Observations of the Isotropy of the Cosmic Microwave Background Radiation

Abstract

Over the past decade the anisotropy of the microwave background radiation has emerged as a field of fundamental importance to astrophysics due to its significance in theories of galaxy formation and in the quest for the physical origin of fluctuations. The history of the development of this subject may be divided into three phases. In the first phase, lasting up until about 1982, searches for anisotropy were carried out on angular scales ranging from two arc minutes to 180°, and, apart from the dipole term due to the peculiar velocity of the Earth relative to the Hubble flow (e.g. Fixsen et al 1983, Lubin et al 1985, Halpern et al 1985, Strukov & Skulachev 1988, Meyer et al 1991b, Smoot et aI 1991b), no anisotropies were detected down to the level of AT/T ≈ 10^(-4)-10^(-3) expected from simple scenarios for galaxy formation. In the second phase, from ~1983 to ~1989, sensitivities were improved by a full order of magnitude, but still no intrinsic anisotropies were detected. Over the same period many theoretical models were eliminated and new models were explored which predicted fractional anisotropies approaching 10^(-6). We are presently in the third phase: Sensitivities on all angular scales have reached levels where confusing signals due to discrete sources or diffuse Galactic emission due to dust, free-free emission and synchrotron radiation-dominate the measurements and have to be removed before the intrinsic anisotropy can be studied.