I Observations fit poorly with the Standard Model of Cosmology

[Buzz Bloom]

TL;DR Summary

This is a conclusion from a study by the University of Bonn. From Sci-Tech Daily, the following is the headline. Breaking Cosmology: Too Many Disk Galaxies – “A Significant Discrepancy Between Prediction and Reality”

Reference:
https://scitechdaily.com/breaking-c...t-discrepancy-between-prediction-and-reality/

The following are quotes from the reference.

1. The Standard Model of Cosmology describes how the universe came into being according to the view of most physicists. Researchers at the University of Bonn have now studied the evolution of galaxies within this model, finding considerable discrepancies with actual observations. . . . The results have now been published in the Astrophysical Journal.

2. Large discrepancy between prediction and reality
. . .
The calculations are based on the Standard Model of Cosmology; they show which galaxies should have formed by today if this theory were correct. The researchers then compared their results with what is currently probably the most accurate observational data of the real Universe visible from Earth.
“Here we encountered a significant discrepancy between prediction and reality,” Haslbauer says: “There are apparently significantly more flat disk galaxies than can be explained by theory.” However, the resolution of the simulations is limited even on today’s supercomputers. It may therefore be that the number of disk galaxies that would form in the Standard Model of Cosmology has been underestimated. “However, even if we take this effect into account, there remains a serious difference between theory and observation that cannot be remedied”, Haslbauer points out.

Here is The Astrophysics Journal reference:
https://iopscience.iop.org/article/10.3847/1538-4357/ac46ac

 

[PeterDonis]

Large discrepancy between prediction and reality

I think there is quite a bit that this particular paper might be leaving out. (From what I can tell on a quick skim, the paper's authors favor MOND, so what they say should be taken with that in mind.) Any calculation of what kinds of galaxies we should expect to see at particular redshifts must depend on some model of how galaxies form; however sophisticated the computer models of $\Lambda CDM$ are, I doubt they are good enough to calculate the details of galaxy evolution solely using fine-grained dynamics from initial conditions soon after the Big Bang. I would assume that these computer models are more coarse-grained than that and contain assumptions regarding how galaxies will evolve as gravitational clumping occurs. But such assumptions could be changed without changing the overall framework of the $\Lambda CDM$ model.

 

[kimbyd]

I think there is quite a bit that this particular paper might be leaving out. (From what I can tell on a quick skim, the paper's authors favor MOND, so what they say should be taken with that in mind.) Any calculation of what kinds of galaxies we should expect to see at particular redshifts must depend on some model of how galaxies form; however sophisticated the computer models of $\Lambda CDM$ are, I doubt they are good enough to calculate the details of galaxy evolution solely using fine-grained dynamics from initial conditions soon after the Big Bang. I would assume that these computer models are more coarse-grained than that and contain assumptions regarding how galaxies will evolve as gravitational clumping occurs. But such assumptions could be changed without changing the overall framework of the $\Lambda CDM$ model.

This is my general feeling as well. The number of disk galaxies is highly dependent upon the dynamics of normal matter, and normal matter is monstrously complicated. Effects like the brightness of the accretion disk of a galaxy's central black hole and the number of supernovae in the galaxy can have dramatic impacts on the dynamics of the galaxies. Neither effect can be simulated using N-body simulations.

 

[ohwilleke]

The problem with ΛCDM are myriad and involve many independent issues anyone of which is a serious blow to the model. Collectively, they robustly demonstrate that the "Standard Model of Cosmology" which is the current paradigm (as much as anything because no consensus alternative has been developed), is deeply flawed.

A good place to begin is with this review article:

The dark energy plus cold dark matter (ΛCDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of Universe and its evolution with time. Yet on length scales smaller than ∼1 Mpc and mass scales smaller than ∼10¹¹M⊙, the theory faces a number of challenges. For example, the observed cores of many dark-matter dominated galaxies are both less dense and less cuspy than naively predicted in ΛCDM. The number of small galaxies and dwarf satellites in the Local Group is also far below the predicted count of low-mass dark matter halos and subhalos within similar volumes. These issues underlie the most well-documented problems with ΛCDM: Cusp/Core, Missing Satellites, and Too-Big-to-Fail. The key question is whether a better understanding of baryon physics, dark matter physics, or both will be required to meet these challenges. Other anomalies, including the observed planar and orbital configurations of Local Group satellites and the tight baryonic/dark matter scaling relations obeyed by the galaxy population, have been less thoroughly explored in the context of ΛCDM theory. . . .

James S. Bullock, Michael Boylan-Kolchin, "Small-Scale Challenges to the ΛCDM Paradigm" (July 13, 2017, last updated September 2, 2019) arXiv 1707.04256

See also:

We present rotation curve fits to 175 late-type galaxies from the Spitzer Photometry & Accurate Rotation Curves (SPARC) database using seven dark matter (DM) halo profiles: pseudo-isothermal (pISO), Burkert, Navarro-Frenk-White (NFW), Einasto, DC14, cored-NFW, and a new semi-empirical profile named Lucky13. We marginalize over stellar mass-to-light ratio, galaxy distance, disk inclination, halo concentration and halo mass (and an additional shape parameter for Einasto) using a Markov Chain Monte Carlo method. We find that cored halo models such as the DC14 and Burkert profiles generally provide better fits to rotation curves than the cuspy NFW profile. The stellar mass-halo mass relation from abundance matching is recovered by all halo profiles once imposed as a Bayesian prior, whereas the halo mass-concentration relation is not reproduced in detail by any halo model. We provide an extensive set of figures as well as best-fit parameters in machine-readable tables to facilitate model comparison and the exploration of DM halo properties.

Pengfei Li, Federico Lelli, Stacy McGaugh, James Schombert, "A comprehensive catalog of dark matter halo models for SPARC galaxies" (January 28, 2020). arXiv 2001.10538

"Here we present a new challenge for CDM by showing that some of Milky Way's satellites are too dense, requiring the formation masses and redshifts of halos in CDM not compatible with being a satellite." Mohammadtaher Safarzadeh, Abraham Loeb "A New Challenge for Dark Matter Models" arXiv:2017.03478 (July 7, 2021).

"Simulations predict dark-matter dominated systems with stellar-to-total enclosed mass ratios that are a factor of 1.5-2 smaller than real galaxies at all radii. This is an alternative manifestation of the `failed feedback problem'" (Marasco 2020).

Other phenomena that are hard to explain with the ΛCDM model are (1) dwarf galaxies with almost no apparent dark matter, (2) the behavior of wide binary star systems in which the stars appear to be bound by a gravitational force stronger than predicted by general relativity (also here), (3) the fact that inferred dark matter halo shapes differ from the theoretically predicted NFW distribution (see, e.g., Bullock (2017) and Li (2020), quoted above), (4) the fact that inferred dark matter distributions are tightly linked in a predictable manner to observed ordinary matter distributions even at a fine scaled level that is basically impossible to explain with truly collisionless dark matter particles that interact exclusively through gravity in a manner prescribed by General Relativity; this is also the case in galaxy clusters; (5) the systemic variation in the amount of apparent dark matter in elliptical galaxies, or why spiral galaxies have smaller proportions of ordinary matter than elliptical galaxies in same sized inferred dark matter halos, or why thick spiral galaxies have more inferred dark matter than thin ones or why the number of satellite galaxies is related to budge mass in spiral galaxies; (6) deficits of X-ray emissions in low surface brightness galaxies; (7) it predicts too few galaxy clusters; and (8) it gets globular cluster formation wrong (see also here).

Dark matter can't explain bulge formation in galaxies: Alyson M. Brooks, Charlotte R. Christensen, "Bulge Formation via Mergers in Cosmological Simulations" (12 Nov 2015). Conversely, "There is a significant deficit of intrinsically thin disk galaxies, which however comprise most of the locally observed galaxy population." Moritz Haslbauer, Indranil Banik, Pavel Kroupa, Nils Wittenburg, Behnam Javanmardi, "The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology" arXiv:2202.01221 (February 2, 2022) (published at 925 ApJ, 183 (2022) DOI: 10.3847/1538-4357/ac46ac.

void galaxies in the observation sample seem to have generally larger mean-distance than simulated ones at any given void size. In addition, observed void galaxies tend to reside closer to the void center than those in the simulation. This discrepancy is also shown in the density profile of voids. Regardless of the void size, the central densities of real void profiles are higher than the ones in the predicted simulated catalog.

Saeed Tavasoli, "Void Galaxy Distribution: A Challenge for ΛCDM" arXiv:2109.10369 (September 21, 2021) (Accepted in ApJ Letter) DOI: 10.3847/2041-8213/ac1357.

The LambdaCDM model of cosmology predicts a characteristic scaling relationship between galactic cluster size and velocity dispersion. Reality doesn't agree. The power law should have an exponent of three in the LambaCDM model. The power law actually differs from that by more than four sigma at 4 ± 0.1. Yong Tian, Han Cheng, Stacy S. McGaugh, Chung-Ming Ko, Yun-Hsin Hsu "Mass-Velocity Dispersion Relation in MaNGA Brightest Cluster Galaxies" arXiv:2108.08980 (August 20, 2021) (published in 24 The Astrophysical Journal Letters 917).

There is also evidence of an "external field effect" that is too strong to be explained by tidal forces found in the ΛCDM model.

There are also some issues with the ΛCDM model when compared with observational data at the larger cosmology scale. Most notably, (1) galaxies form too quickly (the impossible early galaxies problem), (2) 21cm wavelength radiation data that demonstrates the temperature of the universe at 180 million to 280 million years after the Big Bang, seems inconsistent with the model because the universe was much colder than predicted and is instead consistent with a no dark matter hypothesis, (3) the velocity of colliding galaxy clusters is also too often higher than it predicts, and (4) the ΛCDM model predicts that the most massive galaxies should be several hundred times more common than they are and it is harder for astronomers to miss hundreds of huge galaxies than it is for them to miss large numbers of tiny ones.

And another cosmology scale problem is that: "[T]he direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a p-value of 10^{-4} (3.9 sigma), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance LambdaCDM model." (Secrest 2020).

A January 13, 2021 article in issue 358 of BBC Science Focus Magazine entitled "The Cracks in Cosmology: Why Our Universe Doesn't Add Up?" by Marcus Chown identifies three notable recent flaws in this model in observations newly made in 2020. First, he points to the gravitational lensing of subhalos in galactic clusters recently observed to be much more compact and less "puffy" than LambdaCDM would predict. Secondly, he points to a KIDS telescope observation of very large scale structure which shows it to be 8.3% smoother (i.e. less clumpy) than predicted by LambdaCDM. Third, he points to the Hubble tension (see, e.g., here) that shows that Hubble's constant, which is a measure of the expansion rate of the universe, is about 10% smaller when measured via cosmic microwave background radiation (with a small margin of error) than when measured by a wide variety of measures at times much more removed from the Big Bang that the time at which the cosmic microwave background came into being.

 

[PeroK]

Other phenomena that are hard to explain with the ΛCDM model are (1) dwarf galaxies with almost no apparent dark matter,

This strikes me as a very specific anomaly that, at first sight, may have causes other than a fundamental problem with the large-scale cosmological model. The question is whether this is more of a problem for MOND and rival theories? Are the dynamics of this galaxy compatible with GR and no local dark matter? If so, then we only need an explanation for how it lost its dark matter. But, if the dynamics of this galaxy are incompatible with MOND, then that maye be genuinely inexplicable.

 

[ohwilleke]

This strikes me as a very specific anomaly that, at first sight, may have causes other than a fundamental problem with the large-scale cosmological model. The question is whether this is more of a problem for MOND and rival theories? Are the dynamics of this galaxy compatible with GR and no local dark matter? If so, then we only need an explanation for how it lost its dark matter. But, if the dynamics of this galaxy are incompatible with MOND, then that maye be genuinely inexplicable.

First of all, it is entirely possible that an observation that is a problem for CDM is also a problem for other models. I am not trying to compare or rank different approaches, just trying to address observational evidence challenges to the prevailing CDM paradigm.

In MOND, it is explained if the external field effect applies and can explain this result in some cases. Basically, the presence of a gravitational field from a nearby massive object that includes a galaxy that would otherwise exhibit dark matter phenomena in MOND and some other modified gravity theories, do not show those effects if the external gravitational field combined with the local gravitational field exceed the MOND threshold. To quote Stacy McGaugh:

The importance of the EFE in dwarf satellite galaxies is well documented. It was essential to the a priori prediction of the velocity dispersion in Crater 2 (where MOND correctly anticipated a velocity dispersion of just 2 km/s where the conventional expectation with dark matter was more like 17 km/s) and to the correct prediction of that for NGC 1052-DF2 (13 rather than 20 km/s). Indeed, one can see the difference between isolated and EFE cases in matched pairs of dwarfs satellites of Andromeda. Andromeda has enough satellites that one can pick out otherwise indistinguishable dwarfs where one happens to be subject to the EFE while its twin is practically isolated. The speeds of stars in the dwarfs affected by the EFE are consistently lower, as predicted. For example, the relatively isolated dwarf satellite of Andromeda known as And XXVIII has a velocity dispersion of 5 km/s, while its near twin And XVII (which has very nearly the same luminosity and size) is affected by the EFE and consequently has a velocity dispersion of only 3 km/s.

In some circumstances, a galaxy with less than the expected amount of dark matter could be explained in CDM theories with tidal stripping effects. But this is more challenging to do. It is a problem for CDM theories because galaxies without dark matter shouldn't form in the first place. Yet, it is quite difficult to devise circumstances where it would have enough DM to form, but would lose DM but not stars, particularly while remaining gas dominated (as all "no dark matter" candidates galaxies to date have been). Electromagnetically neutral atoms or molecules of interstellar gas should be affected by gravitational tidal stripping in a manner similar to dark matter.

Ultra-diffuse galaxy AGC 114905 is the only case to date of what has been claimed to be a dark matter phenomena free galaxy that is isolated from a body that could give rise to an external field effect (there are have a couple of other no dark matter galaxies discussed in the quote above, but only in places where an EFE could be present). But increasingly, AGC 114905 is looking like it was a case of systemic error in the measurement of its inclination angle of the observed disk-like galaxy. This error appears to have distorted efforts to characterize its dynamics and made it only look like it lacked dark matter phenomena.

For example, a recent article to be published in the same journal where the original claim was made noted that if the original claim was true that the galaxy would be wildly unstable contrary to other indicators that suggest it is a stable system that has existed for a long time. See J. A. Sellwood and R. H. Sanders, "The ultra-diffuse galaxy AGC 114905 needs dark matter" arXiv:2202.08678 (February 17, 2022) (submitted to MNRAS) https://doi.org/10.48550/arXiv.2202.08678

Finally, I'll concede that it is possible that upon further investigation one or more of the observational issues with CDM may prove to have other explanations, although I have no particular reason to doubt anyone of them at this time. But since many of these observational evidence challenges to CDM are independent of each other in terms of the source evidence that supports them (ruling out instrumental type systemic errors or fluke random alignments of bodies that create a misleading impression, for example) and in terms of the conceptual problems presented, the overall conclusion that CDM is deeply flawed is quite robust.

It is no secret that I do have a preferred theory, backed by peer reviewed published work. But I am not trying to advocate for that theory, or any other particular theory, in this thread.

 

[PeroK]

the overall conclusion that CDM is deeply flawed is quite robust.

I think that's far too strong. CDM is an unproven hypothesis. There's a world of difference. It's not credible to me that so many mainstream cosmologists would waste their time with a theory that is patently unworkable.

It is no secret that I do have a preferred theory, backed by peer reviewed published work. But I am not trying to advocate for that theory, or any other particular theory, in this thread.

MOND, is another unproven hypothesis. You may not have advocated this theory directly, but your posts here are hardly an unbiased summary of modern cosmology.

I don't have to be an expert in the field to realize that any galaxy with little or no dark matter (in the CDM model) is a serious problem for MOND that then requires an additional parameter. See, for example:

https://arxiv.org/abs/1912.08592

My (biased) amateur assessment is that if tomorrow CDM was dropped and MOND became the predominant theory then the cracks in MOND, re its inability to explain everything in modern cosmology, would soon open up; and "deeply flawed" would be an understatement.

 

[ohwilleke]

A new CDM problem raised in a paper accepted a MINRAS:

Examining a catalogue of isolated galaxy pairs, a preferred orbital intervelocity of ~150 km/s was recently reported. This discovery is difficult to reconcile with the expectations from Newtonian numerical simulations of cosmological structure formations.

 

[PeterDonis]

A new CDM problem raised in a paper accepted a MINRAS:

As I pointed out in post #2, this might just mean that we shouldn't place too much faith in numerical simulations of structure formation. Any such simulation must involve many assumptions that could be varied without changing the basic structure of the Lambda CDM model. Also, I'm confused about the "Newtonian" part, since structure formation takes place in an expanding universe and the effects of that expansion, i.e., of curved spacetime geometry that cannot be captured in a Newtonian approximation, would be expected to be significant.

 

[ohwilleke]

I think that's far too strong. CDM is an unproven hypothesis. There's a world of difference. It's not credible to me that so many mainstream cosmologists would waste their time with a theory that is patently unworkable.

CDM is a spherical cows theory. It is a very crude approximation of reality that gets a few general trends (and the CMB spectrum) right (which has fewer moving parts and hence is less impressive than it seems), but it gets almost all of the details wrong to the point which it is very unpredictive.

Also, your faith that mainstream physicists won't work with a patently unworkable theory is ill founded. There are plenty of mainstream physicists working on technicolor (a composite Higgs boson theory discredited when the Higgs boson was discovered), plenty who are convinced that supersymmetry is just around the corner, working on string theory that has zero experimental support, etc.

Focusing in -

I don't know how many papers I've seen recently that are convinced the GeV scale supersymmetric WIMP dark matter continues to be a well motivated theory, usually citing to really old papers in their introductions or to more recent papers citing those really old papers, when direct detection experiments and collider experiments have pretty much completely ruled it out.

It is a straightforward conclusion that can be calculated analytically that truly collisionless dark matter particles form dark matter halos of a particular shape, called the NFW distribution after the initials of the authors of the paper that first did the calculation. Probably two-thirds or three-quarters of CDM papers assume the NFW distribution for dark matter halos. But, almost every single paper analyzing galaxy and galaxy cluster dynamics and lensing with the observational power to do so, for at least a decade, has found that NFW distributions for dark matter halos do not remotely reflect reality. If you are trying to infer a dark matter distribution this isn't what you get. Part of it is lazy. An NFW distribution can be calculated analytically, is easy to work with, and you can pretty much cut and past that part of your analysis from lots of other published papers.

But it also reflects the fact that astronomers and astrophysicists don't read each other's work and don't have a good grasp of the overall literature in their field, except their narrow, narrow, sub-sub-field. There are also canonical dark matter particle distributions that do a much better job of fitting the empirical evidence of how dark matter particles are distributed, even though few of the other distributions are anything more than phenomenological fits without a whole lot of theoretical motivation. The other canonical dark matter halo shapes are also easy enough to work with and easy to cut and paste analysis from based upon similar papers using them. But most papers that use NFW distributions either don't acknowledge the problem with doing that or incorrectly assert that the observational evidence supports the notion that dark matter has an NFW distribution.

Needless to say, these mainstream astrophysicists who can't even be bothered to take seriously the strong evidence that NFW distributions don't work, have virtually no awareness of understanding of the modified gravity theory alternatives or even of significantly different dark matter particle alternatives. They've heard of MOND but don't really understand its subtleties or its limitations. They also think that MOND is the only modified gravity theory out there when this simply isn't the case.

This said, the amount of work done with simple cold dark matter models of the kind assumed in LambdaCDM has fallen off a lot. Recognizing the utter failure to LambdaCDM to get small scale structure (i.e. galaxy and galaxy cluster scale) right, self-interacting dark matter (SIDM) models that add an additional Yukawa force between dark matter particles, warm dark matter and axion-like dark matter models (that rely on quantum effects arising from their very small masses to solve some of the problems), and models like millicharged dark matter (that adds a force with a strength between the weak force and gravity to address the fact that dark matter haloes that are inferred are highly correlated with ordinary matter distributions beyond what gravity could produce) have received far more attention now.

The only reason the LambdaCDM is still the paradigm is because no one has done enough work in another theory to blow it away on all fronts. It is a weak king that still remains on the throne because no one has managed to gain enough supports to topple it, and because a lot of mainstream senior scholars locked into the theory when the evidence against it hadn't been developed yet and can't easily transition to another radically different approach.

MOND, is another unproven hypothesis. You may not have advocated this theory directly, but your posts here are hardly an unbiased summary of modern cosmology.

First of all, MOND is a phenomenological toy model with a range of applicability limited to weak fields up to the galaxy scale.

Second of all, as I've made very clear I am not advocating for MOND per se, and everyone, including its advocates don't believe that it is a complete rigorous description of the fundamental physics of gravity.

Third, dismissing it as an "unproven" hypothesis is far off the mark. MOND has made far, far more correct ex ante predictions in its domain of applicability, using very large data sets, than any CDM theory, in a wide variety of contexts. CDM in not very predictive and when it does make predictions is frequently and unpredictably wrong.

In terms of number of well proven accurate predictions relative to magnitude of changes from GR/SM MOND is much better by far than any other contender, despite being admittedly incomplete and lacking a firm theoretical basis. Generalizations of MOND even replicate the CMB patterns and get the time of galaxy formation and the 21cm EDGES results right. It gets the 2D spatial alignment issue right. It gets the relative velocity of isolated galaxy pairs right. It gets the dark matter free galaxies that are subject to the external field effect prediction right. It got the dark matter phenomena dominance of isolated dwarf galaxies right. It predicts the rotation curves of virtually all galaxies from tiny to huge right with a single fixed parameter. It gets the link between baryonic matter distributions and dark matter effects right. And, it is doing the vast majority of that with theory set forth in a short paper in 1983.

But its proponents do read the DM literature. They do acknowledge that simple toy-model MOND underestimates dark matter effects in galaxy clusters.

What MOND is to people who view it correctly is a compact summary of a lot of phenomena that isn't all that hard to replicate and build upon in a relativistic theoretically well grounded theory, of which there are maybe half a dozen or a dozen whose tires have been kicked at little to make sure that they work. And, some of those relativistic generalizations fail some of the time, but none of the fail in so many ways, so badly, a simple LambdaCDM.

Similarly, while MOND doesn't explain the Bullet Cluster, several gravity based theories that replicate MOND's successes do, and CDM is also challenged by the Bullet Cluster.

I don't have to be an expert in the field to realize that any galaxy with little or no dark matter (in the CDM model) is a serious problem for MOND that then requires an additional parameter. See, for example:

https://arxiv.org/abs/1912.08592

Only to the extent that the external field effect isn't present. As noted above, it is present in multiple analyzed cases and the remaining one is probably a methodological error. The paper linked, for example, considers NGC 1052-DF2 but doesn't look at MOND at all and doesn't consider the possibility of an external field effect which actually does explain that particular galaxy.

There isn't a single contradictory result yet.
https://arxiv.org/abs/1912.08592

My (biased) amateur assessment is that if tomorrow CDM was dropped and MOND became the predominant theory then the cracks in MOND, re its inability to explain everything in modern cosmology, would soon open up; and "deeply flawed" would be an understatement.

Less than you would think, but nobody is trying to do that. There are modified gravity theories and more elaborate DM particle theories inconsistent with the LambdaCDM model that do much better.

 

[ohwilleke]

I'm confused about the "Newtonian" part, since structure formation takes place in an expanding universe and the effects of that expansion, i.e., of curved spacetime geometry that cannot be captured in a Newtonian approximation, would be expected to be significant.

The issue isn't really structure formation. The issue is that at the scale of two galaxies, a scale at which LambdaCDM is using Newtonian approximations and which is in a regime when GR curved spacetime effects aren't supposed to be significant, you see an effect that no DM theory of any kind predicted.

It is worth observing, as you note, that even if you have a dark matter particle with specified properties, your DM particle theory still isn't complete. It needs a structure formation theory to go with it and explain how the particles ended up being arranged the way that they have. But there is really nothing out there in structure formation theory to explain this result and it doesn't show up in any simulations done or analytical predictions, even though it is manifestly expected and predicted decades in advance in MOND - despite its incompleteness and failure to consider relativistic effects.

 

[PeterDonis]

CDM is a spherical cows theory.

Personal opinions are off limits here. We understand that you don't like the CDM model, but please stick to giving references and making statements based on what they say.

 

[PeterDonis]

The issue isn't really structure formation.

It is according to the paper you referenced. The paper explicitly says that the observations it describes are hard to reconcile with Newtonian numerical simulations of structure formation.

 

[PeroK]

There's a world of difference between "CDM is an unproven hypothesis that may turn out to be wrong". Which I suspect most professional cosmologists would accept. And, "CDM is an unworkable theory that can't possibly be right", which I understand is your point of view.

Your antagonism towards CDM has, I suggest, very little rational basis.

 

[ohwilleke]

There's a world of difference between "CDM is an unproven hypothesis that may turn out to be wrong". Which I suspect most professional cosmologists would accept. And, "CDM is an unworkable theory that can't possibly be right", which I understand is your point of view.

Your antagonism towards CDM has, I suggest, very little rational basis.

To be clear, there are potential dark matter particle theories that could be right. But the CDM model is a very specific subset of those theories that proposes a single type of dark matter particle that is collisionless, has interactions with ordinary matter by means other than gravity no stronger than the weak force, is constant in amount from shortly after the Big Bang, and has a characteristic range of mean velocities.

The nice thing about this is that it is pretty easy to model. The bad thing about it is that there are lots and lots of specific observations, cited in my post, the contradict what this straightforward model predicts. It gets the formation time frame for galaxies wrong. It gets the average temperature of the universe at cosmic dawn wrong, it gets the halo mass function wrong, it gets the observed shape of inferred dark matter halos wrong, it get the relative velocities of isolated binary galaxies wrong, it gets the mix of galaxy types wrong, it contradicts the prediction of 2D alignment of galaxy systems wrong, it gets wide binary stars wrong, it gets the circumstances under which galaxies can be expected to show no dark matter effects wrong, it gets the distribution of galaxy clusters velocities relative to each other wrong, it gets the tightness of the correlations between baryonic matter distributions and inferred dark matter distributions wrong, etc.

 

[PeterDonis]

the CDM model is a very specific subset of those theories that proposes a single type of dark matter particle that is collisionless, has interactions with ordinary matter by means other than gravity no stronger than the weak force, is constant in amount from shortly after the Big Bang, and has a characteristic range of mean velocities.

More precisely, the CDM model that cosmologists are currently using has these properties. But that does not mean cosmologists are claiming that this model is right, just that it is the simplest and easiest to use model and so they are using it while research continues into the question of whether a more complex model might eventually be required. Saying that a more complex model might be required, which is the most that you can actually say based on the observations you mentioned, is a much, much weaker claim than the claim you are trying to make, which is that the whole idea of dark matter must be wrong and we need to switch to a completely different theoretical framework like MOND.

 

[ohwilleke]

Saying that a more complex model might be required, which is the most that you can actually say based on the observations you mentioned, is a much, much weaker claim than the claim you are trying to make, which is that the whole idea of dark matter must be wrong and we need to switch to a completely different theoretical framework like MOND.

I am not making nearly so expansive a claim. I am merely claiming that the LambdaCDM model with its six or seven parameters and footnoted assumptions about what CDM means in the context of that model is wrong, not that the whole idea of dark matter, in general, has been disproven. This very specific model has not been nearly as successful as it widely claimed when you consider the evidence contrary to this specific model, as well as the evidence supporting this specific model.

For examples of some of the main deviations from the LambdaCDM model that have been proposed and considered seriously by dark matter particle theorists, such as warm dark matter, self-interacting dark matter, wave-like dark matter (including axion-like particles), and phase changing dark matter, see this Snowmass 2021 paper on "Astrophysical and Cosmological Probes of Dark Matter".

Unfortunately, this very specific narrow sense LambdaCDM model aka the Standard Model of Cosmology, is still receiving wide use with a widespread lack of acknowledgment by many cosmologists of its accumulating problems, as much out of lack of a good understanding of the implications of the larger literature and a lack of desire to interrupt their momentum as anything, which is bad science, but a sociological reality.

I personally don't think that the idea of dark matter is correct, but that is neither here nor there and I don't claim that anyone should believe something based merely on my own gut instincts.

I am only arguing in this thread that the simple model of dark matter in LambdaCDM is not correct. A more complicated model of dark matter (e.g. some form of self-interacting dark matter, or a form of dark matter that interacts with ordinary matter via a fifth force) could conceivably fit the data.

This is still quite a big change in theoretical framework from LambdaCDM. But I'm not claiming or insisting that a modified gravity solution is the only possible solution, although I do insist and claim that some form of gravitational law solution is a possible solution which hasn't been ruled out as many LambdaCDM proponents claim and sincerely believe in good faith, even though the points that they rely on for that belief aren't correct.

For a long time, cosmologists dismissed these flaws as "small scale" problems that can be resolved with a better model of galaxy formation. But now that the cosmology scale problems of LambdaCDM are becoming apparent, and the galaxy formation models are becoming more realistic, and the observations continue to grow exponentially in volume, this story no longer works.

One of the reasons to care about the distinction between the narrow form LambdaCDM Standard Model of Cosmology and the more involved forms that generally either have multiple kinds of DM, or different phases of DM (like the the generalized Chaplygin gas model), or non-SM forces in addition to DM particles (like SIDM or millicharged dark matter), is that this, at a minimum, eliminates the Occam's Razor preference for a very basic LambdaCDM model of DM over a modified gravity solution, since adding new DM sector specific forces, for example, is equivalent in model complexity, to modifying gravity. It puts all of the possibilities that haven't been ruled out on a level playing field.

 

[Davephaelon]

The one detail of galactic dynamics that strikes me as either very difficult, or impossible, to explain with LambdaCDM is Renzo's Rule, where 'bumps' in the rotation curve velocity correlate with the luminosity at that particular radius from the galaxy's center. A vast halo of putative dark matter would presumably wash out such fine details. But I suppose it is possible that this apparent difficulty for the Concordance Model has been addressed in some paper that I am not aware of.

 

[Drakkith]

Third, dismissing it as an "unproven" hypothesis is far off the mark. MOND has made far, far more correct ex ante predictions in its domain of applicability, using very large data sets, than any CDM theory, in a wide variety of contexts. CDM in not very predictive and when it does make predictions is frequently and unpredictably wrong.

I have a difficult time believe this. Do you have a reference supporting this?

 

[PeterDonis]

Renzo's Rule

I have only seen this rule claimed in papers that support MOND. I would want to see some indication from cosmologists who are not MOND proponents that this "rule" is actually a valid feature of the data before putting too much weight on it.

 

[Davephaelon]

Peter Donis:

I don't see any reason to question the accuracy of charts that illustrate Renzo's Rule in papers written by MOND proponents. I assume those charts are based on raw data that is available to everyone. Perhaps those who are not MOND proponents have simply ignored this correlation.

 

[PeterDonis]

I don't see any reason to question the accuracy of charts that illustrate Renzo's Rule in papers written by MOND proponents.

That's not quite the issue I'm raising. Taken at face value, Renzo's rule says that the galaxy rotation curves exactly follow the distribution of visible matter. But if that were actually true, not only would we not need dark matter as a hypothesis, we would not need MOND either! Dark matter and MOND both started out as hypotheses to explain observed discrepancies between galaxy rotation curves and the distribution of visible matter.

So the fact that I see MOND proponents arguing that Renzo's rule is a reason to disbelieve the dark matter hypothesis looks odd to me, and without some kind of countervailing input from the other side of the issue, I am very skeptical about putting much weight on it.

 

[Davephaelon]

"But if that were actually true, not only would we not need dark matter as a hypothesis, we would not need MOND either!"

I got a little confused about how to use the quote feature, so just did it manually. My understanding from looking at various charts is that Renzo's Rule is superimposed on the existing non-Newtonian portion of the velocity curves of galaxies (and maybe the normal curve area as well, but everything is too squished together to see detail). So you would still need something, either DM, or modification of gravity, to account for the higher-than-expected velocity profiles of stars in the outer portions of galaxies.

 

[PeterDonis]

My understanding from looking at various charts is that Renzo's Rule is superimposed on the existing non-Newtonian portion of the velocity curves of galaxies (and maybe the normal curve area as well, but everything is too squished together to see detail). So you would still need something, either DM, or modification of gravity, to account for the higher-than-expected velocity profiles of stars in the outer portions of galaxies.

If that's the case, then Renzo's rule gives us no help in deciding whether DM or a modification of gravity is a better explanation. So again I find it odd that only one side of the debate (MOND) is talking about the rule. Without countervailing input from the other side of the debate, I am skeptical about putting much weight on it.

 

[PeterDonis]

Taken at face value, Renzo's rule says that the galaxy rotation curves exactly follow the distribution of visible matter.

To be clear, it's not me saying this: the proponents of MOND are saying this, and they're saying it as an argument against the DM hypothesis. But they don't talk at all about the fact that it's an argument against the MOND hypothesis to the same extent (to whatever extent the rotation curves match the distribution of visible matter). This is what seems odd to me and makes me want to see input from the other side of the debate.

 

[PeroK]

To be clear, it's not me saying this: the proponents of MOND are saying this, and they're saying it as an argument against the DM hypothesis. But they don't talk at all about the fact that it's an argument against the MOND hypothesis to the same extent (to whatever extent the rotation curves match the distribution of visible matter). This is what seems odd to me and makes me want to see input from the other side of the debate.

I couldn't find much on Renzo's rule outside papers on MOND. I'm not sure to what extent it's generally accepted. The Wikipedia page seems to present a balanced view on MOND. For example, re Renzo's rule it says:

"Milgrom's law fully specifies the rotation curve of a galaxy given only the distribution of its baryonic mass. In particular, MOND predicts a far stronger correlation between features in the baryonic mass distribution and features in the rotation curve than does the dark matter hypothesis (since dark matter dominates the galaxy's mass budget and is conventionally assumed not to closely track the distribution of baryons). Such a tight correlation is claimed to be observed in several spiral galaxies, a fact which has been referred to as "Renzo's rule"

The page also summarises the main objections to MOND.

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

 

[PeterDonis]

MOND predicts a far stronger correlation between features in the baryonic mass distribution and features in the rotation curve than does the dark matter hypothesis

While this is true, at least for the usual kind of dark matter hypothesis, it is also true that a straight standard gravity model of a galaxy (i.e., standard GR or its Newtonian approximation) predicts the same kind of strong correlation here that MOND does, since the rotation curve in both cases is determined solely by the distribution of visible matter. So this correlation by itself is not evidence for MOND. It could be evidence against DM, or at least against the usual kind of DM hypothesis, if it were validated.

 

[Davephaelon]

Well, as I said the actual data shows a correlation between luminosity and rotational velocity piggybacking on top of the anomalously high velocity of stars, once a point is reached where the intrinsic acceleration drops to MOND's characteristic accelerations scale a0. I can't imagine that its a matter of who presents the data. I too would like to see LCDM proponents/experts weigh in on this. But I can't see how they would not see the correlation.

Now LCDM has some really good feathers in its cap, like the CMB power spectrum, galaxy cluster rotation curves, and especially the Bullet Cluster. Non of these, to my knowledge, can be explained by basic MOND, and as you may have already guessed I'm a fan of MOND. This is disappointing. I will admit that the Bullet Cluster is a hard nut to crack in anything but LCDM. In LCDM it comes out quite naturally - the two clusters long ago collided, the clusters sailing easily through the collision zone carrying their Dark Matter with them, while their respective gas clouds experienced a proverbial mash up heating them to very high temperature and leaving them separated from their former hosts.

There are some details in this Bullet Cluster evolution scenario that I am uncertain about such as whether the Dark Matter was only associated with the individual galaxies, or some of it was distributed throughout the cluster and even a bit beyond the cluster. Then there is the lensing data illustrated by jagged, concentric rings enveloping the two individual clusters. I assume that each 'jag' represents a single lensed background object, and the straight lines join these together. I also don't know if anyone has ever calculated the actual total baryonic-plus-DM mass for each cluster, or the masses of the left-behind gas clouds. I did do some preliminary searches on google,

 

[PeterDonis]

the actual data shows a correlation between luminosity and rotational velocity

That's basically what "Renzo's rule" is, yes.

piggybacking on top of the anomalously high velocity of stars

I have not seen this claimed in discussions of Renzo's rule. Those discussions only mention the correlations described above. Note that there are galaxies that do not have anomalous rotation curves to begin with. I have not seen any discussion in the literature (but of course there is a lot of literature I have not read) about whether the "Renzo's rule" correlations are mainly seen in those galaxies, or whether they are also seen in galaxies that do have anomalous rotation curves.

once a point is reached where the intrinsic acceleration drops to MOND's characteristic accelerations scale a0.

As I understand the "Renzo's rule" observations, this is false: the correlations in question are throughout the entire rotation curve, not just the outer region where the acceleration drops below the MOND threshold.

 

[PeterDonis]

I can't imagine that its a matter of who presents the data.

I would rather not try to "imagine" at all. I would rather see people on both sides of the debate weigh in and give their observations and arguments. All kinds of things can go sideways when we're talking about data that's difficult to collect in the first place and that has a lot of possible sources of error.

Perhaps an example to compare with is the OPERA experiments a few years ago that appeared to show neutrinos going faster than light. The actual explanation eventually turned out to be a very subtle kind of experimental error; it took, IIRC, months of discussion among various experts in various fields to find it. During all that time, people all over the Internet were "imagining" things, all of which turned out to be irrelevant. Of course nobody could imagine at the outset how the data could have been in error--that was why the OPERA team reached out to the scientific community in the first place, because they knew how extraordinary actual FTL neutrinos would be but they also knew that there were lots of possible ways that they couldn't imagine that there could be some kind of error in the results, and they wanted to make sure.

 

[Davephaelon]

On your point 1 no problem, agreed. On point 2, I was just going by the visual appearance of the charts that I've seen. On point 3, I actually wasn't certain if the rule applied to the Newtonian region, so that was my mistaken assumption.

 

[ohwilleke]

I have a difficult time believe this. Do you have a reference supporting this?

Re a priori predictions, see, e.g., http://astroweb.case.edu/ssm/papers/GalMONDreview.pdf

Stacy McGaugh, "Predictions and Outcomes for the Dynamics of Rotating Galaxies" (review, 24 pages, 218 references)

A review is given of a priori predictions made for the dynamics of rotating galaxies. One theory — MOND — has had many of its predictions corroborated by subsequent observation. While it is sometimes possible to offer post hoc explanations for these observations in terms of dark matter, it is seldom possible to use dark matter to predict the same phenomena.

 

[ohwilleke]

I have only seen this rule claimed in papers that support MOND. I would want to see some indication from cosmologists who are not MOND proponents that this "rule" is actually a valid feature of the data before putting too much weight on it.

FWIW, Renzo's rule was not from a MOND paper. It was from: Dark Matter in Galaxies; Ryder, S.; Pisano, D.;. Walker, M.; Freeman, K., Eds., 2004, Vol. 220, IAU Symposium, p. 233, [astro-ph/0311348]. The paper is:

https://arxiv.org/abs/astro-ph/0311348

[Submitted on 14 Nov 2003]

The visible matter - dark matter coupling​

Renzo Sancisi (INAF - Osservatorio Astronomico di Bologna, Italy; Kapteyn Astronomical Institute, The Netherlands)

In the inner parts of spiral galaxies, of high or low surface brightness, there is a close correlation between rotation curve shape and light distribution. For any feature in the luminosity profile there is a corresponding feature in the rotation curve and vice versa. This implies that the gravitational potential is strongly correlated with the distribution of luminosity: either the luminous mass dominates or there is a close coupling between luminous and dark matter. In a similar way, the declining rotation curves observed in the outer parts of high luminosity systems are a clear signature of the stellar disk which either dominates or traces the distribution of mass.

The notion that the baryons are dynamically important in the centres of galaxies, including LSBs, undermines the whole controversy over the cusps in CDM halos and the comparison with the observations. If the baryons dominate in the central regions of all spirals, including LSBs, how can the CDM profiles be compared with the observations? Alternatively, if the baryons do not dominate but simply trace the DM distribution, why, in systems of comparable luminosity, are some DM halos cuspy (like the light) and others (also like the light) are not?

 

[PeterDonis]

The paper is

Hm, it looks like this paper is saying the rule only applies in the inner parts of spiral galaxies. I'll take a closer look when I get a chance.

 

[PeterDonis]

This thread has run its course and is now closed.