2.3. Structure Formation

The third part of Table 1 refers to tests of the Friedmann-Lemaître model from the condition that it admit a theory for the origin of cosmic structure: galaxies and all that. Here the danger is that we are testing two theories, cosmology and structure formation. Within the gravitational instability picture the latter requires a prescription for the important dynamical actors - it may include cold dark matter, massive neutrinos, cosmic strings, or other fields - and the character of the departures from homogeneity at high redshift. A commonly discussed model assumes Gaussian adiabatic fluctuations in cold dark matter, baryons, massless neutrinos, and the thermal cosmic background radiation (the CBR). That leaves one free function, the power spectrum of the initial mass density fluctuations, to fit to two functions, the spectra of fluctuations in the present space distribution of the mass and in the present angular distribution of the CBR. It is impressive that we can adjust the one function to match both sets of observations. But as discussed in Section 3.4 we do not yet have firm evidence that the initial conditions are Gaussian, or that they are adiabatic, or that the model takes account of all the important dynamical actors. We expect to have a check: if precision measurements in progress of the CBR and the large-scale matter distributions match in all detail the predictions of one of the simple models now under discussion, for reasonable values of the cosmological parameters, it will make believers of us all (or at least many of us). But before deciding to become a believer it might be wise to wait to see what the measurements reveal.