(by Pavel Kroupa, 10th Nov. 2020, 09:00)
We have not blogged for some time and an update on some of the developments concerning The Dark Matter Crisis has been posted here. Below are recent scientific developments which strongly suggest that the standard model of cosmology (the SMoC) which relies on the existence of cold or warm dark matter (C/WDM) particles is not a correct description of the observed Universe. Note that the SMoC which is based on the hypothesis that cold dark matter particles exist comprises the currently widely accepted LCDM cosmological model, while the SMoC which assumes warm dark matter particles exist constitutes the currently less popular LWDM cosmological model. The difference of both models in terms of structure formation and the type of galaxies formed is minimal, which is why both are referred to as the SMoC.
Why has the Cosmology Crisis become catastrophic?
- First of all, C/WDM particles have still not been found after more than 40 years of searching! The account of the situation published on October 11th, 2020, on the Triton Station by Stacy McGaugh is worth reading. Stacy writes “… the field had already gone through many generations of predictions, with the theorists moving the goal posts every time a prediction was excluded. I have colleagues involved in WIMP searches that have left that field in disgust at having the goal posts moved on them: what good are the experimental searches if, every time they reach the promised land, they’re simply told the promised land is over the next horizon?“. In view of the available evidence challenging the existence of C/WDM particles, it is stunning to read “The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is by now well established” in Sec. 26.1.1 of the 2018 version of the Review of Particle Physics. Some five years ago I had dared to suggest to the editors and section authors to change this very statement to “The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is currently a leading hypothesis” or similar, but the short reply was quite unpleasant. It is unfortunate that only the cosmological argument leads one to the C/WDM particle hypothesis, there being no independent (non-cosmological and non-astronomical) evidence. Such evidence could have come from indications in the Standard Model of Particle Physics, for example, but this is not the case. Put in other words, if we had not known about cosmology or galaxy rotation curves, we would not be contemplating C/WDM particles. Thus, by the astronomical evidence having gone away (follow the Dark Matter Crisis), the physicists are left with nothing apart from belief. I would argue that the words “belief” and “opinion” should be banned from the language of natural sciences. Note that the situation is different for the fast collisionless matter (FCM, or “hot dark matter”) which appears in MOND-cosmological models (Angus 2009). Independetly of the astronomical evidence, the experimental fact that neutrinos have mass and oscillate suggests the existence of an additional sterile neutrino. Candidates for FCM particles thus arise independently of astronomy or cosmology. FCM particles do not affect galaxies as they are too low mass, so even at their maximum allowed phase space density as set by the Tremaine-Gunn limit, they cannot be dynamically relevant to the masses of galaxies. Returning to the SMoC: the lack of experimental verification of C/WDM particles comes in hand with additional failures of the SMoC:
- Testing for the presence of the speculative C/WDM particles through the very well understood physical mechanism of Chandrasekhar dynamical friction leads to the conclusion that the dynamical friction through the putative dark matter halos around galaxies which are, in the SMoC, made up of C/WDM particles, is not evident in the data (Angus, Diaferio & Kroupa 2011; Kroupa 2015; Oehm & Kroupa 2017). That is, a galaxy which falls towards another galaxy should be slowed down by its dark matter halo, and this slow-down is not seen. The galaxies pass each other with high velocities, like two stars passing each other on hyperbolic orbits, rather than sinking towards each other to merge. This evidence for the non-existence of C/WDM halos around galaxies is in-line with the above mentioned lack of experimental detections (point 1 above). Customarily, an image of two strongly interacting galaxies is automatically interpreted as being a galaxy merger. But this is an over-interpretation of such images, since the implied mergers are not happening in the frequency expected in the standard dark-matter-based theory. Renaud et al. (2016) calculate ant document the theoretical description of an observed strongly interacting galaxy pair in the C/WDM framework and in MOND. Indeed, that the population of galaxies does not evolve significantly since a redshift of one has been found by Hoffmann et al. (2020) and has already been described by Kroupa (2015). This lack of evolution and the hugely vast preponderance of disk galaxies, of which a large fraction is without bulges, means that galaxies merge rarely as mergers nearly always transform the involved disk galaxies into earlier types of galaxies (disks with massive bulges, or even S0 or elliptical galaxies).
- The Hubble tension is now much discussed. The Hubble Tension comes about as follows: the Hubble constant we should be observing today can be calculated assuming the standard dark-matter based SMoC is correct and that the Cosmic Microwave Background (CMB) is the photosphere of the Hot Big Bang (but see also point 6 below). The actually measured present-day value, as obtained from many independent techniques including supernovae 1a standard candles, gravitational lensing time delays, and mega-masers, comes out to be significantly larger though. The evidence is compiled in Haslbauer et al. (2020). The observer today sees a more rapidly expanding Universe than is possible according to the SMoC. More on the Hubble tension below (point 7).
- The planes of satellites (or disk of satellites) problem has worsened: Our own Milky Way has been found to have a more-pronounced disk of satellite galaxies around it than thought before (Pawlowski & Kroupa 2020; Santos-Santos, Dominguez-Teneiro & Pawlowski 2020). Andromeda has one (Ibata et al. 2013, Sohn et al. 2020) and the nearby Centaurus A galaxy too (Mueller et al. 2018). The majority of other galaxies also show evidence for such planes or disks of satellites (Ibata et al. 2015). That the three nearby major galaxies simultaneously show such disks of satellite galaxies, and that disks of satellite systems are indicated by the majority of more distant galaxies, where the SMoC expects such satellite planes only in very rare cases (Pawlowski et al. 2015; Pawlowski 2018), eliminates with de facto complete confidence (i.e. much more than 5sigma) the SMoC, given that the satellites are in the great majority of cases ancient and void of gas such that they must have orbited their hosts many times. The Milky Way satellites also seem to be on almost circular orbits, strongly suggestive of a dissipative origin (Cautun & Frenk 2017) similar to the process that forms solar systems.
- Astronomical data have uncovered, with extremely high confidence (more than 5sigma), that the strong equivalence principle is violated on the scale of galaxies (Chae et al. 2020 ), exactly in-line with a central expectation by MOND (Milgrom 1986), and in contradiction to the SMoC. While apparently not receiving much attention (e.g. via news coverage), this work by Chae et al. (2020) is a game-changer, a break-through of the greatest importance for theoretical physics. Independent evidence for the violation of the strong equivalence principle is also evident in asymmetrical tidal tails around globular clusters (Thomas et al. 2018). Gravity therefore behaves non-linearly on galaxy scales, preventing a simple addition of the fields contributed by different masses. This is a consequence of the corrected, generalised Poisson equation (Bekenstein & Milgrom 1984) which these authors point out is also found in classical theories of quark confinement.
- Possibly a “nuclear bomb” nuked standard cosmology: Although the SMoC is only valid if the Universe is transparent, observations show there to be dust between galaxies. This intergalactic dust is ancient, and it radiates as it is heated by photons from the surrounding galaxies. Vaclav Vavrycuk (2018) has added all photons from this dust in an expanding Universe (i.e., in the past the intergalactic dust density was higher in a warmer Universe) and found the photon emission received by us to be very (nearly exactly) comparable to the measured CMB with the correct temperature of about 2.77K. For an explanation of his research paper see this YouTube video by MSc student Rachel Parziale at Bonn University. Note that the measured weak but large-scale magnetic fields around galaxy clusters and voids produce a correlated polarisation signal. The total number of infrared photons received at Earth is an integral over the time evolving density distribution along the line of sight such that the observed mass distribution within a small redshift around us should not correlate with the overall fluctuation of photon intensity seen in projection on the sky. The calculations by Vavrycuk thus suggest that CMB=cosmological dust emission, rather than being the photosphere of the Hot Big Bang. CMB research comprises an incredibly precise science, but the role of intergalactic dust needs to be considered very carefully and by avoiding pre-conceptions. Note that even if only a few per cent of the CMB were to be due to ancient intergalactic dust, then this would already bring down the SMoC.
- The Universe around us contains far too few galaxies out to a distance of about 0.3 Gpc. This Keenan-Barger-Cowie (KBC) void falsifies the SMoC at more than 6sigma confidence. The KBC void kills the SMoC because the SMoC relies on the Universe starting off isotropically and homogeneously with the observed CMB fluctuations at the redshift z=1100 boundary condition about 14Gyr ago and cannot evolve density differences to the observed KBC under-density at z=0 which is the present time. Combined with the Hubble tension, the SMoC is falsified with more than 7sigma confidence. Newtonian gravitation plus the hypothetical C/WDM particles are together nowhere near strong enough to generate the observed density contrasts and the observed velocity differences between neighbouring Gpc-scale volumes. The next blog by Moritz Haslbauer will explain this situation. Note that here we still treat the CMB as the photosphere of a Hot Big Bang, but this may need to be reconsidered (see point 6 above).
- The SMoC relies on the Universe having no curvature, but Di Valentino, Melchiorri & Silk (2020) find the enhanced lensing amplitude in CMB power spectra to imply a closed and thus curved Universe. However, this could be related to structure formation being more efficient than is possible in the SMoC (see point 7 above).
- Cosmic isotropy is challenged at the 5sigma confidence level by X-ray selected galaxy clusters (Migkas et al. 2020), with the implication that the Universe appears to expand faster in a certain direction. A discussion of this evidence is provided by Scientific American. Cosmic isotropy is also challenged by the significant evidence for a dipole in the number counts of quasars beyond redshift one (Secrest et al. 2020). Independently of this, Javanmardi et al. (2011) also found evidence for a directionally dependent expansion rate.
- Last for now but not least, the observation of massively interacting galaxy clusters such as the El Gordo cluster at high redshift (z=0.87) independently falsifies the SMoC with more than 6sigma confidence. In the SMoC, galaxy clusters cannot grow to such masses by this redshift – there is not enough time, or alternatively, Newtonian gravitation is too weak even with the help of the hypothetical C/WDM particles. This is shown by Asencio, Banik & Kroupa (2020). Elena Asencio is researching for her MSc thesis in the SPODYR group in Bonn.
Combining the above KBC void/Hubble Tension/El Gordo falsifications with the previously published tests (Kroupa et al. 2010, Kroupa 2015; see the figure below taken from Kroupa 2012) means that it has become, by now, wrong to still consider the standard dark-matter based cosmological model, the SMoC, as being relevant for describing the Universe. The falsification of the SMoC has reached well above the 7 sigma confidence — Remember: the Higgs Boson was accepted as having been discovered once the experimental confidence rose to 5sigma. It is important to emphasise that independent tests on very different scales lead to the same result, the SMoC being ruled out by many tests with more than 5sigma confidence.
The above list, but more importantly, the very high significance of the results, seem to indicate that a paradigm change may be under way in the sense that our current understanding of the Universe may be entirely rewritten at a very fundamental level. This is already indicated by gravitation being Milgromian. The paradigm shift would be epochal (see also this previous blog on the historical context) if the suggestion by Vavrycuk concerning the physical nature of the CMB were correct (point 6 above) because in this case our very concept of a Hot Big Bang and the origin of matter would be up in the air. There is independent evidence that a once-in-a-century paradigm shift may be under way: the Universe is much more structured than allowed by the SMoC. Thus, the Local Group of Galaxies (on a scale of 3Mpc across, Pawlowski, Kroupa & Jerjen 2013 ) shows a frightening symmetry in its matter arrangement (I call this frightening because there is currently no known theory to explain this distribution of matter). The arrangement of galaxies (Peebles & Nusser 2010) in the nearby cosmological volume (20Mpc across) does not correspond to the SMoC model and these very galaxies show a history of star-formation which appears to be far too tuned and non-varying (Kroupa et al. 2020). This begs the question how they manage to do so? The entire local Universe appears to be engaged in a significant bulk flow generated by major voids and over-densities (Haslbauer et al. 2020; Hoffmann et al. 2020).
Galaxies provide formal and precise observational data that allow us to correct the work of Newton and Einstein on gravitation, who did not have these data at their disposal. Rather, they formulated the currently assumed theories of gravitation subject to Solar System constraints only, which are now many decades if not centuries old. In his book “A Philosophical Approach to MOND“, David Merritt (2020)addresses the formal philosophical measures concerning how the Newtonian/Einsteinian formulation of gravitation needs to be assessed in terms of its success in describing the observed Universe in comparison with the correction to the law of gravitation through incorporation of galaxy data as formulated by MilgrOmiaN Dynamics (MOND). (Next sentence added Jan 3rd, 2021:) In Merritt (2017) we read his conclusion “The use of conventionalist stratagems in response to unexpected observations implies that the field of cosmology is in a state of ‘degenerating problemshift’ in the language of Imre Lakatos.” This would tend to close a circle: if Newtonian/Einsteinian gravitation needs to be revised, then we cannot use Einsteinian gravitation to formulate the evolution of the Universe, which opens the whole issue of how it started, what are the boundary conditions and how does it evolve? The Catastrophic Crisis in Cosmology (i.e. the fact that the observational data do not fit to the SMoC) is thus merely exactly the statement that we may well be in the process of a very major paradigm shift.
The big challenge for the future will be to find out how the Universe truly does work. The next blog by Moritz Haslbauer will indicate how a step towards this goal might have been achieved by Haslbauer, Banik & Kroupa (2020).
In The Dark Matter Crisis by Pavel Kroupa. A listing of contents of all contributions is available here.