Kroupa On Dark Matter v. MOND

Pavel Kroupa, a leading MOND researcher, published an article in the IAI news, directed at a broader audience on July 12, 2022 forthrightly titled "Dark Matter Doesn't Exist" that makes some less commonly discussed arguments for this position. It states in some notable parts:

This dark-matter-based model (for each gram of normal matter there are 25 grams of the exotic dark matter) is about 20 years old, but the strong belief in the scientific community that dark matter exists goes back 30 years. . . .

The many searches worldwide for evidence of dark matter particles, going on since at least 30 years, have come up empty handed. . . .

there is a simple test that these scientists are ignoring and which has already been applied and it tells us that dark matter does not exist. This test goes back to Subrahmanyan Chandrasekhar who, in 1943, showed that a massive body (e.g. a dwarf galaxy) that moves through a background of comparatively low mass particles (e.g. dark matter particles), will slow down. This process of "Chandrasekhar dynamical friction" is exceedingly well understood. . . .With my collaborators and students, we have applied a number of observed galaxy systems to the calculations of Chandrasekhar dynamical friction we would expect to see if dark matter existed, and in all and every case it turns out that the slow-down is not in the data. . . . Rather than observing the slow down of galaxies through Chandrasekhar dynamical friction, we observed a speed-up as the galaxies fall towards each other. This is the same as two stars that fall towards each other in a star cluster. They get faster until they pass each other and then they recede again from each other. . . .

satellite galaxies are typically orbiting their host galaxies in vast disks of satellites, much like the planets orbit the Sun in one plane, while according to the dark matter models, they should be orbiting in all possible directions. . . .

the research community has largely ignored these falsifications

Three other very major tests of the dark-matter based models have been published recently:

(A) One test relies on how rapidly a dark-matter-filled universe can form extremely massive clusters of galaxies that also penetrate each other. The El Gordo galaxy cluster is immensely heavy, weighing a thousand times the mass of the Milky Way and Andromeda together. This cluster is actually composed of two such clusters which have formed and transgressed through each other at a time when the Universe was only half its present age. It turns out that the dark-matter-based models cannot, under any circumstances, grow such massive clusters and also have them falling through each other by that time, falsifying the dark-matter based models rigorously.

(B) Astronomers have also discovered that the local Universe expands more rapidly than the distant Universe. This problem, known as the "Hubble Tension", has triggered many concurrent conferences and hugely long texts written by hundreds of scientists in which all possible solutions are being discussed and explained. Very exotic dark-matter-based models are being developed, with additional processes being speculated to act on dark matter (e.g. dark matter could be decaying, there could be dark photons) or that dark energy has some complex time behaviour or multiple dark forms. Impressive is that this vast expert community, that includes or is driven by major-prize-winning scientists, is entirely ignoring the obvious solution to the Hubble Tension: we are in a region spanning more than a billion lightyears across which contains fewer galaxies by about a factor of two than should be there. Galaxies in this void fall towards its sides (like apples falling to the ground) which is why we witness an apparently faster expanding Local Universe. While this "KBC Void" naturally accounts for the Hubble Tension, the KBC Void is entirely incompatible with the dark-matter-based models because these constitute a model universe which is homogeneous and isotropic on scales larger than a few dozen million lightyears.

(C) Another test of the dark matter models is to compare the thickness of galaxies with those observed in the real Universe in which more than 90 per cent of all galaxies are very thin spiral, or disk, galaxies. In the dark matter models galaxies grow over time mostly by merging with other galaxies. These galaxy-crashes typically destroy the thin disks. Our sophisticated analysis of thousands of observed galaxies show the dark matter based models to be totally incompatible with the real Universe, as the model produces galaxies that are typically too roundish compared to the profusive thin disk galaxies in the real Universe. 

Other problems between the real Universe and the dark-matter models include massive galaxies to have been observed at an early time at which they should not yet exist, 

that modern observations tell us there to be dust between galaxies which challenges the interpretation of the cosmic microwave back ground as being the photosphere of the Hot Big Bang and that the cosmic microwave background has features in it that are incompatible with an inflationary origin, suggest that the Universe is structured on all scales (like a fractal perhaps) such that it may be understandable in terms of dust emission rather than a Hot Big Bang.

Three implications arise from the above:

(a) Modern cosmological theory is totally wrong and we need to develop a new theory based on MOND. MOND is a modern non-relativistic theory of gravitation which extends that of Newton by incorporating data from galaxies which were neither available to Newton nor to Einstein, both of whom had to base their deductions on data limited to the Solar System only. All predictions made 40 years ago by Mordehai Milgrom in the foundation papers have been verified, and in Prague and Bonn we (with Nils Wittenburg and Nick Samaras) are now performing the first ever full cosmological calculations with star formation of a MOND universe. 

MOND comes from a simple space-time scale symmetry and may be a consequence of the quantum vacuum, opening a possible path towards unifying gravitation with standard-model particle physics. A major recent review for further in-depth reading has just been published.

Some of the criticisms are familiar, but a few are less commonly discussed. 

The case against dark matter from a lack of evidence of "Chandrasekhar dynamical friction"is a rarely raised by solid and fairly global and robust challenge of a wide variety of dark matter paradigms.

The argument that too many galaxies are thin disks for a LambdaCDM galaxy assembly history to make sense is also a good and rarely raised argument.

The Hubble tension argument is less compelling, because there are so many competing and plausible explanations for it from systemic measurement biases, to overlooked GR effects, to various forms of new physics and it needs to be sorted out and lots of researchers are working in good faith to do that.

The other criticisms are long standing and familiar (and his list omits some of my favorites).

Kroupa's article lacks a bit in not recognizing the importance of having a fundamental foundation for MOND which is a non-relativistic toy model in its original form and which needs to be generalized into a relativistic theory that can be integrated more easily with existing Standard Model and GR physics (which may not even require modification of GR). In truth, there are many ways to create models that largely recreate MOND's results.

It also lacks a bit for failing to recognize that dark matter particle theories are evolving to address their short fallings with more elaborate models, setting up something of a straw man argument.

Greater clarity on what exactly is being falsified would be helpful and is possible to do.

Still, on the merits, for reasons that extend beyond those in the article itself, I do think that he is right that there is no dark matter and the scientific community is too locked into the dark matter paradigm, even though I would make some of those arguments more tightly and would focus on somewhat different conflicts that he does.

I'll close this post by recapping his references, highlighting the most critical ones, such as those that support the stronger points above:

[1] "Galaxies as simple dynamical systems: observational data disfavor dark matter and stochastic star formation" Kroupa, P., 2015CaJPh..93..169K

[2] "Fast galaxy bars continue to challenge standard cosmology" Roshan, M. et al., 2021MNRAS.508..926R

[3] "The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology" Kroupa, P., 2012PASA...29..395K

[4] "Phase-Space Correlations among Systems of Satellite Galaxies" Pawlowski, M., 2021Galax...9...66P

[5] "Are Disks of Satellites Comprised of Tidal Dwarf Galaxies?" Bilek, M., et al., 2021Galax...9..100B

[6] "A massive blow for ΛCDM - the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology" Asencio, E. et al., 2021MNRAS.500.5249A

[7] "The KBC void and Hubble tension contradict ΛCDM on a Gpc scale - Milgromian dynamics as a possible solution" Haslbauer, M. et al., 2020MNRAS.499.2845H

[8] "The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology" Haslbauer, M., et al., 2022ApJ...925..183H

[9] "The Impossibly Early Galaxy Problem" Steinhardt, C.L., et al., 2016ApJ...824...21S

[10] "Universe opacity and CMB" Vavrycuk, V., 2018MNRAS.478..283V

[11] "CMB anomalies after Planck" Schwarz, D., et al., 2016CQGra..33r4001S

[12] "A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis." Milgrom, M., 1983ApJ...270..365M

[13] "A modification of the Newtonian dynamics - Implications for galaxies." Milgrom, M., 1983ApJ...270..371M

[14] "A modification of the newtonian dynamics : implications for galaxy systems." Milgrom, M., 1983ApJ...270..384M

[15] "The Mond Limit from Spacetime Scale Invariance" Milgrom, M., 2009ApJ...698.1630M

[16] "The modified dynamics as a vacuum effect" Milgrom, M., 1999PhLA..253..273M

[17] "From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity" Banik, I. and Zhao, H.S., 2022Symm...14.1331B

[18] "A Philosophical Approach to MOND: Assessing the Milgromian Research Program in Cosmology" Merritt, D., 2020, Cambridge University Press, ISBN: 9781108492690, 2020