Throwback Thursday: The Whole Story on Dark Matter

When things don’t add up, it’s a great sign that something amazing is right around the corner.

Every Thursday, we take an older post from the Starts With A Bang archives and update it for the present day. After yesterday’s post on The Death of Dark Matter’s #1 Competitor, there was no better choice than to tell you the whole story on the most mysterious, ubiquitous source of matter permeating our Universe.

“Science progresses best when observations force us to alter our preconceptions.” -Vera Rubin

I want you to think about the Universe. The whole thing; about everything that physically exists, both visible and invisible, about the laws of nature that they obey, and about your place in it.

It’s a daunting, terrifying, and simultaneously beautiful and wondrous thing, isn’t it?

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

After all, we spend our entire lives on one rocky world, that’s just one of many planets orbiting our Sun, which is just one star among hundreds of billions in our Milky Way galaxy, which is just one galaxy among hundreds of billions that make up our observable Universe.

Yes, we’ve learned an awful lot about what’s out there and our place in it. As best as we can tell, we’ve learned what the fundamental laws are that govern everything in it, too!

Image credit: Mark Garlick / Science Photo Library, retrieved from the BBC.

As far as gravitation goes, Einstein’s theory of general relativity explains everything from how matter and energy bend starlight to why clocks run slow in strong gravitational fields to how the Universe expands as it ages. It is arguably the most well-tested and vetted scientific theory of all time, and every single one of its predictions that has ever been precision-tested has been verified to be spot-on.

Image credit: Contemporary Physics Education Project, via

On the other hand, we’ve got the standard model of elementary particles and interactions, which explains everything known to exist in the Universe, and all the other (nuclear and electromagnetic) forces that they experience. This, also, is arguably the most well-tested and vetted scientific theory of all time.

And you would think that if our understanding of things were perfect, if we knew all about the structure of the Universe, the matter in it, and the laws of physics that it obeyed, we’d be able to explain everything. Why? Because all you’d have to do is start out with some set of initial conditions — immediately following the Big Bang — for all the particles in the Universe, apply those laws of nature that we know, and see what it turns into over time! It’s a hard problem, but in theory, it should be not only possible to simulate, it should give us a sample Universe that looks just like the one we have today.

Image credit: ESA and the Planck Collaboration.

But this isn’t what happens. In fact, this can’t be the way it happens at all. This picture I painted for you above is all true, on the one hand, but we also know that it isn’t the whole story. There are other things going on that we don’t fully understand.

Here, as best as I can present the full history in a single blog post, is the whole story.

As we come forward from the event of the Big Bang, our Universe expands and cools, while the entire time it experiences the irresistible force of gravity. Over time, a number of extremely important events happen, including, in chronological order:

  1. the formation of the first stable atomic nuclei,
  2. the formation of the first neutral atoms,
  3. the formation of stars, galaxies, clusters, and large-scale structure,
  4. and the slowing expansion of the Universe over its entire history.

If we know what’s fundamentally in the Universe and the physical laws that everything obeys, we’ll arrive at quantitative predictions for all of these things, including:

  1. what nuclei form and when they do so in the early Universe,
  2. what the radiation from the last-scattering-surface, when the first neutral atoms are formed, looks like in great detail,
  3. what the structure of the Universe, from large scales down to small scales, looks like both today and at any moment in the Universe’s past,
  4. and how the scale, size, and number of objects in the observable Universe have evolved over its history.

We have made observations measuring all four of these things, quantitatively, extremely well. Here’s what we’ve learned.

Image credit: NASA / Goddard Space Flight Center / WMAP101087.

What we consider to be normal matter, that is, stuff made up of proton, neutrons and electrons, is highly constrained by a variety of measurements. Before any stars formed, the nuclear furnace of the very early Universe fused the first protons and neutrons together in very specific ratios, depending on how much matter and how many photons there were at the time.

What our measurements tell us, and they’ve been verified directly, is exactly how much normal matter there is in the Universe. This number is incredibly tightly constrained to be — in terms that might be familiar to you — about 0.262 protons + neutrons per cubic meter. There could be 0.28, or 0.24, or some other number in that range, but there really couldn’t be more or less than that; our observations are too solid. (And since we know the size of the Universe today, we know the mean density of normal matter!)

Image credit: Ned Wright, via his cosmology tutorial.

After that, the Universe continues to expand and cool, until eventually the photons in the Universe — which outnumber the nuclei by more than a billion-to-one — lose enough energy that neutral atoms can form without immediately being blasted apart.

When these neutral atoms finally form, the photons are free to travel, uninhibited, in whatever direction they happened to be moving last. Billions of years later, that leftover glow from the Big Bang — those photons — are still around, but they’ve continued to cool, and are now in themicrowave portion of the electromagnetic spectrum. First observed in the 1960s, we’ve now not only measured this Cosmic Microwave Background, we’ve measured the tiny temperature fluctuations — microKelvin-scale fluctuations — that exist in it.

Image credit: ESA and the Planck Collaboration.

These temperature fluctuations, and the magnitudes, correlations and scales on which they appear, can give us an incredible amount of information about the Universe. In particular, one of the things they can tell us is what the ratio of total matter in the Universe is to the ratio of normal matter. We would see a very particular pattern if that number were 100%, and the pattern we do see looks nothing like that.

Here’s what we find.

Image credit: Planck Collaboration: P. A. R. Ade et al., 2013, A&A Preprint.

The necessary ratio to achieve this particular patter of wiggles is about 5:1, meaning that only about 16% of the matter in the Universe can be normal matter. This doesn’t tell us anything what this other 84% is, except that it isn’t the same stuff we’re made out of. From the Cosmic Microwave Background alone, we only know that it exerts a gravitational influence like normal matter, but it doesn’t interact with electromagnetic radiation (photons) like normal matter does.

You can also imagine that we’ve got something wrong about the laws of gravity; that there’s some modification we can make to it to mimic this effect that we can re-create by putting in dark matter. We don’t know what sort of modification could do that (we haven’t successfully found one, yet), but it is conceivable that we’ve just got the laws of gravity wrong. If a modified theory of gravity could explain the fluctuations of in the Microwave Background without any dark matter at all, that would be incredibly interesting.

But if there really is dark matter, it could be something light, like a neutrino, or something very heavy, like a theorized WIMP. It could be something fast-moving, with a lot of kinetic energy, or it could be something slow-moving, with practically none. We just know that all of the matter can’t be the normal stuff we’re used to, and that we’ve come to expect. But we can learn more about it by simulating how structure — stars, galaxies, clusters, and large-scale structure — forms in the Universe.

Because the types of structures you get out — including what types of galaxies, clusters, gas clouds, etc. — exist at all times in the Universe’s history. These differences don’t show up in the Cosmic Microwave Background, but they do show up in the structures that form in the Universe.

What we do is take a look at the galaxies that form in the Universe and see how they cluster together: how far away from a galaxy do I have to look before I see a second galaxy? How early in the Universe do large galaxies and clusters form? How quickly do the first stars and galaxies form? And what can we learn about the matter in the Universe from this?

Image credit: Chris Blake and Sam Moorfield, via

Because if the dark matter — which doesn’t interact with light or normal matter — has lots of kinetic energy, it will delay the formation of stars, galaxies, and clusters. If the dark matter has some but not too much, it makes it easier to form clusters, but still hard to form stars and galaxies early on. If the dark matter has virtually none, we should form stars and galaxies early. Also, the more dark matter there is (relative to normal matter), the more smooth the correlations will be between galaxies on different scale, while the less dark matter there is means that the differences in correlations between different scales will be very stark.

The reason for this is that early on, when clouds of normal matter starts to contract beneath the force of gravity, the radiation pressure increases, causing the atoms to “bounce back” on certain scales. But dark matter, being invisible to photons, wouldn’t do this. So if we see how big these “bouncing features” are, known as baryon acoustic oscillations, we can learn whether there’s dark matter or not, and — if it’s there — what its properties are. The thing we construct, if we want to see this, is just as powerful as the graph of the fluctuations in the microwave background, a couple of images above. It’s the much lesser-known but equally important Matter Power Spectrum, shown below.

Image credit: W. Percival et al. / Sloan Digital Sky Survey.

As you can clearly see, we do see these “bouncing” features, as those are the wiggles in the curve, above. But they’re small bounces, consistent with 15-to-20% of the matter being “normal” matter and the vast majority of it being smooth, “dark” matter. Again, you might wonder if there isn’t some way we could modify gravity to account for this type of measurement, rather than introducing dark matter. We haven’t found one yet, but if such a modification were found, it would be awfully compelling. But we’d have to find a modification that works for both the matter power spectrum and the cosmic microwave background, the way that a Universe where 80% of the matter is dark matter works for both.

This is from the structure data on large scales; we can also look on small scales, and see whether small clouds of gas, in-between us and very distant, bright objects from the early Universe, are thoroughly gravitationally collapsed or not; we look at the Lyman-alpha forest for this.

Image credit: Bob Carswell.

These intervening, ultra-distant clouds of hydrogen gas teach us that, if there is dark matter, it must have very little kinetic energy. So this tells us that either the dark matter was born somewhat cold, without very much kinetic energy, or it’s very massive, so that the heat from the early Universe wouldn’t have much of an effect on the speed it was moving millions of years later on. In other words, as much as we can define a temperature for dark matter, assuming it exists, it’s on the cold side.

But we also need to explain the smaller-scale structures that we have today, and examine in gory detail. This means when we look at galaxy clusters, they, too, should be made of 80-85% dark matter and 15-20% normal matter. The dark matter should exist in a big, diffuse halo around the galaxies and the clusters. The normal matter should be in a couple of different forms: the stars, which are extremely dense, collapsed objects, and the gas, diffuse (but denser than the dark matter) and in clouds, populating the interstellar and intergalactic medium. Under normal circumstances, the matter — normal and dark — is all held together, gravitationally. But every once in a while, these clusters merge together, resulting in a collision and a cosmic smash-up.

Image composite credits: 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.

The dark matter from the two clusters should pass right through one another, because dark matter doesn’t collide with normal matter or photons, as should the stars within the galaxies. (The stars not colliding is because the cluster collision is like firing two guns loaded with bird-shot at one another from 30 yards away: every single pellet should miss.) But the diffuse gas should heat up when they collide, radiating energy away in the X-ray (shown in pink) and losing momentum. In the Bullet Cluster, above, that’s exactly what we see.

Image credit: NASA/CXC/STScI/UC Davis/W.Dawson et al., retrieved from Wired.

Ditto for the Musket Ball Cluster, a slightly older collision than the Bullet Cluster, that’s just recently analyzed. But others are more complicated; cluster Abell 520, for example, below, is still being scrutinized, as the source of gravitational lensing doesn’t appear to be 100% correlated with where the mass is expected to be.

Image credit: NASA / CXC / CFHT / UVic. / A. Mahdavi et al.

If we look at the individual components, you can see where the galaxies are (which is also where the dark matter ought to be), as well as the X-rays, which tell us where the gas is, you’d expect the lensing data — which is sensitive to the mass (and hence, dark matter) — to reflect that.

But we can go to even smaller scales, and look at individual galaxies on their own. Because around every single galaxy, there should be a huge dark matter halo, comprising approximately 80% of the mass of the galaxy, but much larger and more diffuse than the galaxy itself.

Image credit: ESO / L. Calçada.

Whereas a spiral galaxy like the Milky Way might have a disc 100,000 light-years in diameter, its dark matter halo is expected to extend for a few million light-years! It’s incredibly diffuse because it doesn’t interact with photons or normal matter, and so has no way to lose momentum and form very dense structures like normal matter can.

What we don’t yet have any information about, however, is whether dark matter interacts with itself in some way. Different simulations give very different results, for example, as to what the density of one of these halos ought to look like.

Image credit: R. Lehoucq et al.

If the dark matter is cold and doesn’t interact with itself, it should have either an NFW or a Moore-type profile, above. But if it is allowed to thermalize with itself, it would make an isothermal profile. In other words, the density doesn’t continue to increase as you get close to the core of a dark matter halo that’s isothermal.

Why a dark matter halo would be isothermal isn’t certain. Dark matter could be self-interacting, it could exhibit some sort of exclusion rule, it could be subject to a new, dark-matter-specific force, or something else that we haven’t thought of yet. Or, of course, it could simply not exist, and the laws of gravity that we know could simply need modification. On galactic scales, this is where MOND, the theory of Modified Newtonian Dynamics, really shines.

Image credit: University of Sheffield.

While the NFW and Moore profiles — the ones that come from the simplest models of Cold Dark Matter — don’t really match up with the observed rotation curves very well, MOND fits individual galaxies perfectly. The isothermal halos do a better job, but lack a compelling theoretical explanation. If we only based our understanding of the “missing mass” problem — whether there was extra, “dark” matter, or whether there was a flaw in our theory of gravity — on individual galaxies, I would likely side with the MOND-ian explanation.

So when you see a headline like Serious blow to dark matter theories?, you already have a hint that they’re looking at individual galaxies. Let’s look at one from two years ago as an example.

Image credit: ESO / L. Calçada.

A team of researchers took a look at stars relatively close to our solar neighborhood, and looked for evidence of this inner distribution of mass from the theoretical dark matter halo. You’ll notice, looking a couple of images up, that only the simplest, completely collision-less models of Cold Dark Matter give that large effect in the cores of dark matter halos.

So let’s take a look at what the survey shows.

Image credit: C. Moni Bidin et al., 2012.

Indeed, the simple (NFW and Moore) halo profiles are highly disfavored, as many studies before have shown. Although this is interesting, because it demonstrates their insufficiency on these small scales in a new way.

So you ask yourself, do these small-scale studies, the ones that favor modified gravity, allow us to get away with a Universe without dark matter in explaining large-scale structure, the Lyman-alpha forest, the fluctuations in the cosmic microwave background, or the matter power spectrum of the Universe? The answers, at this point, are no, no, no, and no. Definitively. Which doesn’t mean that dark matter is a definite yes, and that modifying gravity is a definite no. It just means that I know exactly what the relative successes and remaining challenges are for each of these options. It’s why I unequivocally state that modern cosmology overwhelmingly favors dark matter over modified gravity, and that was before the binary pulsar measurements ruled out the most viable possibility of modified gravity.

Image credit: NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer, via

But I also know — and freely admit — exactly what it will take to change my scientific opinion of which one is the leading theory. And you’re free to believe whatever it is you like, of course, but there are very good reasons why the modifications to gravity that one can make to have gravity succeed so well without dark matter on galactic scales fail to address the other observations without also including dark matter.

And we know what it isn’t: it isn’t baryonic (normal matter), it isn’t black holes, it isn’t photons, it isn’t fast-moving, hot stuff, and it probably isn’t simple, standard, cold and non-interacting stuff either, like most WIMP-type theories hope for.

Image credit: Dark Matter Candidates, retrieved from IsraCast.

I think it’s likely to be something more complicated than the leading theories of today. Which isn’t to say that I think I know exactly what dark matter is or how to find it. I’m even sympathetic to certain degrees of skepticism expressed on that account; I don’t think I would claim to be 100% certain that dark matter is right and our theories of gravity are also right until we can verify dark matter’s existence more directly. But, if you want to reject dark matter, there’s a whole host of things you’ll need to explain some other way. Don’t completely ignore large-scale structure and the need to address it; that’s a surefire way to fail to earn my respect, and the respect of every cosmologist who studies it.

And that’s, as best as I can express it in a single blog post, the whole story on dark matter. I’m sure there are plenty of comments; let the fireworks begin!

Have your say and weigh in at the Starts With A Bang forum on Scienceblogs!

Next Story — The rarest light elements in the Universe
Currently Reading - The rarest light elements in the Universe

Understanding the cosmic origin of all the elements heavier than hydrogen can give us a powerful window into the Universe’s past, as well as insight into our own origins. Image credit: Wikimedia Commons user Cepheus.

The rarest light elements in the Universe

There’s a big gap between Helium and Carbon. Come find out why!

“And argon, krypton, neon, radon, xenon, zinc and rhodium,
And chlorine, cobalt, carbon, copper, tungsten, tin and sodium.
These are the only ones of which the news has come to Harvard,
And there may be many others, but they haven’t been discarvard.”
Tom Lehrer

Immediately after the Big Bang, before the first stars in the Universe ever formed, the Universe consisted of hydrogen (element #1), helium (element #2) and pretty much nothing else. Despite originating from an incredibly hot, dense state, arbitrarily heavy elements weren’t created early on the same way they’re made today in stars. Despite being hot enough to make pretty much anything, the early Universe makes almost nothing for one simple reason: if it was hot-and-dense enough to fuse elements together in the very early stages, it was also hot enough to blast those composite elements apart again.

It’s only when the Universe has cooled enough that elements aren’t immediately split apart — a little more than three minutes in — that we can build our way up the periodic table.

The initial nucleosynthesis reaction chain that produces deuterium, helium-3 and helium-4 in the early Universe. Image credit: Wikimedia Commons user Joanna Kośmider, with modifications by E. Siegel.

But even after just a few minutes, conditions are so low in energy that 99.999999% of the elements cap out at helium. And we don’t make anything new beyond that until we begin to form stars. Although the first stage of stellar burning always involves fusing hydrogen into helium in a star’s core, the stars that are massive enough (more than about 40% as massive as our Sun) will eventually build their way up the periodic table:

  • When the star’s core runs out of hydrogen fuel, it contracts and heats up.
  • When it reaches a temperature of about 100 million K, the helium ignites.
  • With that ignition, helium burning commences, where three helium atoms fuse together to create carbon (element #6), releasing energy in the process.
A new star cluster full of bright, giant stars that will produce copious amounts of carbon (and more) in their cores. Image credit: ESO / G. Beccari, via

This is the process at play in red giant stars, with more massive stars creating elements such as nitrogen, oxygen, neon, magnesium, silicon, sulphur and iron-cobalt-and-nickel. In addition, stellar burning also produces free neutrons, which can combine with the pre-existing elements to climb up the periodic table one element at a time, all the way up to elements like lead and bismuth (elements #82 and #83). And finally, the absolute most massive stars will die in a spectacular supernova explosion, leading to a runaway fusion reaction that — in principle — should produce everything that’s known in the periodic table and beyond, creating every element possible.

The nebula from supernova remnant W49B, still visible in X-rays, radio and infrared wavelengths. Image credit: X-ray: NASA/CXC/MIT/L.Lopez et al.; Infrared: Palomar; Radio: NSF/NRAO/VLA.

Every element possible, that is, except the three we skipped. You see, the Universe starts off with hydrogen and helium, all stars produce helium, and then stars over a certain mass threshold produce carbon, nitrogen, oxygen and lots of heavier elements. But carbon was already element #6; what about lithium, beryllium and boron (elements #3, #4 and #5)? When we look out at the Universe and the Solar System, and ask what the abundances of the elements are, we notice there’s a tremendous gap between helium and carbon, like those three elements are incredibly suppressed.

Image credit: Wikimedia Commons user MHz`as, with data from Katharina Lodders (2003). The Astrophysical Journal 591: 1220–1247.

You can’t make those elements by fusing lighter ones together, since adding hydrogen to helium would create lithium-5, which is unstable, and adding two heliums together would create beryllium-8, which is unstable. (In fact, all nuclei with a mass of 5 or 8 are unstable.) You can’t make them from stellar reactions involving elements like carbon or above, since those only create heavier elements, not lighter ones. In fact, you can’t make the first of the heavier-than-helium elements in stars at all.

A model of a plant cell, with primary and secondary cell walls. Without boron, plant cell walls wouldn’t exist. Image credit: Caroline Dahl, under a c.c.a.-s.a.-3.0 license.

And yet, lithium, beryllium and boron not only exist, but boron in particular is vital for life-as-we-know-it on Earth. Without boron, there would be no such thing as a cell wall, and hence, no such thing as a plant. (For some of us the lithium batteries in our cellphones may be equally as indispensable!)

Yet plants exist, lithium, beryllium and boron exist, and so somehow, these elements must have been created. The keys, believe it or not, are the most energetic sources of particles in the Universe: black holes, neutron stars, supernovae and active galaxies. When these cosmic catastrophes ignite, become active or even explode, they don’t just emit particles. They emit the highest energy particles in the known Universe.

Image credit: NASA / JPL-Caltech; Chandra / Spitzer / Hubble composite of the Cassiopeia A supernova remnant.

And when those energetic particles (known as cosmic rays) strike a heavier element — one created in a star — it can blast it apart, creating a cascade of lower-mass particles. This process, known as spallation, is how the lithium, beryllium and boron found on Earth was formed, and the only reason why these elements can be found at all on our planet. These three elements are by far the rarest of all the light elements, and this process is the only reason they’re around at all. The next time you see a plant, think not only of the evolutionary story that allowed it to be so, but the cosmic one, that enabled the elements essential to it to even exist. Without the most catastrophic, energetic events in the Universe, three of the lightest elements, lithium, beryllium and boron, simply would not be.

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — Science: where finding nothing is the biggest victory of all
Currently Reading - Science: where finding nothing is the biggest victory of all

A Higgs boson decaying into Fermions — taus and bottom quarks — in 2012 at the CMS detector. The Higgs boson is the last new particle discovery, and it completes the Standard Model. Image credit: CERN / CMS collaboration.

Science: where finding nothing is the biggest victory of all

“Eureka!” is not always as powerful as “that’s what I thought!”

“Reality is what kicks back when you kick it. This is just what physicists do with their particle accelerators. We kick reality and feel it kick back. From the intensity and duration of thousands of those kicks over many years, we have formed a coherent theory of matter and forces, called the standard model, that currently agrees with all observations.” -Victor J. Stenger

Reading the science news over the past month, you might conclude that it’s been one defeat after another for physics. After all:

Yet these apparent defeats are merely a thin veil covering the greatest truth of all: physics really is incredibly well-understood.

Image credit: E. Siegel, of the known particles in the Standard Model. This is still everything that’s been directly discovered.

From a particle physics perspective, the Standard Model describes all the normal matter we’ve ever observed or detected directly. Combined with General Relativity, our leading theory of gravitation, the four fundamental forces describing all the particles and their interactions — the strong nuclear, electromagnetic, weak nuclear and gravitational forces — are almost completely understood. To be honest, they’re so well understood that most people take this for granted.

The fabric of spacetime, illustrated, with ripples and deformations due to mass. Image credit: European Gravitational Observatory, Lionel BRET/EUROLIOS.

General Relativity was formulated in the 1910s; the Standard Model’s predictions were finalized in the 1960s. For the past 50 years, the greatest novel ideas in theoretical physics, from grand unification to neutrinoless double beta decay to extra dimensions to supersymmetry, have all failed to turn up a direct experimental signature of a new particle or interaction beyond the known forces. We’ve reached the point where the only the most esoteric questions respecting the matter we know, questions concerning the gravitational field of an electron passing through a double slit or the information from the particles falling into a black hole, for example, aren’t answered by our current theories.

We don’t know what happens at the singularity inside a black hole, either, but we’re a long way from gathering experimental data about that! Image credit: NASA, via

If you put any Standard Model particle or set of particles out into the Universe and subject it to any sort of conditions at any energies, if you allow it to collide in a controlled or uncontrolled fashion with low energy, high energy or ultra-high energy particles, you can describe every single interaction by these simple sets of laws. If you shield these particles from everything you can conceive of in the Universe, you see exactly what these laws predict.

The key thing — the greatest victory — is that we don’t see anything elseat all.

The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding. Images credit: André David, via Twitter at

We don’t see any unexpected or unanticipated decays. We don’t see any collisions whose properties we can’t explain. We don’t see the violation of laws or symmetries we don’t expect. We don’t see even tiny amounts of a signal that’s forbidden by the Standard Model or General Relativity. We don’t see proton decay; we don’t see flavor-changing-neutral-currents; we don’t see CPT violation; we don’t see anything move faster than the speed of light in a vacuum. And when it comes to the new things that we do see, they’re exactly in line with what’s predicted, from gravitational frame-dragging to the Higgs boson decaying exactly as we expect to pulsars spinning in perfect time to gravitational radiation matching up perfectly with what Einstein predicted 101 years prior.

The inspiral and merger gravitational wave signal extracted from the event on December 26, 2015. Image credit: Figure 1 from B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 241103 — Published 15 June 2016.

There are lots of stories out there about how physics is broken, and how we need a massive new breakthrough or a paradigm shift to keep the enterprise of new discoveries going. What nonsense! The truth is that the laws of physics we have in place are the most successful sets of laws we’ve ever come up with. They’ve been tested more robustly than any other set of laws ever, and they’ve passed every single one. We might not understand why the laws are the way they are, or why the Universe comes with certain properties that the Standard Model and General Relativity don’t have an explanation for, such as:

  • dark matter,
  • dark energy,
  • the tiny, non-zero masses of neutrinos,
  • or the matter/antimatter asymmetry of the Universe.

There’s more to learn, for certain. There are more questions out there to be answered. What these null results — these non-discoveries — are telling us is something phenomenal and profound: that physics isn’t over and done, but rather that the hints of what comes next requires looking far, far deeper than we’re presently looking. That means higher energies, larger telescopes, more particle collisions, more sensitive detectors, more significant digits closer to the speed of light or absolute zero and quite likely better, newer ideas than the ones we’ve been pursuing fruitlessly for so long.

The farthest galaxy ever spectroscopically confirmed. To push the frontiers even farther, we’ll need to go even deeper into the Universe. Image credits: NASA, ESA, and A. Feild (STScI).

It’s a great opportunity for creative scientists and a wonderful time to be alive. We can honestly look back and marvel at how far we’ve come, what we’ve already discovered, and how miraculously well it all works. At the same time, we can look ahead at the great mysteries in front of us, and contemplate just what marvelous secrets about nature they might hold when we finally unlock them. In the meantime, it’s no nightmare that these answers remain hidden; it’s merely the latest, greatest challenge that the Universe has presented us with. The fact that we’ve found nothing new that’s compelling so far means our journey must continue. The promised land is still ahead.

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — How did the Universe get its first supermassive black holes?
Currently Reading - How did the Universe get its first supermassive black holes?

Illustration of the distant galaxy CR7, which last year was discovered to house a pristine population of stars formed from the material direct from the Big Bang. Image credit: M. Kornmesser / ESO.

How did the Universe get its first supermassive black holes?

How they get so big so fast is a mystery. Could “direct collapse” be the solution we’re seeking?

“For something to collapse, not all systems have to shut down. In most cases, just one system is enough.” -Robert Kiyosaki

At the center of almost every large galaxy lies a supermassive black hole, millions or even billions of times the mass of our Sun in scale. Our Universe has been around for 13.8 billion years, which you might think is plenty of time to form a black hole that large. Yet the farther and farther back in time we look, every supermassive black hole we measure seems to have roughly the same mass as the ones today. In other words, while the largest-scale structures in the Universe:

  • giant elliptical galaxies,
  • ultra-massive clusters with thousands of times the mass of the Milky Way,
  • and galactic filaments that are hundreds of millions of light years across,

took billions of years to form, there are ultra-massive black holes that have been around as far back as we can see.

An illustrated timeline of the Universe’s history. Image credit: European Southern Observatory (ESO).

They don’t predate stars, but they are found in the earliest massive galaxies we know how to measure. The crazy explanation is that the Universe was born with these cosmic behemoths, but that flies in the face of everything else we know about structure formation, including the magnitudes and mass/size scales of the fluctuations that gave rise to everything we see today. The new physics that would have to be hypothesized to create a Universe that was born with extremely large black holes is not only preposterous, it’s incredibly constrained by observations of the cosmic background light populating the cosmos.

The fluctuations in the Cosmic Microwave Background, as seen by Planck. Image credit: ESA and the Planck collaboration.

But there are two “mundane” explanations, or explanations that don’t involve any new fundamental physics beyond that which we currently know.

  1. A huge set of starbursts — a giant rush of catastrophic star-formation — triggered the formation of a great many stellar-mass black holes in just a few million years. Over time, they merged together and migrated towards the center of the galaxy, giving rise to a supermassive black hole in very short order.
  2. A supermassive black hole formed by direct collapse of matter into a very large, massive black hole all at once, providing the seed for ultramassive black holes to grow over a relatively short timescale.

The first scenario, by a great many people, is thought to be the simplest explanation, since we see plenty of evidence of how this could work, even today.

The starburst galaxy Henize 2–10, located 30 million light years away. Image credit: X-ray (NASA/CXC/Virginia/A.Reines et al); Radio (NRAO/AUI/NSF); Optical (NASA/STScI).

Star formation is known to occur in bursts, with the greatest rates of star formation occurring in the first three billion years of the Universe, and falling ever since. When stars form en masse, they produce stars of all different masses and colors, including copious numbers of stars over 20, 50, 100 or even 200 solar masses. These massive, blue, hot stars are both the brightest and the shortest lived, and they end their lives in core-collapse supernovae, almost all of which give rise to black holes. Because of the dynamics of gravity, the way these most massive objects work is that they interact with the other stars around them, kick them around while sinking to the center of a cluster-or-galaxy, and then merge together. It’s a simple, conservative scenario. But it might also be insufficient.

Last year, the galaxy CR7 was discovered: a strong candidate for having a truly pristine population of stars. Pristine means that this would be the very first time stars were forming inside this galaxy since the Big Bang, and the science supports this view quite strongly. Yet if we look at even this galaxy, we find something spectacular about it: it also exhibits evidence for a supermassive black hole. And while the starburst explanation is tempting, it may not line up completely with what we see. In a paper written earlier this year, scientists Aaron Smith, Volker Bromm and Abraham Loeb came up with a different explanation: perhaps they were seeing the first evidence for a direct-collapse black hole!

The X-ray and optical images of a small galaxy containing a black hole many tens of thousands of times the mass of our Sun. These black holes may have arisen first in the Universe by the direct collapse of matter. Image credit: X-ray: NASA/CXC/Univ of Michigan/V.F.Baldassare, et al; Optical: SDSS; Illustration: NASA/CXC/M.Weiss.

This galaxy, sending light from 13 billion years ago, has to see that light travel throughout the expanding Universe, where its wavelengths get stretched from ultraviolet through the visible portion of the spectrum and all the way into the infrared. Still, the hottest, bluest stars — which it’s rich in — cause intense ultraviolet emission from the atoms present: hydrogen and helium. These emission lines originate from slightly different parts of the galaxy, and thanks to the incredible technology of the 2015 observations detecting them, we were able to determine that the hydrogen emissions appear to be moving quickly, at 160 km/s, relative to the helium emissions. When Smith, Bromm and Loeb try to model these emissions using simulations, they find that a massive source of radiation must be present at the center, creating an ionized bubble and driving an expanding shell of gas out from the center.

A rich nebula of gas, pushed out into the interstellar medium by the hot, new stars formed in the central region. Image credit: Gemini Observatory / AURA.

There are two explanations to consider: either there’s a massive star cluster at an incredibly high temperature of 100,000 K, or there’s a massive black hole driving it. The big difference between the two models is that the massive black hole produces the offset speed between the hydrogen and helium as well as the incredible size of the region (over 50,000 light years!) while the primordial massive star cluster does not.

These are simulations only, however; if you want to confirm your picture, you need evidence to decide one way or the other. The way we’ll be able to determine whether there truly is a massive black hole will be observational, and it will involve looking for the characteristic radio emissions from black holes.

A small section of the Karl Jansky Very Large Array, one of the world’s largest and most powerful arrays of radio telescopes. Image credit: John Fowler, under a cc-by-2.0 license.

The largest, most advanced radio telescope arrays in the world are on the verge of being up to the task! The evidence pointing to the existence of direct-collapse black holes is tantalizing and suggestive, but we’re not over the threshold yet. In order to get there, we need to see the proof. But the theoretical possibility has been raised, and the gauntlet has been thrown down. It’s time to collect the evidence and let nature decide!

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — Cassini prepares for its final, suicidal mission
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A natural color view of Saturn, its rings edge-on and its largest moon, Titan, as viewed from the Cassini spacecraft. Image credit: NASA/JPL/Space Science Institute.

Cassini prepares for its final, suicidal mission

After nearly 20 years in orbit around Saturn, Cassini prepares to say goodbye.

“All the atoms of our bodies will be blown into space in the disintegration of the solar system, to live on forever as mass or energy.” -Carolyn Porco

In 1997, NASA’s Cassini spacecraft was launched for a journey to Saturn, where it would study our Solar System’s ringed world as never before.

Visible and radio images of Saturn’s rings and their structure, as delivered by Cassini. Image credit: NASA/JPL/Space Science Institute.

It delivered beyond our wildest expectations, presenting breathtaking new views of the least dense planet known.

Saturn in eclipse, perhaps the most stunning image of the planet ever taken. Image credit: NASA/JPL/Space Science Institute.

It viewed Saturn in eclipse, discovering two new, outer rings in the process.

An infrared view of Saturn, along with its ring’s shadows on the planet’s atmosphere. Image credit: NASA / JPL / Space Science Institute.

Its infrared eyes viewed Saturn’s hazes beneath the top-level clouds.

A false-color image highlighting Saturn’s hurricane over its north pole, inside the much larger hexagon-shaped feature. Image credit: NASA/JPL-Caltech/SSI.

The north pole of Saturn was found to possess a strange hexagonal storm, thought to be stable over century-long timescales.

Descent Imager/Spectral Radiometer (DISR) image of Titan taken at 2km altitude during the descent. Image credit: ESA/NASA/JPL/University of Arizona.

The Huygens probe released by it descended onto Titan, its largest moon, discovering an incredible landscape, liquid methane lakes and even waterfalls.

Iapetus, the second Saturn moon ever discovered, as imaged by Cassini. Image credit: NASA / JPL-Caltech / Space Science Institute / Cassini.

The mystery of Iapetus, its two-toned moon, was solved as well: dark material from the captured comet, Phoebe, causes the ice on one side to sublimate and settle on the other.

The captured Kuiper Belt object, Phoebe, now one of Saturn’s moons. Image credit: NASA/JPL/Space Science Institute.

Enceladus, an icy, outer moon, was found to contain a subsurface water-ice ocean, which erupts in spectacular geysers.

This is a false-color image of jets (blue areas) in the southern hemisphere of Enceladus taken with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005. Image credit: NASA/JPL/Space Science Institute.

The rings were determined to be made up of 99.9% water-ice, and are at least hundreds of millions of years old.

One of Saturn’s small moons passing in its orbit through a gap in the rings. Image credit: NASA/JPL/Space Science Institute.

And finally, it discovered and viewed the largest storm in the Solar System’s known history: 2011′s Saturnian hurricane.

Image credit: NASA / JPL-Caltech / Space Science Institute, of the great storm’s evolution over a period of 8 months.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words. Cassini will end its mission in 2017 by crashing into Saturn, thereby avoiding any possible contamination of moons with organics on them.

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

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