Introduction

Cosmic microwave background radiation (CMBR) is a relic from the redshift z ≈ 103, beyond which the universe is optically thick in most of the wave bands. This radiation therefore carries vital information about the state of the universe at an epoch that is probably as early as we could probe by direct electromagnetic measurements. A considerable amount of theoretical and observational progress has been achieved in this topic in the past decade, and future observations of CMBR hold the promise for allowing us to determine the parameters of the universe with unprecedented accuracy.

The temperature anisotropies in CMBR and related issues are discussed in this chapter. Anisotropies that are due to peculiar velocities and fluctuations in the gravitational potential are derived in Section 6.3 and discussed in detail in Sections 6.4 and 6.5. The damping of anisotropies and the distortions that arise because of the astrophysical processes are studied in Section 6.7.

Processes Leading to Distortions in CMBR

We have seen in Chap. 4 that the photons in the universe decoupled from matter at a redshift of ~ 103. These photons have been propagating freely in space–time since then and can be detected today. In an ideal Friedmann universe, a comoving observer will see these photons as a blackbody spectrum at some temperature T0.