The Cosmic Background Radiation 1.1 the Cbr Spectrum

Observations of the Cosmic Microwave background have provided many of the most powerful constraints we have on cosmology and events in the early universe. The spectrum and isotropy of CBR have long been a pillar of Big Bang models. The discovery of low levels on anisotropy has provided new information and tools for our understanding of the early universe. Further observations promise to enhance greatly our knowledge of processes in the early universe and cosmological parameters. We can anticipate rapid advance in this eld up to and through the year 2000 which will dramatically focus our eeorts in cosmology during the next millenium. This paper outlines the primary science likely to be discovered and deened by a vigorous airborne and ground-based program which should be strongly supported. If successfully excuted, we an anticipate a measurement of the CBR anisotropy spectrum to within a factor of two of the conndence level unavoidably set by cosmic variance. Even so, observations of the CBR are the best and often the only way to obtain information on many critical parameters so that an ambitious satellite experiment that maps the full-sky to the T=T 10 6 level is well justiied. x1 Introduction From its prediction (Gamow & Alpher & Hermann 1948) and discovery (Penzias & Wilson 1965) the cosmic background radiation (CBR) has been a cornerstone of the Big Bang cosmology and led to its widespread acceptance. In the intervening years improved observations have lead to a better understanding of its role in the early universe and the wealth of information that it carries about the early universe. With this work its importance to cosmology has grown rapidly while the technology to exploit it has kept pace. A major advancement came with the COBE satellite in the early 1990's with a very precise measurement of the spectrum (Mather et al. 1993) as being black body and the discovery of anisotropies (Smoot et al. 1992) at the 10 5 level. This discovery was followed by an explosion of activity and the reports of anisotropies by several groups and a rapid advance in theoretical work. The spectrum and temperature of the cosmic background radiation are key issues in cosmology. The Big Bang model makes the unequivocal prediction that to high order the spectrum will be thermal and that the temperature will vary with redshift as T(z) = (1 + z)T 0. The current COBE FIRAS observations now …