Cosmic Structure Formation and the Background Radiation

I describe our post-COBE theories for how structure developed in the Universe, from the ultra-large scales beyond that httle bit of the Universe accessible to our observations to ‘short-distance’ scales where gaa dynamicrd processes will have been m importmt m gravity in shaping the high redshift objects that formed. In between lie the very large scales whose gravitationrd potential fluctuations are what we believe the COBE DMR experiment has detected; large scrdes probed by grd~y and cluster clustering, by the streaming motions of gd=ies, and, most importantly, by intermediate angle microwave background experiments. Together with COBE, we have hopes that these probes can pin down the parameters of the cosmological model, including the amplitude and shape of the primordid spectrum of gravitational metric fluctuations from whose instability the structure around us springs. An initially scale invariant spectrum or something close to it connects tilrough a hundred-fold extrapolation in wavelength the COBE detection to observations of gaJaxy clustering. Although this is a spectaculm confirmation of the general thrust of the inflation-based theoretical work of the paat decade, and is consistent with the ‘stand~d’ cold dark matter model, albeit with more nonlinear dynamics than most theorists, are comfortable with, reconciliation of the observed large scale power in the galaxy distribution with the COBE amplitude may require variations on the baaic ~heme, involving such exotica as a n~zero cosmological constant, a whiff of hot dark matter (i. e., few eV neutrinos), extended, naturaJ or power law inflation, non-local biming of the galaxy distribution relative to the dark matter distribution, gravity wave induced temperature anisotropies, etc.