On The Dark Matter Crisis, we have already presented numerous problems that appear within the LCDM model of cosmology. Some of these have been given names, like the “Missing Satellites Problem”, where LCDM predicts more dark matter subhaloes around the Milky Way than there are observed satellite galaxies, which are expected to trace them. Or the “Missing Baryons Problem”: from cosmological predictions we expect a certain density in the baryonic, luminous and thus in principle observable matter. But when you add up all the visible matter you observed, you only get 10-40 per cent of what you expect. The larger fraction is missing.
Even the ongoing non-detection of the DM particle in direct-detection experiments might be seen by some as another of these problems. So, there are several cases in which the model predicts something which then is not observed, thus leading to the 'missing' of that particular entity or observation thereof.
This week, two additional studies claim that even more seems to be missing (when your expectations are based on what LCDM predicts, that is). They both suggest a serious lack in the amount of expected dark matter on two very different size-scales: the local universe and our immediate neighborhood within the Milky Way.
Dark Matter missing in ... the Local Universe
In the work titled “Missing Dark Matter in the Local Universe”, Igor D. Karachentsev has looked at a sample of 11,000 galaxies in the local Universe around the MW. He has summed up the masses of individual galaxies and galaxy-groups and used this to test a very fundamental prediction of LCDM.
The idea is as simple as it is brilliant: cosmology has precise predictions as to what is the content of our universe. In particular, it predicts the density of matter to be Ωm,glob = 0.28 +- 0.03 (83 per cent of this in dark, 17 per cent in luminous matter). Now, to test this, all you have to do is to sum up all the mass within a certain volume of space, and you can estimate the actual density of mass within that volume. To be sure that your volume is representative, it needs to be large. If you only sum over, say, a sphere of 100 kpc in diameter, the density strongly depends on whether you have a galaxy in this volume or not. Karachentsev chose to use a volume with a radius of 50 Mpc around the MW. On this size-scale, the density is expected to fluctuate by only 10 percent, a reasonably low value in astronomy. The scale can thus be assumed to be representative and you should observe the mass density predicted by LCDM.
Except that you do not.
Karachentsev reports that the average mass density is only Ωm,loc = 0.08 +- 0.02, a factor of 3-4 lower than predicted and can not be explained by the uncertainties in the data or prediction. As most of the mass-content in the Universe is supposed to be dark matter, this means that most dark matter is missing in this volume.
It is not straight-forward to interpret this result, except that it might be a serious problem for LCDM. In the paper three solutions within the framework of standard dark matter cosmology are suggested. First of all, we might resort to the unsatisfying claim that the local Universe is exceptionally non-representative of the Universe as a whole. We would then sit in a local void, a very large under-dense region of the Universe. Unfortunately, as Karachentsev states in his paper, this is in contradiction to observations. The other two suggested solutions are based on the idea that maybe not all mass is counted. Dark matter is defined to be an elusive thing, after all. Dark halos might be more extended than predicted in the models, pushing it outside the virial radius of a halo, the region in which observations can indirectly 'measure' it from the dynamics. However, taking this as a solution to the observed mass-deficit “clearly contradicts the existing observational data”, as Karachentsev states in his work. But maybe much of the dark matter is hiding somewhere else? Karachentsev suggests it to be in massive dark clumps not filled with galaxies (he calls them 'dark attractors'), and thus is invisible to us when looking for galaxies only. But how could these dark clumps, with masses of galaxy-clusters, remain dark? You would need to separate the baryonic, luminous matter from a large bunch of dark matter to make sure no galaxies from in the dark attractor.
In any case, these suggestions require modifications to the behavior of dark matter because their processes are not predicted in current models. None of these possibilities seem very attractive, leaving us with the conclusion that, assuming we live in a LCDM universe, a large fraction of the dark matter is gone missing.
Dark Matter missing in ... the Solar Neighborhood
The amount of dark matter in the solar neighborhood was investigated in the work “Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood” by Christian Moni Bidin and collaborators. For a short introduction, you can have a look at this proceedings paper, and yesterday, the ESO also issued a press release about this work, titled “Serious Blow to Dark Matter Theories?”.
In their work, Moni Bidin et al. have looked at a sample of 400 red giant stars close to the Sun at vertical distances of 1.5 to 4 kpc above the MW disc. In addition to the stellar 3D positions, they have derived three-dimensional kinematics for these stars. From this data, they estimate the dynamical surface mass density of the MW within this range in heights from the disc. This surface mass density should be the sum of all mass, visible and dark. But it turns out, according to their analysis, that the visible mass alone is already a perfect fit to the observed value. According to the authors, no additional mass is needed (see their plot below).
CAPTION: Upper panel of figure 1 of Moni Bidin et al. (2012). Observational results (black) for the surface mass density within a certain distance from the Galactic plane (x-axis). The dotted and dashed lines show the 1- and 3-sigma strip of the observations. The predictions of models (grey) containing a dark matter halo all lie significantly above the observed value, except for the model accounting for visible mass only (labelled VIS).
Their analysis is based on a number of assumptions about the structure of and kinematics in the Milky Way disc, like that the density decays exponentially in both radial and vertical direction, that there is a ﬂat rotation curve, thar there is no bulk motion of stars in vertical or radial direction and so on. It might well be that some of their assumptions are not perfectly valid. However, they have checked that changing one of their adopted input parameters or assumptions can not solve the problem of missing DM. Very exotic hypotheses (they mention an unreasonably thin thick disc as an example) can make their data fit with the expectations from DM models, but such a solution is unsatisfying and rather improbable, according to them.
Taken together, the work suggests that, given their assumptions about the the MW disc, dark matter halos as predicted by current models do not explain the observations. It might be more informative to state it the other way around, though: according to them, the observations can be easily explained with the visible matter of the Milky Way disc alone, there is no need for more.
Note added on 21.05.2012: In a recent posting on astro-ph Bovy & Tremaine point out that the deduced amount of dark matter depends on the assumptions that go into the modelling of the stellar kinematics. They assume Newtonian dynamics to be valid (as Moni Bidin et al. have) but in contradiction to Moni Bidin et al. they show that it is not correct to assume the mean azimuthal velocity is independent of Galactocentric cylindrical radius. Instead, taking the circular velocity to be independent of the radius, Bovy & Tremaine show that the usual local matter density is arrived at. If Milgromian dynamics were correct rather than Newtonian dynamics, then it emerges that the local stellar kinematics ought to show evidence for phantom dark matter (e.g. Fig.12 in Kroupa 2012). We remind the reader that in the past it has been claimed that local stellar kinematics shows evidence for significant amounts of dark matter in the disk of the Milky Way, while more thorough later analysis has found this signal to go away (Kujiken & Gilmore 1989; Kuijken 1991; Flynn & Fuchs 1994). Thus, all in all, the Newtonian analysis by Bovy & Tremaine not only "saved dark matter", but more importantly although unintentionally, Bovy & Tremaine demonstrated consistency of the data with MOND. End Note.
Dark Matter missing in ... well, it is simply not there at all
Indeed, a 50 page review of the observational tests of the standard model has been compiled by Pavel Kroupa in "The dark matter crisis: falsification of the current standard model of cosmology" and will appear in the Publications of the Astronomical Society of Australia (PASA-CSIRO publishing). Using a huge number of different data, Pavel Kroupa performs a strict logical falsification of the currently standard cosmological model, which is based on Einstein's theory of general relativity, concluding that cold or warm dark matter cannot exist.
Note added on 21.05.2012: The implications of the Dual Dwarf Galaxy Theorem of the Kroupa 2012 paper is that cold or warm dark matter cannot be dynamically relevant in galaxies. It then implies that non-Newtonian (e.g. Milgromian) dynamics must be valid. Ironically, when interpreting Milgromian systems with Newtonian eyes, the observes will see evidence for dark matter. However, this is phanotm dark matter and it is exactly coupled to normal matter. That is, phanton dark matter is not constituted of ballistic particles which are on individual orbits within a Newtonian potential. End Note.
A crisis of modern physics
If there is no dynamically relevant cold or warm dark matter then we still need to explain the flat rotation curves of galaxies. This leads to a crisis in modern physics, as our very understanding of space-time and matter are now at stake.
Other posts you might find interesting:
By Pavel Kroupa and Marcel Pawlowski
(19.04.2012): "Dark Matter gone missing in many places: a crisis of modern physics?" on
SciLogs. See the overview of topics in The Dark Matter Crisis.