Testing Models for Structure Formation

I review a number of tests of theories for structure formation. Large-scale flows and IRAS galaxies indicate a high density parameter Ω ≃ 1, in accord with inflationary predictions, but it is not clear how this meshes with the uniformly low values obtained from virial analysis on scales∼1Mpc. Gravitational distortion of faint galaxies behind clusters allows one to construct maps of the mass surface density, and this should shed some light on the large vs small-scale Ω discrepancy. Power spectrum analysis reveals too red a spectrum (compared to standard CDM) on scales λ ∼ 10 − 100 hMpc, but the gaussian fluctuation hypothesis appears to be in good shape. These results suggest that the problem for CDM lies not in the very early universe — the inflationary predictions of Ω = 1 and gaussianity both seem to be OK; furthermore, the COBE result severely restricts modifications such as tilting the primordial spectrum — but in the assumed matter content. The power spectrum problem can be solved by invoking a cocktail of mixed dark matter. However, if gravitational lensing fails to reveal extended dark mass around clusters then we may be forced to explore more radical possibilities for the dark matter.