Brian York's Life

Cold Dark Matter

Written on June 19, 2008

Some of you will already have seen this. Despite all the controversy surrounding Pluto (well, it’s more controversy than you usually get about astronomy), I think that the discovery in that little letter is probably of far greater importance. Of course, explaining why will require a little bit of background.

Many years ago (circa 1934), a smart but somewhat eccentric astronomer (strange how often those go together) named Zwicky pointed out that some of the galaxy clusters we see have too much kinetic energy. Specifically, when you estimate their mass (based on the amount of light present), it’s too small to keep them from flying apart. Of course, this wasn’t too much of a problem, since what it mostly meant was that we weren’t very good at measuring the mass of galaxy clusters based on their light content. But that wasn’t all.

A few decades later (we’re moving into the 1960s, 1970s, and even the early 1980s here), astronomers were using spectrographs on the new 4-m class telescopes to look at galaxies. Specifically, they were measuring rotation curves. This involves looking at the stars a certain distance out from the galactic centre, and measuring how fast they were rotating around the centre. They expected to see a nice falling curve as distance increased, similar to the curve you get when you look at the solar system. The reason you get this nice falling curve is that you can treat a solar system as having an ideal point mass in the centre, and a bunch of other point masses orbiting it which interact only with the central mass, and not with each other (to a first approximation, anyway). When you do that, you discover that the orbital speed depends on the distance away from the centre, and is lower the farther away you go (farther away = less gravitational force = move more slowly). Galaxies also have most of their light concentrated at the centre, so the astronomers were expecting to see the same thing. But they didn’t.

Instead, the orbital velocity levelled off, and became effectively constant. No big deal at first, since you can get that type of velocity curve just by having an even distribution of mass all the way out. Sure, the stars looked denser in the centre, and there looked like there was more gas there, but the arms of a spiral galaxy are still pretty bright, and there must be gas in the arms (because that’s where most of the star formation happens), so the new results were possible. But then people started looking out beyond the disk, where there were only a few stars, no star formation, and effectively no gas. And the rotation curve was still constant.

The only way you can get that, with gravity as we know it, is if there were mass, and quite a lot of it, out even in the galactic halo. But there obviously wasn’t any light there, and light traces mass quite well (at least in the solar system). Still, there seemed no choice but to conclude that there was “dark matter” in the galaxies that were making the rotation curves look odd. But not everyone agreed, of course.

A few astronomers decided that the real solution was to change the way gravity worked. This is called MOND (MOdified Newtonian Dynamics), and it introduces a correcting term that only operates at very large distances, and very low mass concentrations (otherwise it would have shown up in the solar system, and it hadn’t). The first MOND theories were, well, Newtonian (that is they didn’t include relativity), which was an obvious problem, but eventually some relativistic MOND theories were created. And there really wasn’t any way to choose between them, since the dark matter was invisible, and it followed the light distribution closely enough that you couldn’t really distinguish. Sure, dark matter was hierarchical in nature (pretty much none at the solar system scale, a bit at the galaxy scale, a lot at the cluster scale, and quite a lot at higher scales), but MOND also had more effect for larger objects (like clusters), so there still wasn’t any good way to tell.

Then came WMAP (Wilkinson Microwave Anisotropy Probe). Now, COBE (Cosmic Background Explorer) found the cosmic background, but it couldn’t really measure any irregularities in the background. For reference, the biggest source of fluctuations is microwave emission from the Milky Way (so you have to filter that out). Then comes other microwave hotspots (from neutron stars, active galaxies, etc.) (so you have to filter those out). Then comes the cosmic dipole (because the Milky Way is moving relative to the cosmic background, so part of it is red-shifted and part is blue-shifted). Then you have the actual fluctuations, which produce a nice map like this. For reference, the overall temperature of the background is 2.73 K. And the difference in temperature between the hottest (red) and the coolest (blue) points on that map is about 0.00001 K. So finding them wasn’t easy.

Now, the fluctuations look random, but they aren’t. There’s a pattern in the spacing, and it tells us a lot about the early universe. In the early universe, you see, photons, protons, and electrons were in equilibrium. Oh yes, and there was dark matter (in the standard model), which only interacts due to gravity. And the dark matter has a clumpy distribution. So the result is that places with dark matter clumps attract in protons and electrons. But these are constantly absorbing and emitting photons, so the clump also has many more photons that the surrounding universe, and that means light pressure, which pushes away the protons and electrons (but not the dark matter, because it doesn’t interact with light). So these temporary matter clumps are constantly forming and breaking apart as the universe expands and cools.

Then, suddenly, the universe becomes cool enough (and big enough) that the photons that are emitted don’t get absorbed right away. So they escape, and keep on going. The coupling is broken, and nothing’s forcing the protons and electrons to break apart again when they clump together. So these little clumps start interacting with one another, and getting bigger and bigger, and eventually structure is formed (stars, galaxies, clusters, etc.) with larger structures taking more time to form.

And those photons that escaped? They’re the cosmic background. And those fluctuations exist because, when matter and radiation decoupled, things were clumpy. And, by looking at the fluctuations, and how intense they are, and how far apart they are, you can tell a lot about the early universe.

One of the things you can tell is how much matter there was. Another is how much baryonic (non-dark) matter there was, because baryonic matter was interacting with the photons, while dark matter wasn’t. And, when you do the calculations, you find there was about six times as much dark matter as baryonic matter. This is, of course, a problem for MOND people, which they dealt with mostly by ignoring it (really — MOND papers seldom reference the WMAP results except to say vague things about them being unreliable, and there are quite a few MOND papers published in the past year that imply (or even state outright) that the only evidence for dark matter is galactic rotation curves). So dark matter now has a fairly substantial majority of scientists convinced, while MOND seems unable to explain the WMAP data, but people are still pushing it.

Then comes the result I started with. These people looked at two merging galaxy clusters, because they realized something very important: most of the (non-dark matter) mass in the cluster is in the form of very hot plasma. And, while stars (and galaxies, and dark matter) act like collisionless particles when clusters merge, gas acts like a fluid, so the gas from the two clusters would end up colliding and slowing down (which you can see in their figure). So they could see where the gas was, and it wasn’t where the galaxies were. Next, they measured where the mass was through gravitational lensing (gravity bends light, so when you have mass between you and a source of light the source looks slightly brighter because the light has been focussed (so long as the mass is compact enough not to just block the light of course)). And the mass measurements show that the mass is where the galaxies are. Except that it’s already known that the galaxies are much less massive than the plasma, so there must be something else where the galaxies are. Something more massive than the plasma. Something that acts as collisionless particles. Something that doesn’t emit light. Something like dark matter. And this is a vitally important result, because it’s the first time that someone has shown, conclusively, that the mass distribution and the visible mass distribution are not the same. That’s something that doesn’t work at all in any MOND theory (unless you modify MOND to include dark matter, but really what’s the point).

So, after all this, that’s why this result is so significant. It’ll solve a “controversy” that’s been going on almost as long as “what are the diffuse interstellar bands” (which, I point out modestly, I’m currently working on), and much more relevant to cosmology as a whole.

And what about the “cold” in the title? That’s a distinction for another day.

Entry last updated August 27, 2006