Firstly, it can be seen from table
2.2
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
with
. 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 [96,
57,
90
]. These simulations seem to be showing a more peaked CDM
enhancement toward galaxy centres than the previous work [94,
95] 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 [91].
Thirdly, it can be seen that the type Ia supernovae data are
crucial in determining the value of
. 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 [108]. 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 [86,
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
with
(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 [41]; 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
component.
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Experimental Searches for Dark Matter
Timothy J. Sumner http://www.livingreviews.org/lrr-2002-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |