GETTING AROUND COSMIC VARIANCE

Cosmic microwave background (CMB) anisotropies probe the primordial de nsity field at the edge of the observable Universe. There is a limitin g precision (''cosmic variance'') with which anisotropies can determin e the amplitude of primordial mass fluctuations. This arises because t he surface of last scatter (SLS) probes only a finite two-dimensional slice of the Universe. Probing other SLS's observed from different loc ations in the Universe would reduce the cosmic variance. In particular , the polarization of CMB photons scattered by the electron gas in a c luster of galaxies provides a measurement of the CMB quadrupole moment seen by the cluster. Therefore, CMB polarization measurements toward many clusters would probe the anisotropy on a variety of SLS's within the observable Universe, and hence reduce the cosmic-variance uncertai nty. [S0556-2821(97)04820-0].