Introduction to Cosmology

The Cold Dark Matter Paradigm 221

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0

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  • 2 – 10
    log(k Mpc/Ωh^2 )
  
  
  
  

CMB

superclusters

log(

δM/M)

2

clusters

CDM power spectrum

at recombination

galaxies

1

Figure 9.5The theoretical power function푃(푘)in the훬CDM model as a function of the density
fluctuation wavenumber푘in units ofℎ^2 Mpc−^1. This can also be expressed by the angular scale
in degrees or by the linear size퐿of present cosmic structures in units of Mpc훺−m^1 ℎ−^1 .The
crosses indicate the approximate locations of galaxies, clusters, superclusters and the CMB.

Theoretical models predict that the brightest galaxies at푧=3 should be strongly
clustered, which is indeed the case. This comparison is also independent of any
parameter adjustments. In contrast, DM is much more weakly clustered at푧=3than
at푧=0, indicating that galaxies were strongly biased at birth.
In Figure 9.5 we show the theoretical linear power spectrum푃(푘). The real 2dFGRS
galaxy power spectrum data lie so accurately on the theoretical curve in Figure 9.5 that
we have refrained from plotting them. To achieve this success, all the free parameters
have been adjusted.
One important signature of gravitational instability is that material collapsing
around overdense regions should exhibit peculiar velocities and infall leading to
redshift-space distortions of the correlation as shown in Figure 9.3. We have previ-
ously referred to large-scale bulk flows of matter observed within the LSC attributed
to the ‘Great Attractor’, an overdensity of mass about 5. 4 × 1016 푀⊙in the direction
of theHydra–Centauruscluster, but far behind it, at a distance of some 79ℎ−^1 Mpc.
The 2dFGRS has verified that both types of redshift-space distortions occur, the
‘Fingers of God’ due to nearby peculiar velocities and the flattening due to infall at
larger distances. These results are quantified by the parameter value

훽=훺m^0.^6 ∕푏= 0. 43 ± 0. 07. (9.18)