Annu. Rev. Astron. Astrophys. 1993. 31: 689-716
Copyright © 1993 by . All rights reserved

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4.2. Overall Assessment of the Cold Dark Matter Scenario

There is no question about the historical utility of the CDM scenario. The existence of this model sharpened the questions to be asked of both observers and theoreticians. A great deal of cosmology had been taken for granted. The existence of galaxies, clusters, and other structures had been treated as simple fact by most investigators. With the development of quantitative CDM and HDM models, it was realized that such facts must be explained, that the physical mechanisms, which caused the existence of the objects in question, must be elucidated, and that the appropriate theory should be able to reproduce these observations and a host of others. So, primary credit must go to those investigators whose work effectively changed the mind-set of the discipline. Combining the quantitative theories with observational normalization of the spectral amplitude in the linear regime, which is now possible, we can, in principle, predict quantitatively all aspects of an evolving universe on the macroscopic statistical level. Our inability to model the required physical processes, and the limitations of computer hardware and software do not, of course, permit very fine detail to be computed. But the existing models compared with the rapidly improving systematic observations do allow some meaningful comparisons to be made.

If one were to accept the quoted errors of both published observations and theoretical simulations, then the standard (Omega = 1, n = 1, sigma8 = 1) CDM model fails. It predicts relatively too little large-scale structure and too much small-scale structure. It also predicts too high a small-scale velocity dispersion. In addition, it may have some difficulty in predicting late galaxy formation and too much late merging of galaxies. Finally, the evidence for the needed values of Omega0 and H0 is, on balance, not persuasive.

Many of the early exponents of the CDM picture have, in the post COBE world, proposed variants on the standard model. Several of these are attractive and it is possible that one of them will be found to be correct. But, of course, we must also face the simple and plausible possibility that, in the year 1992 we do not know what caused the origin of structure in the universe. It may easily be the case that none of the theories presently being given serious consideration is correct.

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