Annu. Rev. Astron. Astrophys. 1980. 18: 489-535 Copyright © 1980 by Annual Reviews. All rights reserved

1. INTRODUCTION

This article presents a critical review of measurements of the attributes of the cosmic background radiation and indicates the prospects for improved measurements. Since the last review in these volumes (Thaddeus 1972), there has been substantial progress in the held and of course several other review articles (Alpher & Herman 1975, Danese DeZotti 1977, Ulfbeck 1980). An up-to-date review of theoretical work on the background is presented by Sunyaev Zel'dovich (1980) in this volume.

Whether one's taste runs to the presently favored "big bang" cosmology or to more heretical models, the background radiation has become an accepted and essential part of observational cosmology. The radiation has unique and remarkable properties: it is isotropic on both large and small angular scales to a higher degree than any other source in the universe; it exhibits no linear polarization; and it has a spectrum so close to that of a blackbody that one can talk only of deviations from a thermal spectrum. In short, the radiation satisfies beyond almost reasonable expectations the simple hypothesis that it is a remnant of a homogeneous and structureless primeval explosion. The observed blandness and apparent pristine state of the radiation beg for more detail. One hopes that precision observations would show imprints left by early cosmic processes and that observed deviations could be fruitfully interpreted. Studies of the background radiation have passed the discovery phase and are beginning an analytic one.

This review is dedicated to the analytic phase. How well have the spectrum, intensity distribution on large angular scales, and polarization been measured? What limits these measurements and where is there a reasonable chance for improvement?