Tuesday, June 13, 2017

Baryon Effects Can't Save Cold Dark Matter Models

The New Result

Simulations that include only Cold Dark Matter (CDM) don't reproduce something close to reality at the galaxy scale. One argument that this doesn't invalidate CDM models is that interactions between ordinary matter and dark matter (baryon effects) cure this problem and bring CDM models more closely into line with observed reality. CDM models can include baryon effects that can solve the problem, but they have to be highly unrealistic to accomplish this goal.

Now, a realistic, bottom up effort to determine that magnitude of these baryon effects has established that baryon effects don't solve the problems with cold dark matter models that are revealed in simulations using only cold dark matter.

Where Does This Leave Us?

First, dark matter phenomena are absolutely real and everyone agrees that this is the case. But, there is lack a consensus explanation of the mechanism behind these phenomena.

Warm Dark Matter Is Still Viable, But Is Tightly Constrained

This new result leaves basically one viable particle dark matter model: warm dark matter (WDM) with particles with masses of ca. 2-15 keV. There are very tight constraints on the parameter space of WDM, but those constraints still, just barely, leave a narrow mass range open. If one were looking for a theoretical framework to fit WDM into, a gravitino without other SUSY particles, a right handed neutrino, or a sterile neutrino singlet would both be plausible, yet minimal models. But, a neutrino that oscillates with ordinary neutrinos is also extremely constrained in parameter space.

Some Gravity Modification Theories Are Still Viable

There are also multiple gravity modification theories that could explain dark matter phenomena, although some gravity modification theories have been shown to be inconsistent with experiment. (Not, however, due to the Bullet Cluster which actually favors modified gravity theories over particle dark matter theories.)

Self-Interacting Dark Matter Models Are Pretty Much Ruled Out

Models with a 5th force finite but long range Yukawa force transmitted by a massive boson that acts between dark matter particles called self-interacting dark matter (SIDM) looked promising for a while. But, pretty much all plausible models of SIDM have been completely ruled out because sufficiently strong self-interactions should give rise to dark matter annihilations which are not observed. Also, while Occam's razor is pretty neutral between the WDM and gravity modification approaches discussed above, it disfavors SIDM which basically requires both.

Axion Dark Matter Isn't Definitively Ruled Out But Is Materially Constrained

It isn't entirely clear how axion dark matter models measure up - the range of axion dark matter models under consideration is great and the phenomenology seems to be all over the map, and this approach isn't subject to the limitations associated with thermal dark matter models. Most axion dark matter proposals mostly aren't a good fit to the originally proposed particle that would explain why there is no CP violation in the strong force (which I have never found compelling), however. And, this parameter space is still meaningfully constrained.

Not Clear Where Wave Dark Matter Stands

Another proposal which I haven't seen definitively ruled out is wave dark matter.

Cold Dark Matter, Including WIMPs And MACHOs Are Ruled Out

But, pretty much the entire universe of CDM (the subset of CDM called WIMPs is already pretty much ruled out by direct detection experiments which is bad news for SUSY theories which generically predict WIMP dark matter candidates) and MACHOs (such as primordial black holes) has been ruled out in an increasingly convincing way.

Any Dark Matter Candidate Must Be Basically Sterile And Stable At Low Energies

Of course, by hypothesis, any particle that interacts via the electromagnetic force or strong force was ruled out. And, experiments in the parameter range that remains viable for any kind of dark matter particle rules out any weak force interactions comparable to those of the existing Standard Model particles. So, any dark matter candidate needs to be basically completely "sterile" (i.e. without interactions other than Fermi contact interactions), at least at low energies.

Dark matter must also be either completely stable, or "metastable" with a mean lifetime on the order of the age of the universe or more.

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