Earlier simulations comparing cold dark matter models (with dark matter particles that are thermal relics with masses on the order of 1-1,000 GeV), with warm dark matter models (with particles typically around 2 keV), reveal that warm dark matter models produce results much more like what we see in the real work than cold dark matter models.
But, those simulations contained important simplifications, most critically, not meaningfully including the effects of ordinary baryonic matter on the overall evolution the the large scale structure of the universe and particular galaxies.
A new paper argues that many of the problems that the older models found with CDM models relative to WDM models are fixed in more sophisticated new simulations that appropriately consider the impact of a mix of ordinary baryonic matter and dark matter, rather than simply modeling pure dark matter systems.
Meanwhile, an epic 170 paper comprehensively explores alternatives to the standard lambda CDM model of cosmology that modify gravity in the weak gravitational field regime (an idea that started with Milgrom's MOND theory but has many modern variants) rather than relying at all or at least as heavily, on a dark matter hypothesis. Despite its length, it is only a preliminary survey and does not provide a definitive resolution of these questions.
There have been a lot of papers recently by those people who have modified their existing lambdaCDM models by changing the mass of the dark matter so that it is on the order of 2-10 keV.
ReplyDeleteThen, the authors show that 2-10 keV dark matter doesn't do much better than CDM.
The problem is that you have to do more than just change the mass of the dark matter particle. A 2keV fermion dark matter particle is quantum degenerate, which means that its density in the center of galaxies can't become cuspy (as would a 1-100 GeV dark matter particle.)
The way to tell if a paper by these authors is worthwhile to read is to search for the words 'quantum degenerate' or 'fermion.' If you don't see these words, then you know that it's not worth your time to read the paper.
These 'scientists' should ask themselves a simple question.
What keeps 1-100 GeV dark matter particles from clumping together?
We know that ~1 GeV neutrons clump together. If the total mass is less than the Chandrasekhar limit, you get a neutron star. If you are above that limit, you get a blackhole.
This whole Cold Dark Matter idea is ridiculous. The heaviest stable particle is the up quark (with a rest mass ~ MeV) and the heaviest stable particle that doesn't interact via the strong force is the electron (~0.5 MeV.)
A much simpler explanation for dark matter (and dark energy) is that dark matter is a 2-keV nearly-sterile neutrino. Such a particle would be quantum degenerate and would not clump together in the center of galaxies. Slowly over time, the sterile neutrinos decay into many lighter active neutrinos, who are also quantum degenerate. The quantum degenerate light active neutrinos provide the pressure that keeps the universe expanding. In other words, the light active neutrinos are the source of dark energy and the heavier, unstable mostly-sterile neutrinos are the source of dark matter.
This explains why the universe is expanding. As mostly-sterile neutrinos decay into lighter neutrinos, then the density of light neutrinos increases, however, this means that the pressure and entropy would increase. Since the pressure shows up in the General Theory of Relativity, spacetime expands because of the pressure.
Once all of the mostly-sterile neutrinos have decayed, then the main production source of light neutrinos will stop, and the expansion of the universe will slow or stop...but it won't go backwards into a big crunch because there will still be the quantum degeneracy pressure of the light neutrinos.
As for modified Gravity (MOND), this idea is equally out-of-date because there is now proof that there are volumes of space with dark matter but virtually no baryonic matter. The Bullet Cluster (as well as maps of dark matter in the universe where there is no non-dark matter) have effectively killed MOND theories. Plus, there's the fact that you can't just change Einstein's theory of General Relativity.
MOND as well as the cosmoligical constant are ad hoc.
We have explanations for dark energy and dark matter that don't require us to change Einstein's General Theory of Relativity.
It does, however, require us to find these 2-10 keV sterile neutrinos and to prove that there are enough light active, quantum degenerate neutrinos to provide the pressure that causes the universe to expand. This is not an easy task, but it seems achievable in the coming decades or century as we more precisely measure neutrino oscillations.
Phenomena attributed to dark matter are the greatest unsolved problem in fundamental physics.
ReplyDeleteNaval gazing problems like the hierarchy problem, the strong CP problem, the naturalness problem, the baryon asymmetry of the universe, etc. are basically efforts to tell Nature that it screwed up laws of nature that work just fine even if they aren't what one would expect.
Dark matter phenomena, in contrast, are the predominant situation where the Standard Model and GR fail to explain what is observed in a big way. (There are some problems in QCD in which theoretical predictions are also inaccurate but there is very good reason to believe that this has more to do with defective means calculating answers with equations that are fundamentally sound and complete, rather than fundamental flaws in the Standard Model).
There is no definitive answers regarding the mechanism or mechanisms that give rise to this phenomena. The Standard Model doesn't have particles that behave the way that dark matter appears to behave and are numerous enough to account for these effect. The DM candidates supplied by SUSY theories that could perhaps explain what we observe have largely been ruled out by HEP physicists and astronomers. What astronomers observe does not conform to GR when applied to all known baryonic matter in the universe.
To be fair, scientists studying this don't have an easy task. Nearly collisionless particles that interact predominantly only via the weakest force in the universe (gravity) and modifications of gravitational fields where the are already exceedingly weak, the only two possible solutions that even come close to fitting the data, are profoundly difficult to observed directly. Precision observations of faint objects billions of light years away from billions of years ago are not easy to make, and indirectly observing invisible things at those distance and time scales is even harder. Our incomplete knowledge of what's out there in the universe generating cosmic rays and driving galactic and cluster dynamics doesn't help either.
The theories that fit the data best, although none are a perfect fit are single flavor warm dark matter (of the keV sterile neutrino type), single flavor cold dark matter with very dynamic interactions with baryonic matters via gravity in sophisticated models, single flavor cold dark matter with self-interactions mediated by a massive boson that is still lighter than any known massive bosons, and relativistic generalizations of MOND or something roughly equivalent with cluster dark matter perhaps made of neutrinos.
Right or wrong in terms of mechanism, MOND's ability to very parsimoniously predict essentially all galactic scale dark matter phenomena shows that the mechanism must be very simple and intimately related by some means to the distribution of luminous matter (even if simply by virtual of uniformity in the process by galaxies evolve rather than any force modification or direct cause from DM particles).
Ultimately, I think, precision mapping of dark matter halos from their inferred gravitational effects will be the key to narrowing the parameter space and finding the answer which almost necessarily requires new physics.