Pavel Kroupa: Dark Matter, Cosmology and Progress

(July 4th 2010)

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The dark-matter concordance cosmological model fails

The currently (2010) widely accepted/believed description of the birth and evolution of the universe and of its contents is "Lambda Cold Dark Matter Concordance Cosmological Model" (LCDM CCM) (see here ). Often LCDM CCM experts state that the current scientific epoch is that of precision cosmology, because the many parameters that define this model have been measured extremely precisely and because this model excellently accounts for the large-scale structure, as is evident for example in the distribution of galaxies and galaxy-clusters and the microwave back-ground temperature variations.

Within the CCM exotic dark matter needs to be postulated in order to account for the large rotational velocities of stars about their galaxies and for additional mass in galaxy clusters, if the observations are interpreted from the point of view of Newtonian dynamics. The dark matter particles are not part of the Standard Model of Particle Physics, which is otherwise extremely successful in describing all observed matter phenomena. Within the CCM, (exotic) dark matter forms the first gravitationally bound structures which merge until today forming ever larger and more massive dark-matter halos.

Baryonic (i.e. normal) matter falls into these halos forming stars and the visible universe. Within the CCM galaxies form through heavy merging activity, such that lesser (un-merged) sub-halos remain as satellites about the larger more massive halos. As a result of the heavy merging activity, galaxies turn out to be dominated by a central bulge (essentially an elliptical galaxy) and disk galaxies are small and thick. Galaxies like the Milky Way galaxy ought to have thousands of satellites. Among the old, well-known major and presently not well solved challenges for the CCM on galaxy scales are for example the observed small number of satellite galaxies around major galaxies (e.g. the Milky Way has only about 30 satellites), although many standard cosmologists would claim that the missing satellite problem has been solved. And, the observed existence of very extended but thin rotating disk galaxies, of which at least 30% do not have a bulge, cannot be understood. Tom Shanks summarises some of the more fundamental problems with the CCM here .

My own research was very much confined to the early version of the LCDM CCM (mid-1990's) when I began performing numerical experiments on the satellite galaxies of the Milky Way. I was quite happy with the CCM, as everyone else, and did not bother with the fundamental issues raised by some. With time, however, it became apparent that the LCDM CCM accounts poorly for the properties of the satellite galaxies and their distribution about the Milky Way. Warm dark matter models fared no better.

By considering other major galaxies I began to realise that actually I do not know any single galaxy whatsoever which looks like an object that may be described successfully within the framework of the dark-matter CCM (for example in terms of a galaxy's distribution of dark matter within it, or in terms of the thinness and extent of the visible matter in galaxies, or even in terms of the star-formation behaviour of galaxies ).

Perhaps the tide began turning significantly in 1999 when I for the first time heard an excellent talk at Harvard University by Stacy McGaugh on his research on rotationally supported galaxies (see Prof. Dr. Stacy McGaugh for much information on this issue). Stacy explained in a most convincing manner that an alternative description via modified gravity (or "extended gravitational theory") actually leads to a far superior understanding of galactic properties than the CCM. See McGaugh's MOND pages for an introduction to MOND.

Since then it has become rather clear that the CCM (with cold or warm dark matter with at most very weak coupling to baryonic matter) is ruled out as a viable description of the universe. The research paper published in 2010, Local Group tests of dark-matter concordance cosmology: Towards a new paradigm for structure formation, leads to this conclusion. In this research paper five problems for the CCM are found, in addition to the well-documented previously known problems that had mostly not been resolved. Each of the five problems poses a challenge for CCM, and together they exclude it with very high confidence indeed. The Disk-of-Satellite issue is but one of the problems.

Currently (July 2010), it is not evident at all how the properties of dark matter can be adapted such that the resulting distribution of visible matter around and in galaxies can be made consistent with the observations. A very strong coupling between the baryons and dark matter particles would need to be postulated, but this involves introducing a "fifth" force which is only relevant for the dark-matter-baryon interaction and is none of the known three forces with known mediating boson-particles (electromagnetic, weak and strong), which make the Standard Model of Particle Physics so successful.

Thus, in order to keep a cosmology with exotic dark matter and dark energy we would need to postulate a cosmological model which relies on
1) the existence of dark matter,
2) the existence of dark energy,
3) an unknown "dark/fifth force" coupling dark matter and baryons and perhaps
4) an additional unknown "dark coupling" acting only between dark energy and dark matter.

This may be seen as post-adjustment epicycle modelling and these additions of new unknown purely speculative dark components of the universe may be conceived as not being very motivating, especially so since a much more elegant solution is already on the horizon in the form of modified or extended gravity. This may require a modification of Einstein's field equations, but this is not unnatural given that the currently used field equations are of the simplest form anyway and do allow modifications. Some (hot, i.e. relativistic) dark matter particles may be around even if gravity is modified (see the seminal contributions by Angus, Famaey & Diaferio, 2010, MNRAS). Indeed, there is a natural required extension of the Standard Model of Particle Physics which involves the existence of sterile neutrinos to account for the observational fact that the active neutrinos have a mass (which they do not in the pure Standard Model of Particle Physics).

On a philosophical note, it is not at all surprising that astronomers are indeed finding problems in our description of how space-time couples to matter under circumstances when the curvature is very small (weak-field limit). After all, we still do not know, at a fundamental level, how matter couples to space-time and if it is even physically sensible to consider space-time as disjoint from matter as is currently the case (Einstein's field equation for example allows cosmological solutions for universes without any matter content). Is space-time a physical concept independent of matter, or is it an emergent property from matter? In this view space-time without matter would be a physically non-sensical concept.

The years 2008-2010 may stand out

in the historical context, as the transition period when break-through results affirmed the above interpretation of the current lack of understanding of cosmology. Noteworthy is the reported failure to detect dark matter particles by the on-going experiments. The following research papers may be counted as key events in this historic process:

  • Disney, Romano, Garcia-Appadoo, West, Dalcanton & Cortese: Galaxies appear simpler than expected (2008, Nature). Within the CCM, a dark-matter halo with a given mass would contain baryonic galaxies with a large range of properties (as specified by a large range of allowed specific angular momenta, baryon fractions, ages, dark-matter specific angular momenta etc.). This paper demonstrates that real galaxies follow a simple scaling relation and that the large variation of galaxies is not observed. It is completely unknown how the properties of the dark-matter particles would need to be adapted to explain this result within a dark-matter cosmological model. The dark-matter particles would have to be nearly exactly coupled to the baryons to achieve this.

  • Gentile, Famaey, Zhao & Paolo: Universality of galactic surface densities within one dark halo scale-length (2009, Nature). It is shown that the surface density of luminous (normal) matter within the putative (mathematically formal) dark-matter halo scale radius is invariant among galaxies. This result demonstrates a physical coupling between baryons and dark matter particles which is not contained in the CCM, and would only be fixed within the dark-matter hypothesis if a fifth (dark) force is invented. However, the baryon surface density invariance is automatically contained in the alternative-gravity theory (notably Mordehai Milgrom's MOND).

  • Peebles & Nusser: Nearby galaxies as pointers to a better theory of cosmic evolution (2010, Nature). With this review the authors show that the distribution of matter in the Local Volume of galaxies is next-to-incompatible with the expectations from the CCM. Basically, there are too massive isolated galaxies just beyond the edge of the filaments.

  • Kroupa, Famaey, de Boer, Dabringhausen, Pawlowski, Boily, Jerjen, Forbes, Hensler & Metz : Local Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation (2010, A & A). This research paper demonstrates that the Local Group of galaxies is not explainable within the context of the CCM, but that it is very naturally accountable within a modified gravitational framework (without dark matter on galaxy scales).

    Some of my own research is covered in press releases.

  • XENON100 collaboration: First Dark Matter Results from the XENON100 Experiment (2010, arXiv:1005.0380). The XENON100 experiment in Italy fails to detect WIMP scattering events and excludes a favoured cross section with 90% significance. Furthermore, the reported detection of two WIMP events by the cryogenic dark matter search (CDMS) experiment in the USA is falsified, consistent with the reported CDMS result that theirs is a statistically not significant detection.

  • Sawangwit & Shanks: Beam profile sensitivity of the WMAP CMB power spectrum (2010, MNRAS). See also Sawangwit & Shanks and the RAS press release. The authors demonstrate that there is a serious systematic issue with the resolution of the WMAP instrument used to construct the WMAP, which is a key anchor for the high-precision LCDM CCM description of the universe. They demonstrate that the currently permissible re-interpretation of the WMAP data even allow a pure-baryonic (no-dark matter) cosmological model, and that the evidence for dark energy via the Sachs-Wolfe Effect may be absent. That is, WMAP may not be providing any evidence for dark energy.

  • Interestingly, in a 2009 research paper, Pierre-Marie Robitaille finds that a careful review of the COBE instrument reveals numerous problems in these experiments: inadequate antenna characterization and testing, improper treatment of systematic errors, and failure to correctly measure the external calibrator. In addition, he suggests that it appears that the CMB anisotropies represent imaging artifacts generated when the COBE team inappropriately removed a systematic quadrupole signal from underlying random maps.

  • Given the above problems with the CCM, and in particular the surfaced issues with the COBE and WMAP data analysis, it is not permissible any longer to refer to the CCM as anything close to being of high precision nor as a confirmed theoretical construct. Rather, we are currently in the process of experiencing a major re-shaping of our world view but are not yet in the situation of having a successful theory of space-time and matter and the coupling between the two. A historical parallel is immediately evident in that about 100 years ago physicists (e.g. Planck, Einstein, de Broglie, Bohr) were just beginning to realise that the quantum world and the world at very high velocities was very different from the classical physics experienced on a daily basis, but neither the Theory of General Relativity nor Quantum Mechanics had been developed yet to the level known about three decade later. It was becoming clear that dynamics on large and small scales was definitely non-Newtonian.

    Progress and Sociology

    Scientific progress in understanding our world lies at the very foundation of our technological civilization. It is an established fact that without the theoretical research on completely new approaches to problems achieved about 100 years ago in Europe we would not today have mobile phones nor GPS navigation nor the energy sources so badly needed to power our societies. Here the key strides forward were the development of quantum mechanics and of the special theory of relativity at a time when practical aspects of these theoretical break-throughs were not even on the horizon. Thus, break-throughs that were achieved at a cost to society of only the salary of a few theoreticians, today amount to industry worth many billions of Dollars or Euros, and probably significantly more given the dependency of the entire planet-wide civilization on information and high-tech industries.

    How is such progress achieved, and how can it be supported to the best efficiency, given that society nourishes further progress through providing a fraction of its tax revenues?

    The above issues on cosmology are an excellent modern case in point:

    Given that many major problems with the CCM both at the fundamental level and at the level of galactic astrophysics have been known since at least five years if not longer, why is it that a large fraction of the community keeps stressing the excellent agreement between the CCM and observations? A recent research study may perhaps be relevant for this issue: Daniele Fanelli: Do Pressures to Publish Increase Scientists' Bias? An Empirical Support from US States Data (2010).

    The above discourse has already alluded to the notion that true progress on the issues at hand will only be attainable once the vacuum and the coupling of matter to space-time are understood. It is indeed sad to observe from afar that in the US grant-supported research on galactic astrophysics within a modified gravitational framework is all but non-existent. It is, on the other hand, impressive to see research on galactic astrophysics within a modified gravitational framework thriving in Israel with the brilliant theoretical contributions by among others, Milgrom, Beckenstein, Horwitz. In Europe an increasing amount of research is being done in this direction too, and notably in France through the epochal work among others of Combes, Blanchet and Famaey, in Italy by the work of Diaferio, Capozziello, Ciotti, Nipoti and Scarpa, not to mention the ground-breaking research on MOND and extensions of it by Zhao and students in Scotland and Sanders in the Netherlands. For example, a historical conference was held at the end of June 2010 in Strasbourg (MGADS) with the aim of assembling the community developing alternative or extended gravitational theories in the astrophysical and cosmological context, while a previous conference on an alternative view of satellite galaxies had been held in Bad Honnef in Germany in May 2009 ( TDGBonn), where the modified-gravity community was well represented. Furthermore, at the Lorentz Center in Leiden, a conference on New Directions in Modern Cosmology is taking place at the end of September 2010.

    This suggests that a scientific system, as is established in the US, which nearly exclusively relies on peer-review for funding to the extent that the salaries of the scientists significantly depend on successful grant-proposals, is doomed to fail in terms of supporting truly innovative approaches to current problems. In the US it matters too much what the others think, how others judge a researcher's standing, ideas and output. In contrast, research in France and Italy, while under-funded, stands out as not being heavily dependent on grant money, and indeed the researchers are much freer to follow intuitive notions and thus innovative paths to old problems. Concerning the fundamental problems in galactic astrophysics at least, and possibly other research fields as well, Europe (mostly through France) thus appears to be leading significantly ahead of the US.

    But, even in Europe there are many instances that researchers had to distance themselves from MOND-type research as result of a well-founded angst of either not getting the next job or not getting some grant. These are stories worth reporting, as they are very relevant for expenditure of tax-payers money for achieving current scientific progress. Perhaps the most famous story is about Albert Einstein himself, who was not hirable in the academic system for years until he essentially completely revolutionised our world view, both on the quantum and the cosmological scales.

    The current scientific system, as based on a peer-reviewed research-proposal process, contains the potential of failure in that much activity can be kept funded while not leading to major break-throughs at the fundamental level. In Germany for example, the very major fraction of the university resources that are funded by the common hand are given into the possession of a few professors for their entire academic life-span and their own disposal, while the large majority of researchers do not carry any significant weight and do not have a good chance to improve their situation. The current way of using tax-payers money for funding research thus poses, at worst, a possible challenge, and at best merely a hindrance, to continued development of technological societies.

    Prof. Dr. Pavel Kroupa (University of Bonn; )

    Pavel Kroupa and Mordehai Milgrom at MGADS, Strasbourg, 29.06.2010.