Cold Dark Matter Still Doesn't Work, MOND Moves Forward

Stacy McGaugh at Triton Station recaps a long list of problems with cold dark matter (CDM) theories from a decade old review article that are almost all still there and in some cases worse. But these issues for CDM aren't troublesome for MOND for the most part. I have a list of 33 CDM theory problems, but I'm sure that I'm missing some of them from this list (like the far lower than expected number of galaxies in large scale structure voids, which is natural in MOND).

Why the discipline stands by a LambdaCDM theory that is dying from a thousand cuts is beyond me.

Toy-model MOND may not be the final solution, which either is, or looks something like, Deur's gravitational field self-interaction approach (that relies on the non-perturbative behavior of the General Relativity Lagrangian and mimics MOND closing in its domain of applicability). 

The strong tendency of Deur's theory and reality to favor disk-like matter distributions over sphere-like matter distributions (noted at several points in the recap), and the apparently weaker MOND effects outside the galactic plane, both support this approach, as does the relationship between the ellipticity of elliptical galaxies and their apparent dark matter fraction.  

Dark matter phenomena just has to have a gravity-type solution that reproduces MOND in the basic behavior of most galaxies. Dark matter phenomena are wave-like, not particle-like. The evidence of this is overwhelming.

The biggest issue within the MOND community, which was front and center at its 40th anniversary conference, is what the wide binary star system tests show. These tests, potentially, are powerful discriminants between different variants of modified gravity theories. Analyzing the data in a way that can show which properties wide binary stars really have is hard. But it is not an insurmountable problem. The firehose of new, better quality data from a variety of "telescopes" continues to flow and computer processing power to make sense of it continues to improve, so we will get our answers soon enough. 

Some of the problems not on my existing list:

The local void challenge.

Peebles has been pointing out for a long time that voids are more empty than they should be, and do not contain the population of galaxies expected in LCDM. They’re too normal, too big, and gee it would help if structure formed faster. In our review, we pointed out that the “Local Void” hosts only 3 galaxies, which is much less than the expected ∼ 20 for a typical similar void in ΛCDM.

The angular momentum challenge 

During galaxy formation, the baryons sink to the centers of their dark matter halos. A persistent idea is that they spin up as they do so (like a figure skater pulling her arms in), ultimately establishing a rotationally supported equilibrium in which the galaxy disk is around ten or twenty times smaller than the dark matter halo that birthed it, depending on the initial spin of the halo. This is a seductively simple picture that still has many adherents despite never having really worked. In live simulations, in which baryonic and dark matter particles interact, there is a net transfer of angular momentum from the baryonic disk to the dark halo. This results in simulated disks being much too small.

This problem is solved by invoking just-so feedback again. Whether the feedback one needs to solve this problem is consistent with the feedback one needs to solve the cusp-core problem is unclear, in large part because different groups have different implementations of feedback that all do different things. At most one of them can be right. Given familiarity with the approximations involved, a more likely number is Zero.

The pure disk challenge 

Structure forms hierarchically in CDM: small galaxies merge into larger ones. This process is hostile to the existence of dynamically cold, rotating disks, preferring instead to construct dynamically hot, spheroidal galaxies. All the merging destroys disks. Yet spiral galaxies are ubiquitous, and many late type galaxies have no central bulge component at all. At some point it was recognized that the existence of quiescent disks didn’t make a whole lot of sense in LCDM. To form such things, one needs to let gas dissipate and settle into a plane without getting torqued and bombarded by lots of lumps falling onto it from random directions. Indeed, it proved difficult to form large, bulgeless, thin disk galaxies in simulations.

The solution seems to be just-so feedback again, though I don’t see how that can preclude the dynamical chaos caused by merging dark matter halos regardless of what the baryons do.

The stability challenge 

One of the early indications of the need for spiral galaxies to be embedded in dark matter halos was the stability of disks. Thin, dynamically cold spiral disks are everywhere around us, yet Newton can’t hold them together by himself: simulated spirals self destruct on a short timescale (a few orbits). A dark matter halo precludes this from happening by counterbalancing the self-gravity of the disk. This is a somewhat fine-tuned situation: too little halo, and a disk goes unstable; too much and disk self-gravity is suppressed – and spiral arms and bars along with it.

I recognized this as a potential test early on. Dark matter halos tend to over-stabilize low surface density disks against the formation of bars and spirals. You need a lot of dark matter to explain the rotation curve, but not too much so as to allow for spiral structure. These tensions can be contradictory, and the tension I anticipated long ago has been realized in subsequent analyses.

The low surface brightness spiral F568-1 (left) and its rotation curve (right). The heavy line indicates the stellar disk mass required to sustain the observed spiral arms; the light line shows what is reasonable for a normal stellar population for which the galaxy consistent with the BTFR and RAR. We can’t have it both ways; this is the predicted contradiction to invoking dark matter to explain both disk stability and kinematics.

I’m not aware of this problem being addressed in the context of cold dark matter models, much less solved. The problem is very much present in modern hydrodynamical simulations, as illustrated by this figure from the enormous review by Banik & Zhao:

The pattern speeds of bars as observed and simulated. Real bars are fast (R = 1) while simulated bars are slow (R > 2) due to the excessive dynamical friction from cuspy dark matter halos. (Fig. 21 from Banik & Zhao 2022).

The missing baryons challenge

The cosmic fraction of baryons – the ratio of normal matter to dark matter – is well known (16 ± 1%). One might reasonably expect individual CDM halos to be in in possession of this universal baryon fraction: the sum of the stars and gas in a galaxy should be 16% of the total, mostly dark mass. However, most objects fall well short of this mark, with the only exception being the most massive clusters of galaxies. So where are all the baryons? 

All told this adds about four or five problems to my existing 33 problems with Lambda CDM.