12. The mass autocorrelation function and nonbaryonic matter
If the bulk of the nonrelativistic matter, with density parameter
M0 ~
0.25, were baryonic, then under adiabatic initial
conditions the most immediate problem would be the strong
dissipation of primeval mass density fluctuations on the scale of
galaxies by diffusion of radiation through the baryons at
redshifts near decoupling.
(101)
Galaxies could form by fragmentation of the
first generation of protocluster "pancakes," as
Zel'dovich (1978)
proposed, but this picture is seriously challenged by the
evidence that the galaxies formed before clusters of
galaxies. (102)
In a baryonic dark matter model we could accommodate the
observations of
galaxies already present at z ~ 3 by tilting the primeval
mass fluctuation spectrum to favor large fluctuations on small
scales, but that would mess up the cosmic microwave background
anisotropy. The search for isocurvature initial
conditions that might fit both in a baryonic dark matter model
has borne no fruit so far
(Peebles, 1987).
The most important point of this test is
the great difficulty of reconciling the power spectra of matter
and radiation without the postulate of nonbaryonic dark matter.
The CDM model allows hierarchical growth of structure, from
galaxies up, which is what seems to be observed, because the
nonbaryonic dark matter interacts with
baryons and radiation only by gravity; the dark matter
distribution is not smoothed by the dissipation of density
fluctuations in the baryon-radiation fluid at redshifts
z
zeq.
As discussed in Sec. III.D, in the CDM model
the small scale part
of the dark matter power spectrum bends from the primeval
scale-invariant form P(k)
k to
P(k)
k-3, and
the characteristic length at the break scales inversely with
M0
(Eq. [42]). Evidence of such a break in
the galaxy power spectrum Pg(k) has been known
for more than a decade
(103);
it is consistent with a value of
M0 in the
range of Eq. (59).
101 Early analyses of this effect are in Peebles (1965), and Silk (1967, 1968). Back.
102 For example, our Milky Way galaxy is in the Local Group, which seems to be just forming, because the time for a group member to cross the Local Group is comparable to the Hubble time. The Local Group is on the outskirts of the concentration of galaxies around the Virgo cluster. We and neighboring galaxies are moving away from the cluster, but at about 80 percent of the mean Hubble flow, as if the local mass concentration were slowing the local expansion. That is, our galaxy, which is old, is starting to cluster with other galaxies, in a "bottom up" hierarchical growth of structure, as opposed to the "top down" evolution of the pancake picture. Back.
103 The first good evidence is discussed in Efstathiou et al. (1990); for recent examples see Sutherland et al. (1999), Percival et al. (2001), and Dodelson et al. (2002). Back.