4 Some Challenges for the ΛCDM Model

The great concordance of independent cosmological observables from Gpc to Mpc scales lends a certain air of inevitability to the ΛCDM 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 ΛCDM appears as a yet unproven hypothesis that relies heavily on two potentially fictitious invisible entities. Thus, an important test of ΛCDM 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 γ-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 ΛCDM 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).

 4.1 Coincidences
 4.2 Unobserved predictions
 4.3 Unpredicted observations
  4.3.1 Baryonic Tully–Fisher relation
  4.3.2 The role of surface density
  4.3.3 Mass discrepancy-acceleration relation
  4.3.4 Renzo’s rule

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