© CAMBRIDGE UNIVERSITY PRESS 1999

1.7.7 Best Bet CDM-Type Models

As said at the outset, the fact that the original CDM model did so well at predicting both the CMB anisotropies discovered by COBE and the distribution of galaxies makes it likely that a large fraction of the dark matter is cold - i.e., that one of the variants of the SCDM model might turn out to be right. Of these, CHDM is the best bet if ₀ turns out to be near unity and the Hubble parameter is not too large, while CDM is the best bet if the Hubble parameter is too large to permit the universe to be older than its stars with = 1 (e.g., Chs. 4 and 8, Section 11.4).

Both theories do seem less ``natural'' than SCDM, in that they are both hybrid theories. But although SCDM won the beauty contest, it doesn't fit the data.

CHDM is just SCDM with some light neutrinos. After all, we know that neutrinos exist, and there is experimental evidence - admittedly not yet entirely convincing - that at least some of these neutrinos have mass, possibly in the few-eV range necessary for CHDM. Isn't it an unnatural coincidence to have three different sorts of matter - cold, hot, and baryonic - with contributions to the cosmological density that are within an order of magnitude of each other? Not necessarily. All of these varieties of matter may have acquired their mass from (super?)symmetry breaking associated with the electroweak phase transition, and when we understand the nature of the physics that determines the masses and charges that are just adjustable parameters in the Standard Model of particle physics, we may also understand why _c, , and _b are so close. In any case, CHDM is certainly not uglier than CDM.

In the CDM class of models, the problem of too much power on small scales that has been discussed here at some length for ₀ = 0.3 and h = 0.7 CDM implies either that there must be some physical mechanism that produces strong, scale-dependent anti-biasing of the galaxies with respect to the dark matter, or else that higher ₀ and lower h are preferred, with a significant amount of tilt to get the cluster abundance right and avoid too much small-scale power (KPH96). Higher ₀ 0.5 also is more consistent with the evidence summarized above against large and in favor of larger ₀, especially in models such as CDM with Gaussian primordial fluctuations. But then h 0.63 for t₀ 13 Gyr.

Among CHDM models, having N = 2 species share the neutrino mass gives a better fit to COBE, clusters, and small-scale data than N = 1, and moreover it appears to be favored by the available experimental data (PHKC95). But it remains to be seen whether CHDM models can fit the data on structure formation at high redshifts, and whether any models of the CDM type can fit all the data - the data on the values of the cosmological parameters, the data on the distribution and structure of galaxies at low and high redshifts, and the increasingly precise CMB anisotropy data. Reliable data is becoming available so rapidly now, thanks to the wonderful new ground and space-based instruments, that the next few years will be decisive.

The fact that NASA and the European Space Agency plan to launch the COBE follow-up satellites MAP and COBRAS/SAMBA in the early years of the next decade, with ground and balloon-based detectors promising to provide precise data on CMB anisotropies even earlier, means that we are bound to know much more soon about the two key questions of modern cosmology: the nature of the dark matter and of the initial fluctuations. Meanwhile, many astrophysicists, including my colleagues and I, will be trying to answer these questions using data on galaxy distribution, evolution, and structure, in addition to the CMB data. And there is a good chance that in the next few years important inputs will come from particle physics experiments on dark matter candidate particles or the theories that lead to them, such as supersymmetry.

Acknowledgments

This work was partially supported by NASA and NSF grants at UCSC. JRP thanks his Santa Cruz colleagues and all his collaborators, especially Anatoly Klypin, for many helpful discussions of the material presented here. Special thanks to James Bullock, Avishai Dekel, Patrik Jonsson, and Tsafrir Kolatt for reading an earlier draft and of this manuscript and for helpful suggestions for its improvement.