Probing Density Fluctuations at Low and High Redshift

We discuss cosmological inference from galaxy surveys at low and high redshifts. Studies of optical and IRAS redshift surveys with median redshift ${\bar z} \sim 0.02$ yield measurements of the density parameter $\Omega$ and the power-spectrum of density fluctuations, but both cannot easily be related to the properties of the underlying mass distribution due to the uncertainty in the way galaxies are biased relative to the mass distribution. Moreover, currently little is known about fluctuations on scales intermediate between local galaxy surveys ($\sim 100 h^{-1}$ Mpc) and the scales probed by COBE ($\sim 1000 h^{-1}$ Mpc). We focus here on several issues, as examples of clustering on different scales: the extent of the Supergalactic Plane, an optimal reconstruction method of the density and velocity fields, the effect of biasing on determination of $\Omega$ from redshift distortion, the future big surveys SDSS and 2dF (with median redshift ${\bar z} \sim 0.1$) and radio sources and the X-Ray Background as useful probes of the density fluctuations at higher redshift (${\bar z} \sim 1$).