Independently of any dark-matter detections or the success or failure of dark-matter searches
(it is notable that the originally favoured dark matter particles have long ago been excluded through direct searches, as summarised by Prof. Stacy McGaugh),
it is a well known problem that galaxies cannot be reproduced in the standard cosmological (i.e. the LCDM) scenario. In the LCDM model, the mass of the universe consists to about 4 % out of normal (baryonic) matter which we observe, while 96% is in an unknown dark form (about 22 % being the exotic cold dark matter and about 74% being in dark energy).
Since the large fraction of astronomers (about 95 % as is often stated) and physicists are convinced that the LCDM model is the correct description of the cosmological universe, a vast industry has been established world-wide to try to solve the failure of LCDM on galaxy scales. Each year hundreds of research papers are spurned out by excellently funded reseach groups claiming to solve some aspect of the problem.
For example, many research papers deal with the question why there are only about two dozen satellite galaxies around the Milky Way, while there ought to be many thousands of satellite dark matter halos around the Milky Way (the Missing Satellite Problem). Why are the satellite galaxies of the Milky Way distributed so unevenly about the Milky Way, forming a giant disk-like distribution which is nearly perpendicularly oriented to the disk of the Milky Way (the Satellite Anisotropy Problem)? Another issue of focuss is why the dark matter halos inferred from observations have cores, while the LCDM model predicts them to have dense central regions, i.e. cusps (the Core-Cusp Problem). Another issue being worked on in much detail is why there are large thin disk galaxies with no bulges (the Angular-Momentum Problem). Why are disk galaxies, which by the way are the majority of all galaxies, all so simple (the Invariant Galaxy Problem)? Why do large elliptical galaxies appear so rapidly after the Big Bang even though they must have been build-up from pre-existing galaxies in the LCDM model, according to which galaxies form through myriads of mergers of smaller dark matter halos that already have a normal-matter content in the form of stars and gas. Why are the predicted building blocks (these dwarf galaxies) observed to be younger than the large elliptical galaxies, although they should be older because the small dark matter halos form before the large ones (the Downsizing Probem)? Or, why is there an observed strong conspiracy between the distribution of normal (baryonic) matter and dark matter in all disk galaxies? This Conspiracy Problem has not been solved although it is known since decades. And, related to this, why is there no observational evidence for dark matter in dwarf-elliptical galaxies and large elliptical galaxies within their visible regions? Why does dark matter always only appear when the surface density of normal matter falls below a critical level? The surface density is but the gravitational acceleration - so, why should dark matter decide to appear when the acceleration falls below a critical threshold, which happens to be the same for all galaxies (the Dark Matter Emergence Probem)? Note that this is a purely observationally established fact, highlighted recently by Gentile et al.(2009, Nature) without any implication, as yet, of hidden deeper physics.
There is a very large number of research papers dealing with each of these aspects, and by adjusting various parts of the modelling, certain problems can be reduced. Each year many new research papers appear attempting to deal with some of the problems. But the real issue is whether a proposed solution to any one of the problems is consistent with the solution to any one of the other problems.
For example, the formation of stars in small dark matter halos can be delayed by introducing re-ionisation at a special time in the universe. Re-ionisation means the re-heating of the cooled gas after the Big Bang through radiation which is intense enough to knock electrons from the hydrogen nuclei. When an electron gets knocked out, the nucleus and electron depart with much energy, the resulting ionised gas being heated to 10000 Kelvin. This may indeed have happened because luminous sources like the first massive stars or active galactic nuclei or quasars appear. The gas, or more correctly the plasma, is then too hot to be captured by the dwarf dark matter halos existing at that time.
But, is this resulting model then able to explain elliptical galaxies, or, the observed cores of dwarf galaxies, at the same time? And, are the observed ages of stars in dwarf galaxies consistent with this delay through re-ionisation?
So while there is a vast research literature claiming success on individual problems, the large picture is usually not addressed, the cross-referencing to other problems affected by the particular solution is rarely if ever attempted. Given this situation, we decided to approach this issue of consistency by testing the excellent observational data that is available for the Local Group of galaxies.
A view sometimes voiced is that the LCDM model is right and that galaxies are merely non-linear small-scale weather phenomena which cannot be computed adequately so it is not surprising that there are problems. It is sometimes stated that these problems are not major and that the LCDM model does excellently on larger-than-galaxy scales.
There is some truth to these statements, because going from a full cosmological computer simulation down to what happens in individual galaxies requires a tremendous numerical resolution, ranging from Gpc down to pc scales. So often computational short-cuts are required.
It is therefore necessary to devise tests of the theory which capture the essence of the LCDM model. That is, any scrutiny of the LCDM model must not test issues that rely on the details of the star-formation recipes used, but must deal with general aspects related to overall conservation laws which must be valid in the LCDM model as in any other. Such tests lead to the realisation that the failure of the LCDM model on galaxy scales is, figuratively speaking, akin to a weather model which predicts constant snowfall in the Sahara.
PK was recently contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are
A) So the LCDM model fails on scales smaller than about 8 Mpc? (this contribution)
C) What are the three best reasons for the failure of the LCDM model?
E) Why is the main-stream community so reluctant to go along with accepting the failure of LCDM?
This contribution deals with Question A, while upcoming contributions will concentrate on the other questions.
Question A: "So, in your view, LCDM fails on the local level, i.e. on scales below about 8 Mpc in the local universe?"
Answer: Indeed, and with high significance. Do remember that I had worked happily within this framework, that is, I find the original hypothesis of LCDM (i.e. there is cold dark matter) as being reasonable, although I had never bothered to check on inflation or dark energy.
The argument comes two-fold. First there are the properties of the Local Volume of Galaxies. Then there are the properties of the Local Group of galaxies, the latter being contained in the former and providing, not surprisingly, much better data on the individual-galaxy scale.
The Local Volume of galaxies consists of the local region of the universe within a sphere around the Sun with a radius of about 8 Mpc. It contains about 560 known galaxies which are distributed in a large sheet. It also contains a void which is far too empty in comparison with the expectations from the LCDM model. Near the edge of the Local Sheet of galaxies which we inhabit are three major disk galaxies that ought not to be in such isolated regions. In their paper "Nearby galaxies as pointers to a better theory of cosmic evolution" Peebles & Nusser (2010, Naure) write "... the properties of nearby galaxies that can be observed in greatest detail suggest that a better theory would describe a mechanism by which matter is more rapidly gathered into galaxies and groups of galaxies. This more rapid growth occurs in some theoretical ideas now under discussion." On p.568 they state :
"The variety of problems we have considered in the interpretation of
the present baseline motivates serious consideration of adjustments of
the fundamental theory. Any adjustment would have to preserve the
properties of the standard cosmology that agree with the cosmological
tests but that allow new physics operating on the scale of galaxies. The
evidence for more-rapid structure formation agrees with what happens
in modifications of the gravity physics of the general theory of relativity
and in general relativity with a long-range force acting only on
the dark matter."
Effectively these authors state that: Either gravity needs to be modified - for example, in MOND structures do grow more rapdily as has been explicitly computed by Claudio Llinares et al. (2008). Or, an additional "dark force (DF1)" needs to be postulated to act only between dark matter particles leading to the GR+I+DM+DE+DF1 model (a model based on the general theory of relativity + inflation + cold dark matter + dark energy + dark force). A "second dark force (DF2)" must, however, also exist, in order to account for the observed fact that dark matter arranges itself according to the distribution of normal (baryonic) matter - the observed strong conspiracy between baryonic and dark matter is one of the very well known and hitherto not solved problems in LCDM - see Pfenniger & Combes (1994). Following the line of thought of Peebles & Nusser, we would then arrive at the GR+I+DM+DE+DF1+DF2 model. Note that this model of the universe is then made essentially only of unkown stuff. Pfenniger & Combes try to solve the conspiracy by suggesting that the dark matter is in the form of cold molecular-hydrogen clouds which cannot be observed. Needless to say, the Conspiracy Problem (CP) is naturally resolved by MOND - in MOND the CP does not even appear.
The Local Group of galaxies is the local region of the universe within a sphere around the Sun with a radius of about 1.5 Mpc. It contains two major disk galaxies (the Milky Way and Andromeda), two smaller disk galaxies (Triangulum or M33 and the Large Magellanic Cloud) and about 40 known dwarf galaxies most of which are satellites of the two major disks. The Local Group is a gravitationally bound, that is, it is a region which has decoupled from the cosmologically expanding Hubble flow.
Our work shows that the satellite galaxies around Andromeda and the Milky Way cannot be accounted for by the LCDM model, and that Andromeda and the Milky Way are too similar given the large variation of galaxies expected in the LCDM model. This is covered in depth by "Local-Group tests of dark-matter concordance cosmology. Towards a new paradigm for structure formation" (Kroupa et al. 2010) in which the large body of recently published research, which claims that the LCDM model can account for the Local Group satellite galaxies, is scrutinised carefully using a variety of tests.
By Pavel Kroupa and Marcel Pawlowski (17.01.2011): "Question A: Galaxies do not work in LCDM, sociology and majority views" on SciLogs. See the overview of topics in The Dark Matter Crisis.