Introduction

The subject of this chapter is an analysis of one of the most impressive predictions of the modern theory of the structure and evolution of the Universe: the prediction of angular anisotropy in the temperature distribution of the cosmic microwave background on the celestial sphere; we will also analyse observations of this phenomenon. This anisotropy results from the interaction of the primordial radiation background with perturbations of density and velocity of the baryonic matter and with perturbations of the metric; these make an inseparable part of any scenario of structure formation in the expanding Universe. We also use this introductory section to share our personal recollections of the period when the theory of anisotropy of the CMB was being created and the first attempts were being made to detect it by observations. These notes are quite subjective and do not attempt to provide historical analysis. We hope nevertheless that the reader will find them interesting. We wish to point out first of all that while the microwave background itself was discovered by accident, its anisotropy was discovered as a result of well-planned observational searches based on a carefully developed theory.

One of the present authors (I. Novikov) was there at the moment of inception of this field of astrophysics – nowadays a fully-fledged field – and remembers well the history of conception of the theory and the drama of the experimental attempts to detect the anisotropy of primordial radiation at the end of the 1960s and the beginning of the 1970s.