Dark Matter, Yes?

Experimental evidence for the existence of dark matter explained by Sean Carroll in his blog.

Here is a picture of the distribution of the matter based on gravitational lensing. (Of course, labeling it Dark Matter assumes the very thing we are trying to prove, so that is a little disappointing.)


Here is a picture of the distribution of matter based on X-ray observations of the hot gas.


The previous two images superimposed:


The gravitational field, as reconstructed from lensing observations, is not pointing toward the ordinary matter. That’s exactly what you’d expect if you believed in dark matter.

There is no denying the conclusion from the third picture that there is definitely "dark matter" there in the sense that the hot gas does not constitute the bulk of the gravitational mass. However, the conclusion that there is Dark Matter (meaning nonbaryonic cold dark matter, in the sense that the term is usually used in cosmology) depends on the assumption that the hot gas is the bulk of the regular matter and there cannot be nonvisible regular matter causing this effect.

Whatever matter is in the blue areas in the pictures above cannot interact heavily with regular matter (else, it would have been dragged along with the pink areas). This tends to support the hypothesis that it is Dark Matter. However, it is my opinion that our imperfect understanding of the universe simply does not allow us at this time the luxury of treating it as the only possible hypothesis to the exclusion of all others. True Dark Matter (Higgs, Superpartners, WIMPs, etc)? Baryons? Neutrinos? Who knows!

One response to Sean Carroll's post seems worth reproducing here: The history of the Vulcan hypothesis shows the way: the strange movement of Mercury was not caused by something which was huge and strangely impossible to see; it was caused by a small relativistic effect which was hidden in normal situations by the very fact of its weakness. When people start saying that an effect is caused by something three times the mass of the visible universe but which has escaped notice for the whole of history, is it really unreasonable to doubt them? I don’t think so. Dark matter is a fudge; a placeholder until we come up with something sensible.