“By now you must know that your father can never be turned from the Dark Side. So will it be with you.” -Emperor Palpatine, Return of the Jedi
You’ve heard about dark matter. It’s the notion that the Universe is somehow very much different than the small corner of it that we’re most familiar with.
When we look at our Solar System, we can add up all the rocky planets, the gas giants, the asteroids, moons, and comets, as well as the entire Kuiper belt, and find out just how much of our neighborhood is dark.
And we can compare that with the Sun, the one thing in our Solar System that emits its own light. To no one’s surprise, the Sun makes up 99.8% of the mass of our Solar System. (In fact, Jupiter, the next-largest mass, makes up 0.1% on its own, equal to everything other than itself and the Sun combined!)
Not every star is like the Sun, of course; many are bigger and brighter, while the vast majority are smaller and dimmer.
After performing a census of all the stars visible in our night sky, averaging all the light emitted from stars and the masses of all the stars together, we (perhaps surprisingly) find that our Sun gives off less light for its mass than the average of all stars in the galaxy!
The biggest, brightest, most luminous stars dominate the overall light given off by a galaxy, and taking these measurements allows us to construct a mass-to-light ratio for normal, star-dominated matter. It turns out to be about three times as luminous, per solar mass, as our Sun is.
So when we look out at a large object in space, containing anywhere from a few hundred thousand to many trillions of stars, if we measure the amount of light coming from it, we can know how much mass there is in the form of stars. That’s just basic astronomy.
But we also know how gravity works. Whether you see a single spiral galaxy rotating or thousands of galaxies zipping through a cluster, if you can measure the speeds at which this collection of matter moves, so long as you understand the connection between gravity and mass, you can figure out how much total mass is in one of these objects.
In a simple, ideal world, these two ways of measuring the mass — from starlight, on one hand, and from the effects of gravity, on the other — would give you equal numbers.
But they don’t.
They don’t even come close, in fact. Whether you take a single galaxy comparable in size to the Milky Way, or a huge cluster of hundreds or even thousands of galaxies, we find — pretty much every time — that there’s more mass than stars can account for.
How much more, you ask?
A factor of fifty.
“Big deal,” you say, “there are plenty of ways that matter can be there and not emit visible light.”
Of course, you’re right. The Universe ought to be filled with not just planets and and rocky objects, but also with interstellar gas, dust, plasmas, dead stars, black holes, and pretty much anything else you can imagine that’s made up of protons, neutrons, and electrons that isn’t a star.
So we try to add up all the different sources of protons, neutrons, and electrons, and compare it to the total amount of mass we see. The results are staggering.
We look at huge X-ray surveys of galaxy clusters, and determine how much of the total mass is in the form of stars, gas, dust, and everything else that known matter — the stuff made of protons, neutrons, and electrons — can make up. Very consistently, we get a number between 13-17% of the total mass, but no more than that.
Much more than exists in stars, to be sure, by a factor of 5-to-7. But still not nearly enough to account for everything.
Now, here’s where it gets interesting.
Every observation we make supports this picture: 15-17% of the total mass is protons, neutrons, and electrons (henceforth just called baryons). But the total amount of mass — 100% — is made up of mostly some non-baryonic dark matter, about 83-85% of the total, or about five times as much as the normal, baryonic stuff.
For galaxy clusters, this is confirmed by not only the peculiar velocities of individual galaxies and X-ray emissions, but by gravitational lensing, as above, and gravitational redshift, as shown below.
The total amount of matter is simply too great to be accounted for by baryons alone. What’s more than that, this dark matter is distributed much more diffusely — in large, fluffy halos — than the stars or gas that we can see.
But it isn’t just clusters that tell us how much total matter there is, nor are clusters the only source for our information on how many total baryons there are.
We can measure the total amount of matter by studying the peculiar velocities of galaxy pairs.
We can know the total amount of baryons by measuring the abundances of the light elements left over from Big Bang Nucleosynthesis. Again, we get that the total amount of matter is about six times as much as can be present in baryons alone.
We can both measure and simulate the large-scale structure of the Universe, and once again, we find that the only way to get agreement is to have there be about five times as much dark matter as there is baryonic matter.
And within this framework, there are all sorts of different quantities we can use to test this number. Just to name a few: baryon acoustic oscillations, the two-point correlation function, and Silk damping in the matter power spectrum all point towards this same picture of a Universe with about a 5:1 ratio of dark matter to baryonic matter.
And of course, you know of one more.
The pattern of fluctuations seen in the Cosmic Microwave Background, or the leftover radiation from the Big Bang. The scale and magnitude of these microKelvin fluctuations allows us to determine — solely based on the laws of physics — just how much baryonic matter and how much dark matter there is.
And, once again, we find that the total mass of the Universe’s dark matter outnumbers the total mass of the baryonic matter by that familiar ratio of about 5:1.
So, every measurement sensitive to dark matter indicates that it’s there, and there’s about five times as much of it as there is of normal matter: the protons, neutrons, and electrons that make up everything on Earth.
So what is this dark matter? What can we say about its properties?
Well, we can take a look at its temperature, for one. If dark matter is hot, matter has a hard time collapsing and forming structure on smaller scales. But the colder — or having less kinetic energy — this dark matter is, the earlier and earlier we can form structure on the smallest scales.
So if we take very distant light sources — things like quasars — we can look for absorption features on small scales, known as the Lyman-alpha forest. The earlier these features are sharp and present, the colder our dark matter has to be! What do we find?
We find that dark matter can’t be hot. Given that we know the temperature of the Universe from the Cosmic Microwave Background, we can rule out massive neutrinos as a dark matter candidate, because they’d be too light (and therefore too hot, given the ratio of kinetic energy to mass) to form this structure.
But dark matter could be either warm, with masses of somewhere from about 5 keV/c2 up to maybe 1 MeV/c2, or cold, where it would have to have even heavier masses or, like the very light axion, be born without kinetic energy. (Searches for axions have come up disappointingly empty thus far.)
So how would we distinguish warm from cold dark matter?
We can look at the cores of galaxies, and see whether they have cusps, like cold dark matter predicts, and they don’t!
Multiple studies confirm this as well: there are the wrong number of dwarf galaxies for cold dark matter, there’s the wrong amount of power on the smallest scales, and the cores of galaxies have the wrong amount of overall matter density for cold dark matter!
So maybe, you think, dark matter isn’t cold, but warm.
The simulations match up better, and warm dark matter is better for astronomy. Standard cold dark matter doesn’t give you the right predictions on these small scales. But when we look over on the physics side, there are no known particles that could be warm dark matter! What could they be?
Maybe, but even the experiments that indicate them as a possibility (LSND, miniBooNE) give the wrong mass (they’re too light). If there are warm dark matter particles, our electron-positron collider experiments should already have detected them, and yet… nothing.
So what are our other alternatives?
Maybe the dark matter is cold, but it interacts with itself. We know that dark matter doesn’t collide with baryonic matter from experiments like CDMS and Edelweiss, but maybe it collides with other dark matter particles? This would allow us to get dark matter that looks isothermal at its core, rather than the other models that fail to match up with observations.
But we have a spectacular result coming from these colliding galaxy clusters, with the best example being the Bullet Cluster.
If you want dark matter to self-interact enough to make your halos isothermal, it would be too “sticky” to give you the mass distribution found in the Bullet Cluster! So none of the models are entirely satisfying.
These claims are not only inconsistent with one another, but also with much more sensitive direct detection experiments, CDMS and XENON.
Casting even further doubt on their results, these teams don’t even know what they’re detecting. (In other words, they cannot fully — or even primarily — account for their background in the detector.) They do see an annual modulation in the signal, which they attribute to the Earth’s motion through the galaxy’s dark matter. But other experiments refute their results, which in turn are inconsistent with one another’s results.
So putting all of this information together, what does it mean?
Astrophysically, there’s got to be some type of dark matter out there, and it needs to be about five times as abundant as the normal, baryonic matter we have. The indirect evidence is overwhelming. If you try to build a theory without dark matter, you simply have to throw out a whole suite of very well-tested and verified physics, and you still cannot explain all the observations detailed above.
So what do we know about this dark matter? It can’t be hot, and it also can’t be completely cold and collisionless. There are constraints on how hot it can’t be, and also on how large its self-interacting cross section can be.
But, as far as particle candidates go, there isn’t a single compelling one. Warm dark matter seems to be the best astrophysical fit, but as for the type of particle behind the majority of matter in the Universe? We know practically nothing about it. And experimental claims of direct detection are, at best, sketchy. (And at worst, bogus, spurious and/or heavily misinterpreted. As Resonaances notes: “[the] experiment is closing in on IDM; what is not clear is whether that stands for Inelastic or Italian Dark Matter.”)
The mystery continues, and while we don’t have the answers, here, at least, are the pieces of the puzzle. Now, put it together, and the Nobel is yours!