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. ). 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.