The great concordance of independent cosmological observables from Gpc to Mpc scales lends a certain air
of inevitability to the CDM model. If we accept these observables as sufficient to prove the model, then
any discrepancy appears as trivia that will inevitably be explained away. If instead we require a higher
standard, such as positive laboratory evidence for the dark sectors, then
CDM appears as a yet
unproven hypothesis that relies heavily on two potentially fictitious invisible entities. Thus, an important
test of
CDM as a scientific hypothesis is the existence of dark matter. By this we mean not just unseen
mass, but specifically CDM: some novel form of particle with the right microscopic properties and correct
cosmic mass density. Searches for WIMPs are now rather mature and not particularly encouraging.
Direct detection experiments have as yet no positive detections, and have now excluded [19] the
bulk of the parameter space (interaction cross-section and particle mass) where WIMPs were
expected to reside. Indirect detection through the observation of
-rays produced by the
self-annihilation6
of WIMPs in the galactic halo and in nearby satellite galaxies have similarly returned null
results [6, 84, 172] at interestingly restrictive levels. For the most-plausible minimally-supersymmetric
models, particle colliders should already have produced evidence for WIMPs [2, 1, 23]. The right model
need not be minimal. It is always possible to construct a more complicated model that manages to evade all
experimental constraints. Indeed, it is readily possible to imagine dark matter candidates that
do not interact at all with the rest of the Universe except through gravity. Though logically
possible, such dark matter candidates are profoundly unsatisfactory in that they could not
be detected in the laboratory: their hypothesized existence could neither be confirmed nor
falsified.
Apart from this current non-detection of CDM candidates, there also exists prominent observational
challenges for the CDM model, which might point towards the necessity of an alternative model (or, at
the very least, an improved one). These challenges are that (i) some of the parameters of the model appear
fine-tuned (Section 4.1), and that (ii) as far as galaxy formation and evolution are concerned (mainly
processes happening on kpc scales so that the predictions are more difficult to make because the baryon
physics should play a more prominent role), many predictions that have been made were not successful
(Section 4.2); (iii) what is more, a number of observations on these galactic scales do exhibit regularities
that are fully unexpected in any CDM context without a substantial amount of fine-tuning in terms of
baryon feedback (Section 4.3).
http://www.livingreviews.org/lrr-2012-10 |
Living Rev. Relativity 15, (2012), 10
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