Discrepancies within the standard cosmological model

2.3 Discrepancies within the standard cosmological model

Although the argument in Section 2.2 is very persuasive and does present a formally consistent picture there are a number of concerns that continue to require attention.

Firstly, it can be seen from Table 1 that, prior to the latest CMB data, the consistency between BBN and the CMB and LSS constraints was marginal. As a result of this a number of routes that allow for higher baryon density were explored. The most recent of these [48 ] invoked a degenerate BBN scenario in which additional light neutrinos (either sterile or degenerate) are allowed. Consistency with observed CMB anisotropies was obtained for $4 ≤ N ν ≤ 13$ with $2 0.25 ≤ h Ωb ≤ 0.35$ . Such a high baryon density would negate the need for dark matter. While it is comforting (to some) to see the new CMB data apparently removing this discrepancy, the data themselves are still not “high-precision” and some aspects of the data reduction remain uncertain [143 ].

A second issue has arisen from high resolution N-body simulations [95 , 57 , 89 ]. These simulations seem to be showing a more peaked CDM enhancement toward galaxy centres than the previous work [93 , 94 ] and more sub-structure in the CDM halos themselves [61 ]. There is increasing evidence that indeed the predictions are incompatible with observational data [26 , 112 , 21 , 43 , 80 ]. Possible ways of softening the central profile include allowing the dark matter to interact more readily, either with itself (self-interacting CDM [25 ]) or with baryonic matter. Although the N-body simulations themselves appear robust in general, in central regions where there are few “particles” there is the issue of resolution and convergence [90 ].

Thirdly, it can be seen that the type Ia supernovae data are crucial in determining the value of $ΩΛ$ . Central to this is the question of whether the optical light-curves can really be used as standard candles, or whether reddening is playing a role here, as quite small amounts of absorption could significantly affect the results. Use of infrared light curves may well be more reliable [83 ]. This suggestion has been countered recently by the observation of a very high-redshift ( $z ∼ 1.7$ ) supernova which is actually brighter than expected, even in a “no-dust” scenario [107 ]. Its increased brightness is shown to be consistent with an early deceleration phase of the Universe.

Finally, there is a class of model in which gravity itself is assumed to be modified [84 , 85 ]. A large number of effects attributed to dark matter have been addressed using modified gravity [82 ] with the most recent being an analysis of the latest CMB aniostropy data [81 ]. In this latest work it is claimed that $Ω Λ ∼ 1$ with $Ωb ∼ Ωm$ (consistent with standard BBN) is the favoured model. However, the result rests heavily on the apparent absence of a second peak in the CMB data from BOOMERANG [42 ]; MAXIMA-1 data [67 ] are not included. At the moment the totality of CMB data does not constrain the second peak sufficiently strongly to rule out a significant $Ωcdm$ component.