Jaime Rudas said:
In my opinion, the ΛCDM model remains the paradigm because it is, by far, the model that best fits the observations, and no alternative has emerged that even remotely comes close in this regard.
Does ΛCDM really fit the observations well?
(I won't litter this comment with citations for all of them but can easily produce them if desired.)
1. Galaxy formation occurs much sooner than predicted.
2. The predicted value of the growth index in the ΛCDM Model, that measures the growth of large scale structure, is in strong (4.2 sigma) tension with observations, given the model's measured parameters.
3. CDM predicts fewer galaxy clusters than are observed.
4. There are too many colliding clusters and when they are colliding they are on average, colliding at too high relative velocities.
5. Void galaxies are observed to have larger mean-distances from each other at any given void size than predicted by ΛCDM.
6. Voids between galaxies are more empty than they should be, and do not contain the population of galaxies expected in ΛCDM.
See also the KBC void.
7. The gravitational lensing of subhalos in galactic clusters recently observed to be much more compact and less "puffy" than CDM would predict.
8. The 21cm background predictions of the theory are strongly in conflict with the EDGES data as shown in this illustration:
9. The Hubble tension. Many analyses of the data prefer a non-constant amount of dark energy over cosmological history.
10. The σ8 -- S8 -- fσ8 tension
11. Increasing evidence that the universe is not homogeneous and isotropic.
12. ΛCDM provides no insight into the "cosmic coincidence" problem.
13. CDM gets the halo mass function (i.e. aggregate statistical distribution of galaxy types and shapes) wrong.
14. Doesn't adequately explain galaxies with no apparent dark matter and has no means of predicting where they will be found.
15. KIDS evidence of less clumpy structure than predicted.
16. CDM should predict NFW shaped dark matter halos almost universally, but observations show that NFW shaped dark matter halos are rare.
17. CDM predicts cuspy central area of dark matter halos which are not observed. Physically motivated feedback models have failed to explain this observation.
18. CDM predicts more satellite galaxies.
19. CDM fails to predict that satellite galaxies strongly tend to be in the plane of the galaxy.
20. The observed satellite galaxies of satellite dwarf galaxies are one hundred times brighter than ΛCDM simulations suggest that they should be
21. Flat rotation curves in spiral galaxies are observed to extend to about a million parsecs, but CDM predicts that they should fall off much sooner (at most in the tens or hundred of thousands of parsecs).
22. CDM fails to explain by the baryonic Tully-Fischer relationship hold so tightly over so many orders of magnitude or why there is a similar tight scaling law with a different slope in galaxy clusters.
23. Well known scaling laws among the structural properties of the dark and the luminous matter in disc systems are too complex to be arisen by two inert components that just share the same gravitational field as CDM proposes.
24. CDM failed to predict in advance that low surface brightness galaxies appear to be dark matter dominated.
25. CDM erroneously predicted X-ray emissions in low surface brightness galaxies that are not observed.
26. CDM fails to predict the relationship between DM proportion in a galaxy and galaxy shape in elliptical galaxies.
27. CDM doesn't predict the relationship between bulge mass and number of satellite galaxies.
28. CDM predicts that too few metal poor globular clusters are formed.
29. CDM does not explain why globular clusters which are predicted and observed to have little dark matter shown non-Keplerian dynamics.
30. We do not observe in galaxy systems the Chandrasekhar dynamical friction we would expect to see if CDM was as proposed.
31. CDM greatly underestimate the proportion of disk galaxies that have very thin disks.
32. CDM doesn't explain why thick spiral galaxies have more inferred dark matter than thin ones.
33. CDM doesn't predict the absence of inferred dark matter effects in gravitationally bound systems that are within moderately strong gravitational fields of a larger gravitationally bound system.
34. Compact objects (e.g. neutron stars) should show equation of state impacts of dark matter absorbed by them, at rates predicted from estimated dark matter density, that are not observed.
35. ΛCDM extended minimally to include neutrinos is over constrained in light of the latest DESI data with a best fit to negative neutrino mass, or at least a sum of neutrino masses far lower than the minimum neutrino masses inferred from neutrino oscillation.
36. CDM does not itself predict the observation from lensing data that dark matter phenomena appear to be wave-like.
37. Extensive and varied searches for dark matter particle candidates have ruled out a huge swath of the parameter space for these particles while finding no affirmative evidence of any such particles. These searches include direct dark matter detection experiments, micro-lensing, LHC searches, searches for sterile neutrinos, searches for axion interactions, comparisons of the inferred mean velocity of dark matter due to the amount of observed structure in galaxies with thermal freeze out scenarios, searches for signatures of dark matter annihilation, etc.
38. CDM is not the only theory that can accurately produce the observed cosmic background radiation pattern (e.g., at least three gravity based approaches to dark matter phenomena have done so).
39. The angular momentum problem: In CDM, 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 simple picture has never really worked. In CDM 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 that results in simulated disks being much too small.
40. 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?
41. CDM 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, in order to allow for spiral structure. This tension has not been successfully reconciled.
42. Many of the possible CDM particle scenarios disturb the well established evidence of Big Bang Nucleosynthesis.
This list isn't comprehensive, but it is more complete than most (compare, e.g., the
list from Wikipedia).
Three and a half dozen strong tensions or outright conflicts between ΛCDM predictions and observations doesn't sound like a great fit to observations to me.
In fairness, the original ΛCDM model formulated in the late 1990s wasn't intended to be perfect. It was a first approximation that was focused mostly on cosmology observations and was not unduly concerned with galaxy and cluster scale phenomena. The scientists who devised it in the first place knew perfectly well that they were ignoring factors (like neutrinos) that were present and had some effect, but were negligible relative to the precision of astronomy observations available at the time (which were much more crude than recent observations - e.g., they didn't have the JWST or the Hubble Telescope or DESI or 21cm measurements or gravitational wave detectors or decent neutrino telescopes). It wasn't supposed to be the final be all and end all theory and it has had a good run and probably lasted longer as the paradigm than originally expected.
And, again, competing paradigms are like duels to be the head of the tribe. Until you have a particular competitor that is clearly superior enough to displace the leader of the pack, it stays in the lead by default, even if its flaws are myriad. I'm not necessarily saying that the competitor has arrived.
But, I'm also saying that a paradigm that has so many conflicts with observations that it is vulnerable and has reduced credibility. So, it shouldn't be taken as seriously as something like the Standard Model of Particle Physics which has only a handful of recent and relatively minor tensions the currently remain unresolved after half a century of rigorous efforts to poke holes in it.
To circle around to the original question of the Hubble tension, all of these discrepancies, even if they merely require tweaks to the ΛCDM model rather than a wholesale abandonment of it, add credence to the possibility that the ΛCDM model details used to predict the early time Hubble constant from the Cosmic Microwaved Background radiation with high precision, could cause the early time Hubble constant value to be underestimated. This is so despite the assumption that there have been correct CMB measurements by Planck and that those measurements were then correctly inserted into the status quo ΛCDM model. This could easily have resulted in a 2-6% too low early time Hubble constant determination from the CMB measurements.
