where
is the reionisation optical depth,
,
,
r,
are the primodial amplitudes and tilts of the scalar and tensor
inhomogeneities,
b
is a bias factor relating rms galaxy fluctuations to the
underlying rms matter fluctuations,
and
are the contributions to the overall density from curvature and
the cosmological constant,
and
are the physical densities of dark matter (both hot and cold
together) and baryonic matter, and finally
is the fraction of dark matter in the form of hot dark matter.
The primary observational data that they used were all available
CMB data [54] and the recently released IRAS Point Source Catalogue Redshift
Survey (PSCz) [113] from which they derived the large scale structure power
spectrum. Simultaneous fits were then done to these data allowing
all 11 parameters to vary [136].
Table
2.2
shows the acceptable range of values for the key parameters that
came out of those fits. In the table the matter density,
, includes both baryonic matter and dark matter, and moreover the
dark matter can be classed either as ``hot'' or ``cold''
depending on whether it was relativistic or not in the early
Universe. The total dark matter density is
, and
. Of particular note for this review were that the cold dark
matter density is non-zero and that the baryonic density has a
range that just accomodates the constraints from BBN [27
] at its lowest end, but with significantly better fits for
higher values. The hot dark matter density can only be a minor
component.
Very recently there have been significant new CMB data
released from BOOMERANG [97], MAXIMA [64], DASI [77], and CBI [102]. These data have given better definition to the second and
third peaks in the CMB power spectrum. Wang, Tegmark and
Zaldarriaga [143] subsequently repeated the above analysis using a combination of
these and previously available CMB data.
The two left-hand plots in the top row in Figure
1
are the most relevant for the dark matter. We see that the dark
matter density is again non-zero, with a similar range of values
as before, and that the fraction of dark matter as ``hot dark
matter'' is less than 35%, assuming no constraints on the hubble
parameter,
h
. The allowable fraction of hot dark matter drops to only 20% if
the preferred hubble parameter value is imposed. The right-hand
column in table
2.2
lists the quoted 95% confidence limits for comparison with the
earlier analysis. The most striking difference is in the baryon
density. While previously the allowable range of
was only just compatible with the upper limit derived from
BBN [27], it now comfortably embraces it. This is illustrated in the
left-hand panel in figure
2, which shows the combined constraints on the baryonic matter and
dark matter densities. The white central region is the allowed
parameter space when all constraints are applied, except for BBN
of course. Relaxing the constraints by not using the PSCz data
enlarges the allowed region to include the cyan coloured area.
If, in addition, no assumptions are made about the value of the
Hubble constant, then the green area also becomes allowed. If all
constraints are accepted then figure
2
implies there is between 4.5 and 9 times as much dark matter in
the Universe as there is baryonic matter.
Constraints on cosmological models can also be derived from
the observations of high red-shift Type 1a supernovae [59]. When combined with data from the CMB anisotropies, these
limits give reasonable agreement with those cited earlier in
table
2.2
. A recent result from de Bernardis
et al.
[42] is shown in the right-hand panel in figure
2
. This time what is shown are the joint constraints on
and
. The solid curves are the 1 to 3
combined likelihood contours and these can be compared with the
values in the table. A somewhat weaker constraint on the Hubble
constant was used.
Hence, from the above, there does indeed seem to be a
cosmological model that can simultaneously satisfy all the
observational evidence used. The ranges of values for the key
parameters relevant to dark matter searches have been summarized
in table
2.2
. Rotation curves of galaxies can also be explained with this
type of cosmological model. Numerous
N
-body simulations have been performed to verify whether structure
formation occurs properly in a number of different types of
models. Gawiser and Silk [53] reviewed the situation with regard to large-scale structure.
Simulations of gravitational collapse on the scale of galaxies
have resulted in universal rotation curves that match reasonably
well those observed in a wide range of galaxies [94,
95
].
From table 2.2 the main features of the emerging standard cosmology from the point of view of dark matter are:
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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 |