale magnetic field in one pair of colliding clusters is strongly suggestive of the same effects existing in other clusters. If this same type of structure that exists between Abell 0399 and Abell 0401 also exists between other colliding clusters, it could solve this minor anomaly of the Bullet cluster, leaving dark matter as the sole unchallenged explanation for the displacement of gravitational effects from the presence of normal matter.

It’s always an enormous step forward when we can identify a new phenomenon. But by combining theory, simulations, and the observations of other colliding galaxy clusters, we can push the needle forward when it comes to understanding our Universe as a whole. It’s another spectacular victory for dark matter, and another mystery of the Universe that might finally be solved by modern astrophysics. What a time to be alive.

Correction: after a Twitter exchange with one of the study’s scientists, the author regrets to inform the reader that the acceleration imparted by the magnetic fields to the electrons along this intergalactic bridge is likely unrelated to the velocity anomaly of the Bullet cluster. Although both may be explained by hydrodynamic effects, the effects that cause this radio emission and the acceleration of electrons are unrelated to the measured high velocity of the Bullet cluster’s collisional elements and X-ray gas. Ethan Siegel regrets the error.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Scientists Discover Space’s Largest Intergalactic Bridge, Solving A Huge Dark Matter Puzzle was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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The supposed ‘hole in the Universe’ that is touted to be a billion light-years across and contain no matter and emit no radiation. Reality is far more interesting than the lies included in this image’s text. (ESO, WITH TEXT BY IFLS)There aren’t any holes in the Universe at all. What actually exists is far more interesting.

Somewhere, far away, if you believe what you read, there’s a hole in the Universe. There’s a region of space so large and empty, a billion light-years across, that there’s nothing in it at all. There’s no matter of any type, normal or dark, and no stars, galaxies, plasma, gas, dust, black holes, or anything else. There’s no radiation in there at all, either. It’s an example of truly empty space, and its existence has been visually captured by our greatest telescopes.

At least, that’s what some people are saying, in a photographic meme that’s been spreading around the internet for years and refuses to die. Scientifically, though, there’s nothing true about these assertions at all. There is no hole in the Universe; the closest we have are the underdense regions known as cosmic voids, which still contain matter. Moreover, this image isn’t a void or hole at all, but a cloud of gas. Let’s do the detective work to show you what’s really going on.

The dark nebula Barnard 68, now known to be a molecular cloud called a Bok globule, has a temperature of less than 20 K. It’s still quite warm when compared with the temperatures of the cosmic microwave background, however, and is definitely not a hole in the Universe. (ESO)

The first thing you should notice, when you take a look at this image, is that the points of light you see here are numerous, of varying brightnesses, and come in a variety of colors. The brighter ones have diffraction spikes, indicating that they’re point-like (rather than extended) sources. And the black cloud that appears is clearly in the foreground of all of them, blocking all of the background light in the center but only a portion of the light at the outskirts, allowing some of the light to stream through.

These light sources cannot be objects billions of light-years away; they are stars within our own Milky Way galaxy, which itself is only around 100,000 light-years across. Therefore, this light-blocking object has to be closer than those stars are, and has to be relatively small if it’s so nearby. It cannot be a great void in the Universe.

The dusty regions that visible-light telescopes cannot penetrate are revealed by the infrared views of telescopes like the VLT with SPHERE, or, as shown here, with ESO’s HAWK-I instrument. The infrared is spectacular at showcasing the sites of new and future star formation, where the visible light-blocking dust is densest. What appears to be a hole or void in visible light can be seen to be for what it actually is: foreground matter that is simply opaque to certain wavelengths.(ESO/H. DRASS ET AL.)

In fact, this is a cloud of gas and dust that’s a mere 500 light-years away: a dark nebula known as Barnard 68. Over 100 years ago, the astronomer E. E. Barnard surveyed the night sky, looking for regions of space where there was a dearth of light silhouetted against the steady background of the Milky Way’s stars. These “dark nebulae,” as they were originally called, are now known to be molecular clouds of neutral gas, and are sometimes also known as Bok globules.

The one we’re considering here, Barnard 68, is relatively small and nearby:

  • it’s located only 500 light-years away,
  • it’s extremely low in mass, at just twice the mass of our Sun,
  • and it’s quite small in extent, with a diameter of approximately half a light-year.
Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked nearly as much, as the smaller-sized dust grains are too little to interact with the long-wavelength light. At longer wavelengths, more of the Universe beyond the light-blocking dust can be revealed. (ESO)

Above, you can see an image of Barnard 68, the same nebula, in the infrared portion of the spectrum. The particles that make up these dark nebulae are of a finite size, and that size is extremely good at absorbing visible light. But longer wavelengths of light, like infrared light, can pass right through them. In the infrared composite image, above, you can clearly see that this isn’t a void or a hole in the Universe at all, but just a cloud of gas that light can easily pass through. (If you’re willing to look at it properly.)

Bok globules are abundant throughout all gas-rich and dust-rich galaxies, and can be found in many different locations in our own Milky Way, from the dark clouds in the plane of the galaxy to the light-blocking clumps of matter found amidst star-forming and future-star-forming regions.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. While the external environment of these globules may be extremely hot, the interiors can be shielded from radiation and reach very low temperatures indeed. (ESA / HUBBLE & NASA)

So if that’s what this image is actually showing, what about the idea behind the caption: that somewhere out there is an enormous void in the Universe, more than a billion light-years across, that contains no matter of any type and that emits no radiation of any type at all?

Well, there are indeed voids out there in the Universe, but they’re probably not the same as what you might think. If you were to take the Universe as it was when it began — as a nearly perfectly uniform sea of normal matter, dark matter and radiation — you’d be compelled to ask how it evolved into the Universe we see today. The answer, of course, involves gravitational attraction, the expansion of the Universe, radiation and gravitational collapse, star formation, feedback, and time.

While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Neutrinos are ubiquitous, but standard, light neutrinos cannot account for most (or even a significant fraction) of the dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

These ingredients, when subject to the laws of physics over the past 13.8 billion years of our cosmic history, lead to the formation of a vast and intricate cosmic web. Gravitational attraction is a runaway process, where overdense regions not only grow, but grow more rapidly as they accumulate more and more matter. The lower-density regions around them, even from quite a distance away, don’t stand a chance.

Just as the overdense regions grow, the surrounding regions that are underdense, of average density, or even of above-average density (but less “above-average” than the most overdense nearby region) will lose their matter to the denser ones. What we wind up with is a network of galaxies, galaxy groups, galaxy clusters, and large-scale filaments of structure, with enormous cosmic voids between them.

The evolution of large-scale structure in the Universe, from an early, uniform state to the clustered Universe we know today. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. Note that in all cases, small-scale structure arises before structure on the largest scales comes about, and that even the most underdense regions of all still contain non-zero amounts of matter. (ANGULO ET AL. 2008, VIA DURHAM UNIVERSITY)

Does this mean, though, that these cosmic voids are completely empty of normal matter, dark matter, and emit no detectable radiation of any kind?

Not at all. Voids are large-scale underdense regions, but they aren’t completely devoid of matter at all. While large galaxies within them may be rare, they do exist. Even in the deepest, sparsest cosmic void we’ve ever found, there is still a large galaxy sitting at the center. Even with no other detectable galaxies around it, this galaxy — known as MCG+01–02–015 — displays enormous evidence of having merged with smaller galaxies over its cosmic history. Even though we cannot detect these smaller, surrounding galaxies directly, we have every reason to believe they are present.

The galaxy shown at the center of the image here, MCG+01–02–015, is a barred spiral galaxy located inside a great cosmic void. It is so isolated that if humanity were located in this galaxy instead of our own and developed astronomy at the same rate, we wouldn’t have detected the first galaxy beyond our own until the 1960s. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)

We see, in many of these cosmic voids, evidence for molecular clouds of gas that are less dense than the Bok globules we talked about earlier, but still that are dense enough to absorb distant starlight or quasar light. These absorption features tell us, quite definitively, that these voids do contain matter: typically in about 50% the abundance of the average cosmic density.

These are low-density regions, not regions completely devoid of all types of matter.

The light from ultra-distant quasars provide cosmic laboratories for measuring not only the gas clouds they encounter along the way, but for the intergalactic medium that contains warm-and-hot plasmas outside of clusters, galaxies, and filaments. Because the exact properties of the emission or absorption lines are dependent on the fine structure constant, this is one of the top methods for probing the Universe for time or spatial variations in the fine structure constant, as well as the properties of the intervening regions of space. (ED JANSSEN, ESO)

We see evidence for the presence of dark matter as well, as the background starlight shows the effects of both gravitational changes (via the integrated Sachs-Wolf effect) and of weak gravitational lensing. Even the cold spots that appear in the cosmic microwave background can be cross-correlated with these underdense regions.

The magnitude of how cold these cold spots get teach us something very important: these voids cannot have zero matter in them at all. They might have just a fraction of the density of a typical region, but as far as underdensities go, a density that’s ~0% the average density is inconsistent with the data.

The cold fluctuations (shown in blue) in the CMB are not inherently colder, but rather represent regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies and clusters, while the underdense regions will be less likely to do so. The gravitational density of the regions the light passes through as it travels can show up in the CMB as well, teaching us what these regions are truly like. (E.M. HUFF, THE SDSS-III TEAM AND THE SOUTH POLE TELESCOPE TEAM; GRAPHIC BY ZOSIA ROSTOMIAN)

You might, then, begin worrying why we cannot detect any radiation or light of any type from them. It should be true that these regions would emit light. The stars that formed within them must emit visible light; the hydrogen molecules that transition from a spin-aligned state to an anti-aligned state should emit 21-cm radiation; the contracting clouds of gas should emit infrared radiation.

Why don’t we detect it? Simple: our telescopes, at these great cosmic distances, aren’t sensitive enough to pick up photons of such low densities. This is why we have worked so hard, as astronomers, to develop other methods of directly and indirectly measuring what’s present in space. Catching emitted radiation is an extremely limiting proposition, and isn’t always the best way to make a detection.

In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, spanning a billion light-years or more, they all contain matter at some level. Even the void that houses MCG+01–02–015 likely contains small, low surface brightness galaxies that are below the detection limit.(ANDREW Z. COLVIN (CROPPED BY ZERYPHEX) / WIKIMEDIA COMMONS)

It is absolutely true that billions of light-years away, there are enormous cosmic voids in space. Typically, they can extend for hundreds of millions of light-years in diameter, and a few of them might extend for a billion light-years in size or even many billions of light-years. And one more thing is true: the most extreme ones don’t emit any detectable radiation.

But that is not because there is no matter in them; there is. It’s not because there aren’t stars, gas molecules, or dark matter; all are present. You just can’t measure their presence from emitted radiation; you need other methods and techniques, which show us that these voids still contain substantial quantities of matter. And you definitely shouldn’t confuse them with dark gas clouds and Bok globules, which are small, nearby clouds of light-blocking matter. The Universe is plenty fascinating exactly as it is; let’s resist the temptation to embellish reality with our own exaggerations.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

No, This Is Not A Hole In The Universe was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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The inside of the LHC, where protons pass each other at 299,792,455 m/s, just 3 m/s shy of the speed of light. As powerful as the LHC is, the cancelled SSC could have been three times as powerful, and may have revealed secrets of nature that are inaccessible at the LHC. (CERN)If we don’t push the frontiers of physics, we’ll never learn what lies beyond our current understanding.

At a fundamental level, what is our Universe made of? This question has driven physics forward for centuries. Even with all the advances we’ve made, we still don’t know it all. While the Large Hadron Collider discovered the Higgs boson and completed the Standard Model earlier this decade, the full suite of the particles we know of only make up 5% of the total energy in the Universe.

We don’t know what dark matter is, but the indirect evidence for it is overwhelming. Same deal with dark energy. Or questions like why the fundamental particles have the masses they do, or why neutrinos aren’t massless, or why our Universe is made of matter and not antimatter. Our current tools and searches have not answered these great existential puzzles of modern physics. Particle physics now faces an incredible dilemma: try harder, or give up.

The Standard Model of particle physics accounts for three of the four forces (excepting gravity), the full suite of discovered particles, and all of their interactions. Whether there are additional particles and/or interactions that are discoverable with colliders we can build on Earth is a debatable subject, but one we’ll only know the answer to if we explore past the known energy frontier. (CONTEMPORARY PHYSICS EDUCATION PROJECT / DOE / NSF / LBNL)

The particles and interactions that we know of are all governed by the Standard Model of particle physics, plus gravity, dark matter, and dark energy. In particle physics experiments, however, it’s the Standard Model alone that matters. The six quarks, charged leptons and neutrinos, gluons, photon, gauge bosons and Higgs boson are all that it predicts, and each particle has been not only discovered, but their properties have been measured.

As a result, the Standard Model is perhaps a victim of its own success. The masses, spins, lifetimes, interaction strengths, and decay ratios of every particle and antiparticle have all been measured, and they agree with the Standard Model’s predictions at every turn. There are enormous puzzles about our Universe, and particle physics has given us no experimental indications of where or how they might be solved.

The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs Boson, falling at the LHC earlier this decade. All of these particles can be created at LHC energies, and the masses of the particles lead to fundamental constants that are absolutely necessary to describe them fully. These particles can be well-described by the physics of the quantum field theories underlying the Standard Model, but they do not describe everything, like dark matter. (E. SIEGEL / BEYOND THE GALAXY)

It might be tempting, therefore, to presume that building a superior particle collider would be a fruitless endeavor. Indeed, this could be the case. The Standard Model of particle physics has explicit predictions for the couplings that occur between particles. While there are a number of parameters that remain poorly determined at present, it’s conceivable that there are no new particles that a next-generation collider could reveal.

The heaviest Standard Model particle is the top quark, which takes roughly ~180 GeV of energy to create. While the Large Hadron Collider can reach energies of 14 TeV (about 80 times the energy needed to create a top quark), there might not be any new particles present to find unless we reach energies in excess of 1,000,000 times as great. This is the great fear of many: the possible existence of a so-called “energy desert” extending for many orders of magnitude.

There is certainly new physics beyond the Standard Model, but it might not show up until energies far, far greater than what a terrestrial collider could ever reach. Still, whether this scenario is true or not, the only way we’ll know is to look. In the meantime, properties of the known particles can be better explored with a future collider than any other tool. The LHC has failed to reveal, thus far, anything beyond the known particles of the Standard Model. (UNIVERSE-REVIEW.CA)

But it’s also possible that there is new physics present at a modest scale beyond where we’ve presently probed. There are many theoretical extensions to the Standard Model that are quite generic, where deviations from the Standard Model’s predictions can be detected by a next-generation collider.

If we want to know what the truth about our Universe is, we have to look, and that means pushing the present frontiers of particle physics into uncharted territory. Right now, the community is debating between multiple approaches, with each one having its pros and cons. The nightmare scenario, however, isn’t that we’ll look and won’t find anything. It’s that infighting and a lack of unity will doom experimental physics forever, and that we won’t get a next-generation collider at all.

A hypothetical new accelerator, either a long linear one or one inhabiting a large tunnel beneath the Earth, could dwarf the sensitivity to new particles that prior and current colliders can achieve. Even at that, there’s no guarantee we’ll find anything new, but we’re certain to find nothing new if we fail to try. (ILC COLLABORATION)

When it comes to deciding what collider to build next, there are two generic approaches: a lepton collider (where electrons and positrons are accelerated and collided), and a proton collider (where protons are accelerated and collided). The lepton colliders have the advantages of:

  • the fact that leptons are point particles, rather than composite particles,
  • 100% of the energy from electrons colliding with positrons can be converted into energy for new particles,
  • the signal is clean and much easier to extracts,
  • and the energy is controllable, meaning we can choose to tune the energy to a specific value and maximize the chance of creating a specific particle.

Lepton colliders, in general, are great for precision studies, and we haven’t had a cutting-edge one since LEP was operational nearly 20 years ago.

At various center-of-mass energies in electron/positron (lepton) colliders, various Higgs production mechanisms can be reached at explicit energies. While a circular collider can achieve much greater collision rates and production rates of W, Z, H, and t particles, a long-enough linear collider can conceivably reach higher energies, enabling us to probe Higgs production mechanisms that a circular collider cannot reach. This is the main advantage that linear lepton colliders possess; if they are low-energy only (like the proposed ILC), there is no reason not to go circular. (H. ABRAMOWICZ ET AL., EUR. PHYS. J. C 77, 475 (2017))

It’s very unlikely, unless nature is extremely kind, that a lepton collider will directly discover a new particle, but it may be the best bet for indirectly discovering evidence of particles beyond the Standard Model. We’ve already discovered particles like the W and Z bosons, the Higgs boson, and the top quark, but a lepton collider could both produce them in great abundances and through a variety of channels.

The more events of interest we create, the more deeply we can probe the Standard Model. The Large Hadron Collider, for example, will be able to tell whether the Higgs behaves consistently with the Standard Model down to about the 1% level. In a wide series of extensions to the Standard Model, ~0.1% deviations are expected, and the right future lepton collider will get you the best physics constraints possible.

The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding, and yet frustrating at the same time. By the 2030s, the LHC will have approximately 50 times as much data, but the precisions on many decay channels will still only be known to a few percent. A future collider could increase that precision by multiple orders of magnitude, revealing the existence of potential new particles.(ANDRÉ DAVID, VIA TWITTER)

These precision studies could be incredibly sensitive to the presence of particles or interactions we haven’t yet discovered. When we create a particle, it has a certain set of branching ratios, or probabilities that it will decay in a variety of ways. The Standard Model makes explicit predictions for those ratios, so if we create a million, or a billion, or a trillion such particles, we can probe those branching ratios to unprecedented precisions.

If you want better physics constraints, you need more data and better data. It isn’t just the technical considerations that should determine which collider comes next, but also where and how you can get the best personnel, the best infrastructure and support, and where you can build a (or take advantage of an already-existing) strong experimental and theoretical physics community.

The idea of a linear lepton collider has been bandied about in the particle physics community as the ideal machine to explore post-LHC physics for many decades, but that was under the assumption that the LHC would find a new particle other than the Higgs. If we want to do precision testing of Standard Model particles to indirectly search for new physics, a linear collider may be an inferior option to a circular lepton collider. (REY HORI/KEK)

There are two general classes proposals for a lepton collider: a circular collider and a linear collider. Linear colliders are simple: accelerate your particles in a straight line and collide them together in the center. With ideal accelerator technology, a linear collider 11 km long could reach energies of 380 GeV: enough to produce the W, Z, Higgs, or top in great abundance. With a 29 km linear collider, you could reach energies of 1.5 TeV, and with a 50 km collider, 3 TeV, although costs rise tremendously to accompany longer lengths.

Linear colliders are slightly less expensive than circular colliders for the same energy, because you can dig a smaller tunnel to reach the same energies, and they don’t suffer energy losses due to synchrotron radiation, enabling them to reach potentially higher energies. However, the circular colliders offer an enormous advantage: they can produce much greater numbers of particles and collisions.

The Future Circular Collider is a proposal to build, for the 2030s, a successor to the LHC with a circumference of up to 100 km: nearly four times the size of the present underground tunnels. This will enable, with current magnet technology, the creation of a lepton collider that can produce ~1⁰⁴ times the number of W, Z, H, and t particles that have been produced by prior and current colliders. (CERN / FCC STUDY)

While a linear collider might be able to produce 10 to 100 times as many collisions as a prior-generation lepton collider like LEP (dependent on energies), a circular version can surpass that easily: producing 10,000 times as many collisions at the energies required to create the Z boson.

Although circular colliders have substantially higher event rates than linear colliders at the relevant energies that produce Higgs particles as well, they begin to lose their advantage at energies required to produce top quarks, and cannot reach beyond that at all, where linear colliders become dominant.

Because all of the decay and production processes that occur in these heavy particles scales as either the number of collisions or the square root of the number of collisions, a circular collider has the potential to probe physics with many times the sensitivity of a linear collider.

A number of the various lepton colliders, with their luminosity (a measure of the collision rate and the number of detections one can make) as a function of center-of-mass collision energy. Note that the red line, which is a circular collider option, offers many more collisions than the linear version, but gets less superior as energy increases. Beyond about 380 GeV, circular colliders cannot reach, and a linear collider like CLIC is the far superior option. (GRANADA STRATEGY MEETING SUMMARY SLIDES / LUCIE LINSSEN (PRIVATE COMMUNICATION))

The proposed FCC-ee, or the lepton stage of the Future Circular Collider, would realistically discover indirect evidence for any new particles that coupled to the W, Z, Higgs, or top quark with masses up to 70 TeV: five times the maximum energy of the Large Hadron Collider.

The flipside to a lepton collider is a proton collider, which — at these high energies — is essentially a gluon-gluon collider. This cannot be linear; it must be circular.

The scale of the proposed Future Circular Collider (FCC), compared with the LHC presently at CERN and the Tevatron, formerly operational at Fermilab. The Future Circular Collider is perhaps the most ambitious proposal for a next-generation collider to date, including both lepton and proton options as various phases of its proposed scientific programme. (PCHARITO / WIKIMEDIA COMMONS)

There is really only one suitable site for this: CERN, since it not only needs a new, enormous tunnel, but all the infrastructure of the prior stages, which only exist at CERN. (They could be built elsewhere, but the cost would be more expensive than a site where the infrastructure like the LHC and earlier colliders like SPS already exist.)

Just as the LHC is presently occupying the tunnel previously occupied by LEP, a circular lepton collider could be superseded by a next-generation circular proton collider, such as the proposed FCC-pp. However, you cannot run both an exploratory proton collider and a precision lepton collider simultaneously; you must decommission one to finish the other.

The CMS detector at CERN, one of the two most powerful particle detectors ever assembled. Every 25 nanoseconds, on average, a new particle bunch collides at the center-point of this detector. A next-generation detector, whether for a lepton or proton collider, may be able to record even more data, faster, and with higher-precision than the CMS or ATLAS detectors can at present. (CERN)

It’s very important to make the right decision, as we do not know what secrets nature holds beyond the already-explored frontiers. Going to higher energies unlocks the potential for new direct discoveries, while going to higher precisions and greater statistics could provide even stronger indirect evidence for the existence of new physics.

The first-stage linear colliders are going to cost between 5 and 7 billion dollars, including the tunnel, while a proton collider of four times the LHC’s radius, with magnets twice as strong, 10 times the collision rate and next-generation computing and cryogenics might cost a total of up to $22 billion, offering as big a leap over the LHC as the LHC was over the Tevatron. Some money could be saved if we build the circular lepton and proton colliders one after the other in the same tunnel, which would essentially provide a future for experimental particle physics after the LHC is done running at the end of the 2030s.

The Standard Model particles and their supersymmetric counterparts. Slightly under 50% of these particles have been discovered, and just over 50% have never showed a trace that they exist. Supersymmetry is an idea that hopes to improve on the Standard Model, but it has yet to make successful predictions about the Universe in attempting to supplant the prevailing theory. However, new colliders are not being proposed to find supersymmetry or dark matter, but to perform generic searches. Regardless of what they’ll find, we’ll learn something new about the Universe itself. (CLAIRE DAVID / CERN)

The most important thing to remember in all of this is that we aren’t simply continuing to look for supersymmetry, dark matter, or any particular extension of the Standard Model. We have a slew of problems and puzzles that indicate that there must be new physics beyond what we currently understand, and our scientific curiosity compels us to look. In choosing what machine to build, it’s vital to choose the most performant machine: the ones with the highest numbers of collisions at the energies we’re interested in probing.

Regardless of which specific projects the community chooses, there will be trade-offs. A linear lepton collider can always reach higher energies than a circular one, while a circular one can always create more collisions and go to higher precisions. It can gather just as much data in a tenth the time, and probe for more subtle effects, at the cost of a lower energy reach.

This diagram displays the structure of the standard model (in a way that displays the key relationships and patterns more completely, and less misleadingly, than in the more familiar image based on a 4x4 square of particles). In particular, this diagram depicts all of the particles in the Standard Model (including their letter names, masses, spins, handedness, charges, and interactions with the gauge bosons: i.e., with the strong and electroweak forces). It also depicts the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry, and how the properties of the remaining particles change as a consequence. Note that the Z boson couples to both quarks and leptons, and can decay through neutrino channels. (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

Will it be successful? Regardless of what we find, that answer is unequivocally yes. In experimental physics, success does not equate to finding something, as some might erroneously believe. Instead, success means knowing something, post-experiment, that you did not know before you did the experiment. To push beyond the presently known frontiers, we’d ideally want both a lepton and a proton collider, at the highest energies and collision rates we can achieve.

There is no doubt that new technologies and spinoffs will come from whichever collider or colliders come next, but that’s not why we do it. We are after the deepest secrets of nature, the ones that will remain elusive even after the Large Hadron Collider finishes. We have the technical capabilities, the personnel, and the expertise to build it right at our fingertips. All we need is the political and financial will, as a civilization, to seek the ultimate truths about nature.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Does Particle Physics Have A Future On Earth? was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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This image is a composite of the Ring Nebula (Messier 57). It was generated by combining new Hubble Wide Field Camera 3 data with observations of the nebula’s outer halo from the Large Binocular Telescope (LBT). Despite its appearances, this object isn’t solely composed of a ring-like structure after all. (HUBBLE DATA: NASA, ESA, C. ROBERT O’DELL (VANDERBILT UNIVERSITY); LBT DATA: DAVID THOMPSON)If we could see in three dimensions instead of two, we’d never have thought otherwise.

Perhaps the most famous sight of a dying star is the Ring Nebula, discovered in 1779.

The Ring Nebula appears to be an enormous gaseous ring surrounding a white dwarf star. This is representative of the fate of Sun-like stars that aren’t part of multi-star systems. Despite its appearances, this isn’t a true ring after all. (NASA, ESA, AND C. ROBERT O’DELL (VANDERBILT UNIVERSITY))

At just over 2,000 light-years distant, it’s the closest dying star to Earth.

In between the 2nd and 3rd brightest stars of the constellation Lyra, the blue giant stars Sheliak and Sulafat (shown prominently here), the Ring Nebula can easily be identified with any telescope or even a pair of binoculars. (NASA, ESA, DIGITIZED SKY SURVEY 2)

Upon observing it, Charles Messier wrote: “it is very dull, but perfectly outlined; it is as large as Jupiter & resembles a planet which is fading.”

This observation originated the misnomer “planetary nebula,” but physically originates when dying stars expel their outer layers.

The elements of the periodic table, and where they originate, are detailed in this image above. While most elements originate primarily in supernovae or merging neutron stars, many vitally important elements are created, in part or even mostly, in these planetary nebulae like the Ring Nebula. (NASA/CXC/SAO/K. DIVONA)

Despite looking very much like a ring to our eyes, the Ring Nebula is anything but.

Planetary nebulae take a wide variety of shapes and orientations depending on the properties of the stellar system they arise from, and are responsible for many of the heavy elements in the Universe. Supergiant stars and giant stars entering the planetary nebula phase are both shown to build up many important elements of the periodic table via the s-process. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

A huge, diffuse set of hydrogen shells surround it, showcasing recently blown-off material as the star dies.

The red outer shells are signs of ionized hydrogen gas, huge and intricate outside the ring itself. Sulfur and Oxygen ions, expelled from the star and prominent in the ring area, are viewed in the other colors shown here. Spectroscopic imaging, where particular emission lines from a specific element, is key to teasing out these features. (D. LÓPEZ (IAC), WHICH IS A. OSCOZ, D. LÓPEZ, P. RODRÍGUEZ-GIL AND L. CHINARRO)

Along our line-of-sight, lobes of low-density gas extend both towards and away from us.

The Helix Nebula, a similar planetary nebula (with a donut-shaped appearance) to the Ring Nebula, has also had its 3D structure mapped out. It, too, is far more intricate than a simple ring explanation would indicate. (NASA, ESA, C.R. O’DELL (VANDERBILT UNIVERSITY), AND M. MEIXNER, P. MCCULLOUGH, AND G. BACON ( SPACE TELESCOPE SCIENCE INSTITUTE))

Our perspective view this structure almost directly down one of its poles, explaining its ring-like appearance.

The Spitzer Space Telescope, looking in infrared light, showcases the temperature of different portions of the Ring Nebula. The inner regions are far hotter, which explains why they’re far brighter. The electrons that were excited or ionized that then fall down in their orbitals is what causes the emission of the light that we can see, and that preferentially happens in the hottest regions. (NASA/JPL-CALTECH/J. HORA (HARVARD-SMITHSONIAN CFA))

In 2013, astronomers used new Hubble data to map out the nebula’s 3D structure.

This schematic shows the geometry and structure of the Ring Nebula (Messier 57) as it would appear if viewed from the side, rather than along our line-of-sight. This shows the nebula’s wide halo, inner region, lower-density lobes of material stretching towards and away from us, and the prominent, glowing disc. (NASA, ESA, AND A. FEILD (STSCI))

The reflective, high-density gas is all most telescopes ever observe.

Through a modest-sized telescope at a dark-sky site, this is what the Ring Nebula will appear to look like through an eyepiece to a human observer. The origin of the name ‘Ring Nebula’ is apparent, but the true story is far more revealing. (CHRIS SPRATT)

But we now know it isn’t a ring at all, but also displays intricate structure, with an outer halo, inner turbulence, lobes and knots.

The different elements (in different colors), the neutral knots of gas (dark globs), and the translucent hue of the inner ring are all artifacts of viewing this intricate 3D structure face-on. The Ring Nebula is no ring at all, nor is it spherical in shape. Its true nature is far more complex, and has taken a variety of observations to reveal. (NASA, ESA, AND C. ROBERT O’DELL (VANDERBILT UNIVERSITY))

This may be the exact fate awaiting the Sun in our distant future.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

The Ring Nebula Is Much, Much More Than A Ring was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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From outside a black hole, all the infalling matter will emit light and always is visible, while nothing from behind the event horizon can get out. But if you were the one who fell into a black hole, what you’d see would be interesting and counterintuitive, and we know what it would actually look like. (ANDREW HAMILTON, JILA, UNIVERSITY OF COLORADO)It’s the ultimate way to go… and yet, it still isn’t what you’d expect.

There are many terrifying ways that the Universe can destroy something. In space, if you tried to hold your breath, your lungs would explode; if you exhaled every molecule of air instead, you’d black out within seconds. In some locations, you’d freeze solid as the heat was sucked out of your body; in others it’s so hot that your atoms would turn into a plasma. But of all the ways the Universe has to dispose of someone, I can think of none more fascinating than to send someone inside a black hole. So does Event Horizon Telescope scientist Heino Falcke, who asks:

[W]hat is it like to be/fall inside a rotating black hole? This is not observable, but calculable… I have talked with various people who have done these calculations, but I am getting old and keep forgetting things.

It’s a tremendously interesting question, and one that science can answer. Let’s find out.

An illustration of heavily curved spacetime, outside the event horizon of a black hole. As you get closer and closer to the mass’s location, space becomes more severely curved, eventually leading to a location from within which even light cannot escape: the event horizon. The radius of that location is set by the mass of the black hole, the speed of light, and the laws of General Relativity alone. In theory, there should be a special point, a singularity, where all the mass is concentrated for stationary, spherically-symmetric black holes. (PIXABAY USER JOHNSONMARTIN)

According to our theory of gravity, Einstein’s General Relativity, there are only three things that determine the properties of a black hole. They are the following:

  1. Mass, or the total amount of matter and the equivalent amount of energy (via E = mc²) that went into both forming and growing the black hole to its present state.
  2. Charge, or the net electric charge that exists in the black hole from all the positively and negatively charged objects that fell into the black hole over its history.
  3. Angular momentum, or spin, which is a measure of the total amount of rotational motion that the black hole inherently has.

Realistically, all the black holes that physically exist in our Universe should have large masses, significant amounts of angular momentum, and negligible charges. This complicates matters tremendously.

When a massive enough star ends its life, or two massive enough stellar remnants merge, a black hole can form, with an event horizon proportional to its mass and an accretion disk of infalling matter surrounding it. When the black hole rotates, the space both outside and inside the event horizon rotates, too: this is the effect of frame-dragging, which can be enormous for black holes. (ESA/HUBBLE, ESO, M. KORNMESSER)

When we typically think of a black hole, we imagine the much simpler kind: one described by its mass only. It has an event horizon that surrounds a single point, and a region surrounding that point from which light cannot escape. That region is perfectly spherical, and has a boundary separating the regions where light can escape from the region where it cannot: the event horizon. The event horizon is located a specific distance (the Schwarzschild radius) away from the singularity in all directions equally.

This is an simplified version of a realistic black hole, but a good place to start thinking about the physics that occurs in two distinct places: outside the event horizon and inside the event horizon.

Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No 3D structures can survive intact. (ASK THE VAN / UIUC PHYSICS DEPARTMENT)

Outside of the event horizon, gravity behaves just like you’d conventionally expect. Space is curved by the presence of this mass, which causes every object within the Universe to experience an acceleration towards the central singularity. If you were to start off a large distance away from this black hole, at rest, and allowed an object to fall into it, what would you see?

Assuming you were able to remain stationary, you’d see this infalling object slowly accelerate away from you, towards this black hole. It would speed up towards the event horizon, remaining the same color, and then something strange would happen. It would appear to slow down, fade away, and get redder in color. It wouldn’t completely disappear though; not quickly, and not ever. Instead, it would just approach that state: getting fainter, redder, and harder to detect. The event horizon is like an asymptote for the object’s light; you’ll always be able to see it if you look hard enough.

This artist’s impression depicts a Sun-like star being torn apart by tidal disruption as it nears a black hole. Objects that have previously fallen in will still be visible, although their light will appear faint and red (easily shifted so far into the red they are invisible to human eyes) in proportion to the amount of time that’s passed since they crossed the event horizon. (ESO, ESA/HUBBLE, M. KORNMESSER)

Now, imagine the same scenario, but this time, don’t imagine you’re observing the infalling object from afar. Instead, imagine that you yourself are the infalling object. The experience you’d have would be extremely different.

The event horizon appears to get much larger far faster than you’d expect, as the curvature of space gets severe. Around the event horizon, space is so distorted that you begin to see multiple images of the outside Universe, as though they were reflected and inverted.

And once you crossed inside the event horizon, you’d not only still see the outside Universe, but a portion of the Universe inside the event horizon. The light you received would blueshift, but then redshift again, as you inevitably fell towards the singularity. In the last moments, space would bizarrely look completely flat.

The physics of this is complicated, but the calculations are straightforward, and were most elegantly performed by Andrew Hamilton of the University of Colorado in a series of papers spanningthe late 2000s to the early 2010s. Hamilton has also created a series of spectacular visualizations on what you would see as you fell into a black hole, based on these calculations.

There are a number of lessons we can learn from examining these results, and many of them are counterintuitive. The way to try and make sense of it is to change the way you visualize space. Normally, we think of space as a stationary fabric and we think of an observer as being “plunked” down somewhere. But inside an event horizon, you’re always in motion. Space is fundamentally in motion — like a moving walkway — continuously, moving everything in it towards the singularity.

Both inside and outside the event horizon, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. (ANDREW HAMILTON / JILA / UNIVERSITY OF COLORADO)

It moves everything so quickly that even if you accelerate directly away from the singularity with an infinite amount of force, you’ll still fall towards the center. Objects from outside the event horizon will still have their light encounter you from all directions, but you’ll only ever be able to see a portion of the objects from inside the event horizon.

The line that defines the boundary between what any observer can see is mathematically described by a cardioid, where the largest-radius component of the cardioid touches the event horizon and the smallest-radius component terminates at the singularity. This means that the singularity, even though it’s a point, does not inevitably connect everything that falls in to everything else. If you and I fall into opposite sides of the event horizon at the same time, we’ll never be able to see each other’s light after the horizon-crossing takes place.

When you fall into a black hole or simply get very close to the event horizon, its size and scale appear much larger than the actual size. To an outside observer watching you fall in, your information would get encoded on the event horizon. What happens to that information as the black hole evaporates is still unanswered. (ANDREW HAMILTON / JILA / UNIVERSITY OF COLORADO)

The reason for this is the always-in-motion fabric of the Universe itself. Inside the event horizon, space moves faster than light, and that’s why nothing can ever escape from the black hole. It’s also why, once inside the black hole, you start seeing bizarre things like multiple images of the same object.

You can understand this by asking a question like, “where is the singularity?”

From inside a black hole’s event horizon, if you move in any direction, you’ll eventually encounter the singularity itself. Therefore, surprisingly, the singularity appears in all directions! If your feet are directly pointed in the direction you’re accelerating, you’ll see them below you, but you’ll also see them above you. All of this is straightfoward to calculate, even though it’s tremendously counterintuitive. And that’s just for the simplified case: the non-rotating black hole.

In April of 2017, all 8 of the telescopes/telescope arrays associated with the Event Horizon Telescope pointed at Messier 87. This is what a supermassive black hole looks like, where the event horizon is clearly visible. Only through VLBI could we achieve the resolution necessary to construct an image like this, but the potential exists to someday improve it by a factor of hundreds. The shadow is consistent with a rotating (Kerr) black hole. (EVENT HORIZON TELESCOPE COLLABORATION ET AL.)

Now, let’s come to the physically interesting case: where the black hole spins. Black holes owe their origin to systems of matter, like stars, which always spin at some level. In our Universe (and in General Relativity), angular momentum is an absolutely conserved quantity for any close system; there’s no way to get rid of it. When a collection of matter collapses down to a radius smaller than the radius of an event horizon, the angular momentum gets trapped inside there, just like the mass does.

The solution we get is now much more complicated. Einstein put forth General Relativity in 1915, and Karl Schwarzschild derived the non-rotating black hole solution a couple of months later, in early 1916. But the next step in modeling this problem in a more realistic fashion — to consider what if the black hole also has angular momentum, instead of mass alone — wasn’t solved until Roy Kerr found the exact solution in 1963.

The exact solution for a black hole with both mass and angular momentum was found by Roy Kerr in 1963, and revealed, instead of a single event horizon with a point-like singularity, an inner and outer event horizon, as well as an inner and outer ergosphere, plus a ring-like singularity of substantial radius. (MATT VISSER, ARXIV:0706.0622)

There are some fundamental and important differences between the more naive, simpler Schwarzschild solution and the more realistic, complex Kerr solution. In no particular order, here are some fascinating contrasts:

  1. Instead of a single solution for where the event horizon is, a rotating black hole has two mathematical solutions: an inner and and outer event horizon.
  2. Outside of even the outer event horizon, there is a place known as the ergosphere, where space itself is dragged around at a rotational speed equal to the speed of light, and particles falling in there experience enormous accelerations.
  3. There is a maximum ratio of angular momentum to mass that is allowed; if there is too much angular momentum, the black hole will radiate that energy away (via gravitational radiation) until it’s below that limit.
  4. And, perhaps most fascinatingly, the singularity at the black hole’s center is no longer a point, but rather a 1-dimensional ring, where the radius of the ring is determined by the mass and angular momentum of the black hole.
Shadow (black) & horizons and ergospheres (white) of a rotating black hole. The quantity of a, shown varying in the image, has to do with the relationship of angular momentum of the black hole to its mass. Note that the shadow as seen by the Event Horizon Telescope of the black hole is much larger than either the event horizon or ergosphere of the black hole itself. (YUKTEREZ (SIMON TYRAN, VIENNA) / WIKIMEDIA COMMONS)

With all this in mind, what happens when you fall inside a rotating black hole? The same thing that happens when you fall into a non-rotating black hole, except that all of space doesn’t behave as though it’s falling towards a central singularity. Instead, space also behaves as though it’s getting dragged around along the direction of rotation, like a spinning vortex. The larger the ratio of angular momentum to mass, the faster it rotates.

While the concept of how spacetime flows outside and inside the (outer) event horizon for a rotating black hole is similar to that for a non-rotating black hole, there are some fundamental differences that lead to some incredibly different details when you consider what an observer who falls through that horizon will see of the outside (and inside) worlds. The simulations break down when you encounter the outer event horizon. (ANDREW HAMILTON / JILA / UNIVERSITY OF COLORADO)

This means that if you see something fall in, you’ll see it get fainter and redder, but also smeared out into a ring or a disk along the direction of rotation. If you fall in, you’ll get whipped around like you’re on some maddening carousel that sucks you towards the center. And when you reach the singularity, it will be a ring; different parts of your body will encounter the singularity — at the inner ergosurface of the Kerr black hole — at different spatial coordinates. As you approach the singularity from inside the event horizon, you’ll gradually become unable to see the other parts of your own body.

The most profound piece of information you should take away from all of this is that the fabric of space itself is in motion, and the event horizon is defined as the location where even if you’re able to move at the ultimate cosmic speed limit — the speed of light — in whatever direction you choose, you will always wind up encountering the singularity.

The visualizations by Andrew Hamilton are the best, most scientifically accurate simulations of what falling into a black hole truly looks like, and are so counterintuitive that all I can truly recommend is that you watch them over and over again until you fool yourself into thinking you understand it. It’s eerie, beautiful, and if you’re adventurous enough to ever fly yourself to a black hole and cross inside the event horizon, it’ll be the last thing you ever see!

Send in your Ask Ethan questions to startswithabang at gmail dot com!

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Ask Ethan: What’s It Like When You Fall Into A Black Hole? was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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This is the Milky Way from Concordia Camp, in Pakistan’s Karakoram Range. While many of the stars seen here may have already died, their stellar remnants continue to shine on. (ANNE DIRKSE / ANNEDIRKSE.COM)13.8 billion years isn’t close to enough time, but if we wait long enough, even our Sun will become one.

The Big Bang happened approximately 13.8 billion years ago, and it might have only taken 50–100 million years to form the very first stars. Ever since then, the Universe has been flooded with starlight. When enough matter — mostly hydrogen and helium gas — gravitates together into a single, compact object, nuclear fusion must take place inside the core, giving rise to a true star.

But as time goes on and fusion continues, eventually that star will run out of fuel. Sometimes, the star is massive enough that additional fusion reactions will take place, but at some point, it all must stop. Even when a star finally dies, however, their remnants will continue to shine. In fact, except for black holes, every remnant ever created still shines today. Here’s the story of how long we’ll need to wait for the first star to truly go dark.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. While the external environment of these globules may be extremely hot, the interiors can be shielded from radiation and reach very low temperatures indeed. (ESA / HUBBLE & NASA)

It all begins from clouds of gas. When a cloud of molecular gas collapses under its own gravity, there are always a few regions that start off just a little bit denser than others. Every location with matter in it does its best to attract more and more matter towards it, but these overdense regions attract matter more efficiently than all the others. Because gravitational collapse is a runaway process, the more matter you attract to your vicinity, the faster additional matter will flow inward.

While it can take millions to tens of millions of years for a molecular cloud to go from a large, diffuse state to a relatively collapsed one, the process of going from a collapsed state of dense gas to a new cluster of stars — where the densest regions ignite fusion in their cores — takes only a few hundred thousand years.

Dark, dusty molecular clouds, like this one within our Milky Way, will collapse over time and give rise to new stars, with the densest regions within forming the most massive stars. (ESO)

Stars come in a huge variety of colors, brightnesses and masses, and a star’s life cycle and fate are determined from the moment of the star’s birth. When you create a new cluster of stars, the easiest ones to notice are the brightest ones, which also happen to be the most massive. These are the brightest, bluest, hottest stars in existence, with up to hundreds of times the mass of our Sun and with millions of times the luminosity.

But despite the fact that the brightest ones are the stars that appear the most spectacular, these are also the rarest stars, making up far less than 1% of all the known, total stars. They are also the shortest-lived stars, as they burn through all the nuclear fuel (in all the various stages) in their cores in as little as 1–2 million years.

Hubble space telescope of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. The hottest, bluest stars are over 200 times the mass of our Sun.(NASA, ESA, AND E. SABBI (ESA/STSCI); ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WIDE FIELD CAMERA 3 SCIENCE OVERSIGHT COMMITTEE)

When these stars, the brightest and most massive ones of all, run out of fuel, they die in a spectacular type II supernova explosion. When this occurs, the inner core implodes, collapsing all the way down to a neutron star (for the low-mass cores) or even to a black hole (for the high-mass cores), while expelling the outer layers back into the interstellar medium.

Once there, these enriched gases will contribute to future generations of stars, providing them with the heavy elements necessary to create rocky planets, organic molecules, and in rare, wonderful cases, life. It is estimated that at least six prior generations of stars contributed to the molecular gas cloud that eventually gave rise to our Sun and Solar System.

When the most massive stars die, their outer layers, enriched with heavy elements from the result of nuclear fusion and neutron capture, are blown off into the interstellar medium, where they can help future generations of starsby providing them with the raw ingredients for rocky planets and, potentially, life. (NASA, ESA, J. HESTER, A. LOLL (ASU))

If you form a black hole from the collapse of a supermassive star, you don’t have to wait very long for it to go dark. In fact, by definition, black holes go almost perfectly “black” immediately. Once the core collapses sufficiently to form an event horizon, everything inside collapses down to a singularity in a fraction of a second. Any remnant heat, light, temperature, or energy in any form in the core simply gets added to the mass of the singularity.

No light will ever emanate from it again, except in the form of Hawking radiation, which is emitted when the black hole decays, and in the accretion disk surrounding the black hole, which is constantly fed and refueled from the surrounding matter. But not every massive star forms a black hole, and the ones that form neutron stars tell a vastly different story.

Forming from the remnant of a massive star that’s gone supernova, a neutron star is the collapsed core that remains behind. (NASA)

A neutron star takes all the energy in a star’s core and collapses incredibly rapidly. When you take anything and compress it quickly, you cause the temperature within it to rise: this is how a piston works in a diesel engine. Well, collapsing from a stellar core all the way down to a neutron star is maybe the ultimate example of rapid compression.

In the span of seconds-to-minutes, a core of iron, nickel, cobalt, silicon and sulfur many hundreds-of-thousands of miles (kilometers) in diameter has collapsed down to a ball just around 10 miles (16 km) in size or smaller. Its density has increased by around a factor of a quadrillion (10¹⁵), and its temperature has grown tremendously: to some 10¹² K in the core and all the way up to around 10⁶ K at the surface. And herein lies the problem.

A neutron star is very small and low in overall luminosity, but it’s very hot, and takes a long time to cool down. If your eyes were good enough, you’d see it shine for millions of times the present age of the Universe. (ESO/L. CALÇADA)

You have all this energy stored within a collapsed star like this, and its surface is so tremendously hot that it not only glows bluish-white in the visible portion of the spectrum, but most of the energy isn’t visible or even ultraviolet: it’s X-ray energy! There is an insanely large amount of energy stored within this object, but the only way it can release it out into the Universe is through its surface, and its surface area is very small. The big question, of course, is how long will it take a neutron star to cool?

The answer depends on a piece of physics that practically isn’t well-understood for neutron stars: neutrino cooling! You see, while photons (radiation) are soundly trapped by the normal, baryonic matter, neutrinos, when generated, can pass right through the entire neutron star unimpeded. On the fast end, neutron stars might cool down, out of the visible portion of the spectrum, after as little as 10¹⁶ years, or “only” a million times the age of the Universe. But if things are slower, it might take 10²⁰-to-10²² years, which means you’ll be waiting for some time.

When lower-mass, Sun-like stars run out of fuel, they blow off their outer layers in a planetary nebula, but the center contracts down to form a white dwarf, which takes a very long time to fade to darkness.(NASA/ESA AND THE HUBBLE HERITAGE TEAM (AURA/STSCI))

But other stars will go dark much more quickly. You see, the vast majority of stars — the other 99+% — don’t go supernova, but rather, at the end of their lives, contract (slowly) down into a white dwarf star. The “slow” timescale is only slow compared to a supernova: it takes tens-to-hundreds of thousands of years rather than mere seconds-to-minutes, but that’s still fast enough to trap almost all the heat from the star’s core inside.

The big difference is that instead of trapping it inside of a sphere with a diameter of only 10 miles or so, the heat is trapped in an object “only” about the size of Earth, or around a thousand times larger than a neutron star. This means that while the temperatures of these white dwarfs can be very high — over 20,000 K, or more than three times hotter than our Sun — they cool down much faster than neutron stars.

An accurate size/color comparison of a white dwarf (L), Earth reflecting our Sun’s light (middle), and a black dwarf (R). (BBC / GCSE (L) / SUNFLOWERCOSMOS (R))

Neutrino escape is negligible in white dwarfs, meaning that radiation through the surface is the only effect that matters. When we calculate how quickly heat can escape by radiating away, it leads to a cooling timescale for a white dwarf (like the kind the Sun will produce) of around 10¹⁴-to-10¹⁵ years. And that will get your stellar remnant all the way down to just a few degrees above absolute zero!

This means that after around 10 trillion years, or “only” around 1,000 times the present age of the Universe, the surface of a white dwarf will have dropped in temperature so that it’s out of the visible light regime. When this much time has passed, the Universe will possess a brand new type of object: a black dwarf star.

The Universe is not yet old enough for a stellar remnant to have cooled enough to become invisible to human eyes, much less to cool all the way to just a few degrees above absolute zero. (NASA / JPL-CALTECH)

I’m sorry to disappoint you, but there aren’t any black dwarfs around today. The Universe is simply far too young for it. In fact, the coolest white dwarfs have, to the best of our estimates, lost less than 0.2% of their total heat since the very first ones were created in this Universe. For a white dwarf created at 20,000 K, that means its temperature is still at least 19,960 K, telling us we’ve got a terribly long way to go, if we’re waiting for a true dark star.

We currently conceive of our Universe as littered with stars, which cluster together into galaxies, which are separated by vast distances. But by time the first black dwarf comes to be, our local group will have merged into a single galaxy (Milkdromeda), most of the stars that will ever live will have long since burned out, with the surviving ones being exclusively the lowest-mass, reddest and dimmest stars of all. And beyond that? Only darkness, as dark energy will have long since pushed away all the other galaxies, making them unreachable and practically unmeasurable by any physical means.

It will take hundreds of trillions of years for the first stellar remnant to cool completely, fading from a white dwarf through red, infrared and all the way down to a true black dwarf. By that point, the Universe will hardly be forming any new stars at all, and space will be mostly black.(USER TOMA/SPACE ENGINE; E. SIEGEL)

And yet, amidst it all, a new type of object will come to be for the very first time. Even though we’ll never see or experience one, we know enough of nature to know not only that they’ll exist, but how and when they’ll come to be. And that, in itself — the ability to predict the far-distant future that has not yet come to pass — is one of the most amazing parts of science of all!

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

When Will The Universe Get Its First ‘Black Dwarf’ Star? was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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The XENON1T detector, with its low-background cryostat, is installed in the centre of a large water shield to protect the instrument against cosmic ray backgrounds. This setup enables the scientists working on the XENON1T experiment to greatly reduce their background noise, and more confidently discover the signals from processes they’re attempting to study. (XENON1T COLLABORATION)If you look everywhere between the numbers 1 and 2, you’ll never find 3.

Let’s say you have an idea about how our physical reality might be different from how we currently conceptualize it. Perhaps you think there are additional particles or interactions present, and that this might hold the solution to some of the greatest puzzles facing the natural sciences today. So what do you do? You formulate a hypothesis, you develop it, and then you try and tease out what the observable, measurable consequences would be.

Some of these consequences will be model-independent, meaning that there will be signatures that appear regardless of whether one specific model is right or not. Others will be extremely model-dependent, creating experimental or observational signatures that show up in some models but not others. Whenever a dark matter experiment comes up empty, it only tests the model-dependent assumptions, not the model-independent ones. Here’s why that doesn’t mean anything for the existence of dark matter.

When you collide any two particles together, you probe the internal structure of the particles colliding. If one of them isn’t fundamental, but is rather a composite particle, these experiments can reveal its internal structure. Here, an experiment is designed to measure the dark matter/nucleon scattering signal. However, there are many mundane, background contributions that could give a similar result. This particular signal will show up in Germanium, liquid XENON and liquid ARGON detectors. (DARK MATTER OVERVIEW: COLLIDER, DIRECT AND INDIRECT DETECTION SEARCHES — QUEIROZ, FARINALDO S. ARXIV:1605.08788)

You can’t get mad at a team for trying the improbable, hoping that nature cooperates. Some of the most famous discoveries of all time have come about thanks to nothing more than mere serendipity, and so if we can test something at low-cost with an insanely high reward, we tend to go for it. Believe it or not, that’s the mindset that’s driving the direct searches for dark matter.

In order to understand how we might find dark matter, however, you have to first understand the full suite of what else we know. That’s the model-independent evidence we have to guide us towards the possibilities of direct detection. Of course, we haven’t yet directly found dark matter in the form of an interaction with another particle, but that’s okay. The indirect evidence all shows that it must be real.

The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs Boson, falling at the LHC earlier this decade. All of these particles can be created at LHC energies, and the masses of the particles lead to fundamental constants that are absolutely necessary to describe them fully. These particles can be well-described by the physics of the quantum field theories underlying the Standard Model, but they do not describe everything, like dark matter. (E. SIEGEL / BEYOND THE GALAXY)

It all starts with the germ of an idea. We can start with the undisputed basics: the Universe consists of all the protons, neutrons and electrons that make up our bodies, our planet and all the matter we’re familiar with, as well as some photons (light, radiation, etc.) thrown in there for good measure.

Protons and neutrons can be broken up into even more fundamental particles — the quarks and gluons — and along with the other Standard Model particles, make up all the known matter in the Universe. The big idea of dark matter is that there’s something other than these known particles contributing in a significant way to the total amounts of matter in the Universe. It’s a revolutionary assumption, and one that might seem like an extraordinary leap.

The very notion of it might compel you to ask, “why would we think such a thing?”

The motivation comes by looking at the Universe itself. Science has taught us a lot about what’s out there in the distant Universe, and much of it is completely undisputed. We know how stars work, for example, and we have an incredible understanding of how gravity works. If we look at galaxies, clusters of galaxies and go all the way up to the largest-scale structures in the Universe, there are two things we can extrapolate very well.

  1. How much mass there is in these structures at every level. We look at the motions of these objects, we look at the gravitational rules that govern orbiting bodies, whether something is bound or not, how it rotates, how structure forms, etc., and we get a number for how much matter there has to be in there.
  2. How much mass is present in the stars contained within these structures. we know how stars work, so as long as we can measure the starlight coming from these objects, we can know how much mass is there in stars.
The two bright, large galaxies at the center of the Coma Cluster, NGC 4889 (left) and the slightly smaller NGC 4874 (right), each exceed a million light years in size. But the galaxies on the outskirts, zipping around so rapidly, point to the existence of a large halo of dark matter throughout the entire cluster. The mass of the normal matter alone is insufficient to explain this bound structure. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

These two numbers don’t match, and the mismatch between the values we obtain for them is spectacular in magnitude: they miss by a factor of approximately 50. There must be something more than just stars responsible for the vast majority of mass in the Universe. This is true for the stars within individual galaxies of all sizes all the way up to the largest clusters galaxies in the Universe, and beyond that, the entire cosmic web.

That’s a big hint that there’s something more than just stars going on, but you might not be convinced that this requires a new type of matter. If that’s all we had to work with, scientists wouldn’t be convinced either! Fortunately, there’s an enormous suite of observations that — when we take it all together — compels us to consider the dark matter hypothesis as extraordinarily difficult to avoid.

The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. The Universe is 75–76% hydrogen, 24–25% helium, a little bit of deuterium and helium-3, and a trace amount of lithium by mass. After tritium and beryllium decay away, this is what we’re left with, and this remains unchanged until stars form. Only about 1/6th of the Universe’s matter can be in the form of this normal (baryonic, or atom-like) matter. (NASA, WMAP SCIENCE TEAM AND GARY STEIGMAN)

When we extrapolate the laws of physics all the way back to the earliest times in the Universe, we find that there was not only a time so early when the Universe was hot enough that neutral atoms couldn’t form, but there was a time where even nuclei couldn’t form! When they finally can form without immediately being blasted apart, that phase is where the lightest nuclei of all, including different isotopes of hydrogen and helium, originate from.

The formation of the first elements in the Universe after the Big Bang — due to Big Bang Nucleosynthesis — tells us with very, very small errors how much total “normal matter” is there in the Universe. Although there is significantly more than what’s around in stars, it’s only about one-sixth of the total amount of matter we know is there from the gravitational effects. Not only stars, but normal matter in general, isn’t enough.

The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. The fluctuations are only tens to hundreds of microkelvin in magnitude, but definitively point to the existence of both normal and dark matter in a 1:5 ratio. (ESA AND THE PLANCK COLLABORATION)

Additional evidence for dark matter comes to us from another early signal in the Universe: when neutral atoms form and the Big Bang’s leftover glow can travel, at last, unimpeded through the Universe. It’s very close to a uniform background of radiation that’s just a few degrees above absolute zero. But when we look at the temperatures on ~microkelvin scales, and on small angular (< 1°) scales, we see it’s not uniform at all.

The fluctuations in the cosmic microwave background are particularly interesting. They tell us what fraction of the Universe is in the form of normal (protons+neutrons+electrons) matter, what fraction is in radiation, and what fraction is in non-normal, or dark matter, among other things. Again, they give us that same ratio: that dark matter is about five-sixths of all the matter in the Universe.

The observations of baryon acoustic oscillations in the magnitude where they’re seen, on large scales, indicate that the Universe is made of mostly dark matter, with only a small percentage of normal matter causing these ‘wiggles’ in the graph above. (MICHAEL KUHLEN, MARK VOGELSBERGER, AND RAUL ANGULO)

And finally, there’s the incontrovertible evidence found in the great cosmic web. When we look at the Universe on the largest scales, we know that gravitation is responsible, in the context of the Big Bang, for causing matter to clump and cluster together. Based on the initial fluctuations that begin as overdense and underdense regions, gravitation (and the interplay of the different types of matter with one another and radiation) determine what we’ll see throughout our cosmic history.

This is particularly important, because we can not only see the ratio of normal-to-dark matter in the magnitude of the wiggles in the graph above, but we can tell that the dark matter is cold, or moving below a certain speed even when the Universe is very young. These pieces of knowledge lead to outstanding, precise theoretical predictions.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

All together, they tell us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no collisional interactions with normal matter; upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once.

However, there should be plenty of dark matter particles passing undetected through Earth, me and you every second. In addition, dark matter should also not collide or interact with itself, the way normal matter does. That makes direct detection difficult, to say the least. But thankfully, there are some indirect ways of detecting dark matter’s presence. The first is to study what’s called gravitational lensing.

When there are bright, massive galaxies in the background of a cluster, their light will get stretched, magnified and distorted due to the general relativistic effects known as gravitational lensing. (NASA, ESA, AND JOHAN RICHARD (CALTECH, USA) ACKNOWLEDGEMENT: DAVIDE DE MARTIN & JAMES LONG (ESA / HUBBLE)NASA, ESA, AND J. LOTZ AND THE HFF TEAM, STSCI)

By looking at how the background light gets distorted by the presence of intervening mass (solely from the laws of General Relativity), we can reconstruct how much mass is in that object. Again, it tells us that there must be about six times as much matter as is present in all types of normal (Standard Model-based) matter alone.

There’s got to be dark matter in there, in quantities that are consistent with all the other observations. But occasionally, the Universe is kind, and gives us two clusters or groups of galaxies that collide with one another. When we examine these colliding clusters of galaxies, we learn something even more profound.

Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do. (X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))

The dark matter really does pass right through one another, and accounts for the vast majority of the mass; the normal matter in the form of gas creates shocks (in X-ray/pink, above), and only accounts for some 15% of the total mass in there. In other words, about five-sixths of that mass is dark matter! By looking at colliding galaxy clusters and monitoring how both the observable matter and the total gravitational mass behaves, we can come up with an astrophysical, empirical proof for the existence of dark matter. There is no modification to the law of gravity that can explain why:

  • two clusters, pre-collision, will have their mass and gas aligned,
  • but post-collision, will have their mass and gas separated.

Still, despite all of this model-independent evidence, we’d still like to detect dark matter directly. It’s that step — and only that step — that we’ve failed to achieve.

The spin-independent WIMP/nucleon cross-section now gets its most stringent limits from the XENON1T experiment, which has improved over all prior experiments, including LUX. While many may be disappointed that XENON1T didn’t robustly find dark matter, we mustn’t forget about the other physical processes that XENON1T is sensitive to. (E. APRILE ET AL., PHYS. REV. LETT. 121, 111302 (2018))

Unfortunately, we don’t know what’s beyond the Standard Model. We’ve never discovered a single particle that isn’t part of the Standard Model, and yet we know there must be more than what we’ve presently discovered out there. As far as dark matter goes, we don’t know what dark matter’s particle (or particles) properties should be, should look like, or how to find it. We don’t even know if it’s all one thing, or if it’s made up of a variety of different particles.

All we can do is look for interactions down to a certain cross-section, but no lower. We can look for energy recoils down to a certain minimum energy, but no lower. We can look for photon or neutrino conversions, but all these mechanisms have limitations. At some point, background effects — natural radioactivity, cosmic neutrons, solar/cosmic neutrinos, etc. — make it impossible to extract a signal below a certain threshold.

The cryogenic setup of one of the experiments looking to exploit the hypothetical interactions between dark matter and electromagnetism, focused on a low-mass candidate: the axion. Yet if dark matter doesn’t have the specific properties that current experiments are testing for, none of the ones we’ve even imagined will ever see it directly. (AXION DARK MATTER EXPERIMENT (ADMX) / LLNL’S FLICKR)

To date, the direct detection efforts having to do with dark matter have come up empty. There are no interaction signals we’ve observed that require dark matter to explain them, or that aren’t consistent with Standard Model-only particles in our Universe. Direct detection efforts can disfavor or constrain specific dark matter particles or scenarios, but does not affect the enormous suite of indirect, astrophysical evidence that leaves dark matter as the only viable explanation.

Many people are working tirelessly on alternatives, but unless they’re misrepresenting the facts about dark matter (and some do exactly that), they have an enormous suite of evidence they’re required to explain. When it comes to looking for the great cosmic unknowns, we might get lucky, and that’s why we try. But absence of evidence is not evidence of absence. When it comes to dark matter, don’t let yourself be fooled.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

This Is Why It’s Meaningless That Dark Matter Experiments Haven’t Found Anything was originally published in Starts With A Bang! on Medium, where people are continuing the conversation by highlighting and responding to this story.

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