# Why we need Dark Matter in the Universe.

Last week, Pamela Gay over at Star Stryder pointed me to a press release which claimed that, among other things, perhaps dark matter wasn’t necessary. So I wrote a guest post on her blog explaining why it was. Apparently, some people still aren’t convinced. So I will lay out for you all the reasons I can think of why we need it, and explain what happens if you try to do without it.

1. Cluster Velocity Dispersions. When we take a look at galaxies, we often find hundreds or even thousands of them clustered together, like in the Coma Cluster. We can measure how quickly those galaxies are moving around their center, and we can determine, based on the laws of gravity, how much mass is in these clusters. When we do these calculations, we find that Ωm, or the amount of total matter in the Universe, is about 20 to 30% of its critical value. (Note: Ωb, or the amount of matter in protons, neutrons, and electrons, is known from Big Bang Nucleosynthesis to be between only 4 and 5%.)

2. The Power Spectrum of the Universe. We can measure how many galaxies there are at a bunch of different positions in the Universe. There are big surveys like the 2-degree Field Galaxy Redshift Survey and the Sloan Digital Sky Survey, which have allowed us to construct maps of where all the galaxies in a large volume of the Universe are (see the image above, where each little dot represents a galaxy). We can then determine, based on the clustering of these galaxies, how much total matterm) and how much normal matterb) there is. We find that Ωm is about 23% and Ωb is about 3 to 4%.

3. The Cosmic Microwave Background. The leftover radiation from the big bang is about 2.725 degrees Kelvin, and it’s approximately that temperature everywhere. But some spots are just a tiny bit hotter or colder (on the order of 10 microKelvins). We can learn a lot about cosmology from these temperature anisotropies, and one of them is how much dark matter and how much normal matter we have in the Universe. What we find from this is that the amount of total matter is about 27% and the amount of normal matter is 4-5%.

4. Gravitational Lensing Data. Again, let’s come back to my favorite nail-in-the-coffin of “we can just change the laws of gravity and do away with dark matter” theories: colliding galaxy clusters. There’s the Bullet Cluster,

there’s Abell 520,

and the cluster CL0024+17,

and the important thing is that we find that we have matter exerting gravity where no normal matter exists. Even if you fine-tune a theory of gravity to work for one of these three geometries, it doesn’t work for the other two. I will sum this up: there is no theory of modified gravity that can explain these three observations without also including dark matter.

5. Rotation of spiral galaxies. When a spiral galaxy rotates, we would expect, if there were no dark matter, for the outer parts to revolve around the center with a much slower velocity than the inner parts, just as the outer planets of our solar system revolve around the Sun with slower velocities than the inner planets. But we don’t see that. In fact, as far as we can tell, the velocities of rotating galaxies remain constant no matter how far out you go. This means either the laws of gravity are wrong for galaxies or there is some extra mass, like dark matter. But if you think the laws of gravity are wrong, I urge you to reread point number 4.

I realize this is especially confusing given the number of crackpots out there, who ignore some of these points and then claim we don’t need dark matter. I realize that there are alternative theories that can explain *some* of these theories, and there may even be alternatives to General Relativity like Bekenstein’s Tensor-Vector-Scalar gravity that can possibly explain points 1, 2, 3, and 5 without dark matter. But point 4 is really the killer. You want there to be gravity where there’s no matter? They haven’t made a theory that can do that in a way that’s consistent with observations, and people have been trying.

There are other tests that provide evidence for dark matter as well, but I think the five above should convince any skeptic of the facts that

• Dark Matter explains all of these observations, while
• modifying gravity cannot explain all of these observations.

Are you convinced? Leave me a comment about it. Still not convinced? Leave me a comment and tell me why!

February 19, 2008

I’m convinced. But what is it? Does it absolutely have to be some particle that isn’t included in the Standard Model? Didn’t you tell me once that you don’t think the LSP is a particularly likely candidate? Have other proposed candidates been worked into gauge theories which we know to be correct (at least to very high precision, if not perfectly)?

2. #2 ethan
February 19, 2008

Ben,

The leading theories all require something beyond the standard model. It could be supersymmetric (the LSP is the lightest supersymmetric particle); it could be a stable relic left over from either inflation or a grand unified theory, it could be from extra-dimensions (the LKP is for the lighest Kaluza-Klein particle), it could be an axion (the Standard Model needs to explain why the strong interactions don’t violate CP; the axion is a model to explain that and it also can make up dark matter), or a lot of other things. Primordial black holes are ruled out as composing all of the dark matter in the mass ranges of interest.

But it isn’t necessarily a particle, either — that’s really all we know. For instance, does gravity really require that there be a graviton to explain it? No, but since we don’t understand gravity on small scales to begin with, it’s a reasonable assumption. But it doesn’t need to be a particle, and dark matter doesn’t need to be one, either. Dark matter could be a fluid of some type, for instance, and that would be consistent with all of the observations. There’s a lot left to uncover, and it would be really interesting to propose a test of it!

The thing that kills most modifications to gauge theories (and this is the same thing that kills most models of supersymmetry) is that there are no flavor-changing neutral currents. In other words, most models predict them (the standard model does not), and the experimental limits on them say that if there are flavor-changing neutral currents, they have to be very, very small. So small that most models of supersymmetry (and many other modifications as well, like technicolor) are ruled out.

That’s as far as I know most of this stuff, though!

Ethan

3. #3 Frank Burdge
February 25, 2008

Ethan, your are most certainly correct about the existence of dark matter and energy. I believe they will both be much better understood within the next 24 months. The interactions of dark matter will undoubtedly explain the exponential inflation epoch of the big bang as well as the the continuing expansion and the current acceleration of the expansion of the universe. Keep up the good work.

Frank