Thursday, June 1, 2023

Cold Dark Matter Still Gets Galaxies Wrong

MOND or another theory giving rise to a comparable effect is necessary and sufficient to explain galaxy dynamics. This is hard to do with dark matter particle theories.
to explain statistically late-type galaxy dynamics within the disk it is necessary and sufficient to explain the RAR and lack of any significant, partially independent correlation. While simple in some modified dynamics models, this poses a challenge to standard cosmology.
From here.

It basically can't be done in a cold dark matter theory.
Before halo compression, high-mass galaxies approximately lie on the observed RAR whereas low-mass galaxies display up-bending "hooks" at small radii due to DM cusps, making them deviate systematically from the observed relation. After halo compression, the initial NFW halos become more concentrated at small radii, making larger contributions to rotation curves. This increases the total accelerations, moving all model galaxies away from the observed relation. These systematic deviations suggest that the CDM model with abundance matching alone cannot explain the observed RAR. Further effects (e.g., feedback) would need to counteract the compression with precisely the right amount of halo expansion, even in high mass galaxies with deep potential wells where such effects are generally predicted to be negligible.
From here and recapitulated here and here.

Cosmology also continue to be a problem for the LambdaCDM model.
We establish a new and cosmological-model-independent method to explore the cosmic background dynamics in this work. Utilizing the latest Pantheon+ type Ia supernova sample and the Hubble parameter measurements, we obtain the values of the Hubble parameter and the deceleration parameter at five different redshift points ranging from 0.2 to 0.6, and find that they can deviate from the predictions of the ΛCDM model at more than 2σ. We further probe the equation of state of dark energy and obtain that a slightly oscillating equation of state of dark energy around the −1 line is favored.
From here.

Milgrom, meanwhile, considers a version of MOND that is matter distribution shape dependent, which was critical to Deur's generalization of the gravitational approach to dark matter to a wider range of applicability.


neo said...

Cold Dark Matter Still Gets Galaxies Wrong

maybe there are many different Dark Matter particles of different mass and effects of which only a small fraction of Dark Matter is Cold Dark Matter by mass

andrew said...

You could have a mix of dark matter particles, some of which are rare or ephemeral.

For example, baryonic cosmology ignores everything but protons, neutrons, electrons, and neutrinos, even though there are hundreds of other hadrons out there which generally only form in extremely high energies and decay exceedingly rapidly.

But, the predominant component of DM can't be CDM-like, which is the core assumption in the LambdaCDM paradigm. So, that paradigm is wrong.

neo said...

But, the predominant component of DM can't be CDM-like, which is the core assumption in the LambdaCDM paradigm. So, that paradigm is wrong.

what about hot and warm and rare or ephemeral dark matter particles mixed with WIMPS so the average yield CDM-like in the LambdaCDM paradigm

andrew said...

"what about hot and warm and rare or ephemeral dark matter particles mixed with WIMPS so the average yield CDM-like in the LambdaCDM paradigm"

It doesn't work. Hot DM looks like a fourth neutrino species which we don't see, and if it is rare it simply doesn't matter much. Ephemeral particles have no significant cosmological impact but generate annihilation signatures which are tightly constrained. WIMPS don't fit the data in ways that adding more to the stew doesn't help.

Even warm dark matter is increasingly ruled out.

Maybe, self-interacting dark matter can work, but efforts to consistently fit the parameters of interaction you need haven't been a great success.

Maybe, axion-like ultralight dark matter can work, but CDM screws that up and decouples from it.

There is really nothing that a CDM component, let alone a SUSY WIMP, would add to fitting the data.

neo said...

doesn't all these papers assume 100% rather than a mixture of various types ?

i.e. 100% warm dark matter or 100% WIMP as oppose to say 10% warm dark matter + 10% WIMP + ....

if only 1% is WIMP that could be why Xenon hadn't seen it

andrew said...

There are papers out there that consider the more than one type of DM models, although they are not as common.

For example, a lot of the primordial black hole literature considers the maximum percentage of DM that can be from PBHs.

A lot of the early WIMP papers likewise considered mixed models but they almost always performed worse than the single DM type models in matching the data (adjusting Chi-squared fits for degrees of freedom in the model as is standard statistical practice).

The best multi-type results come in self-interacting DM models which typically have a fermion DM particle and a massive carrier boson DM particle to carry the self-interaction force (sometimes called a dark photon).

One problem you get is that from, e.g., gravitational lensing of galaxies, you have more or less a fixed budget of total amount of DM. DM mean velocity (i.e. hot v. warm v. cold) drives how much large scale structure there is in a DM particle model.

For a given amount of DM, you can estimate a mean velocity from how much large scale structure there is in galaxies, etc., with one kind of DM particle.

Suppose instead you have two kinds -- one too cold, and one too hot relative to the single estimate. You can tweak the proportions to get the right amount of structure in the universe from hot DM pulling towards less structure and cold pulling towards more structure. But, to do that you need more total DM than you do in a single dominant DM type model, because the cold and the hot DM are cancelling each other out in the amount of structure department.

Another factor that disfavors complex models is that the underlying phenomena is pretty simple. If you use GR, plus MOND in the weak field effectively Newtonian regime of GR, you can explain all gravitational observations from millimeters or less scale in laboratories on Earth to galaxies - all of which follow the radial acceleration relation (RAR) modified by the external field effect.

This doesn't get you to galaxy clusters, but there is a galaxy cluster analog to the RAR which is similar in form with different parameters that can explain all galaxy clusters.

So, with GR + cosmological constant + the MOND constant + one to three galaxy cluster scaling relation constants, you can capture all observed gravitational dynamics from tabletop to galaxy clusters - scores of orders of magnitude of range of applicability in a simple phenomenological fit to some pretty crude formulas.

While this isn't a rigorous proof (although that would be possible, subject to weak assumptions), it doesn't make sense that you would need a lot of different moving parts of a complicated many part model to recreate that.

Deur's model, whether or not it isn't GR, purports to do all of that without a cosmological constant or MOND constant or cluster constant by leveraging the 3D geometry of matter distributions, although there is one physical constant in there that he fits from the MOND data and just assumes can be derived without actually doing so. At least as proof of concept, the astronomy observations can be explained with a model that has very few moving parts.

If a gravity model with very few moving parts can work, then a DM model that produces the same results should also have very few moving parts.