Introduction

At the beginning of the 1980s, when the inflationary theory was first proposed, one of our main goals was to explain the amazing uniformity of the Universe. We were trying to find out why the Universe looks approximately the same in all directions. Of course, locally the Universe does not look uniform — there are such large deviations from uniformity as planets, stars and galaxies. But if one considers the density of matter on scales comparable to the size of the observable Universe, lobs ~ 1028 cm, one finds that this is uniform to an accuracy better than one part in 10 000. The most surprising thing about this is that, according to the standard big bang theory, the distant parts of the Universe which we can see with a powerful telescope were not in causal contact at the time of the big bang and could not have been in such contact until very late stages of cosmic evolution. So one could only wonder what made these distant parts of the Universe so similar to each other.

In the absence of any reasonable explanation, cosmologists invented the so-called ‘cosmological principle’, which claims that the Universe must be uniform. But the Universe is not perfectly uniform, since it contains inho-mogeneities — such as stars and galaxies — which are crucial for life. Because of these small but important violations, the cosmological principle cannot be a true principle of nature, just like a person who takes only small bribes cannot be called a man of principle.

Since the beginning of the 1970s, we have witnessed spectacular progress in the development of cosmology, which started with a breakthrough in the theoretical understanding of the physical processes in the early universe and culminated in a series of observational discoveries. The time is ripe for a textbook which summarizes the new knowledge in a rigorous and yet accessible form.

The beginning of the new era in theoretical cosmology can be associated with the development of the gauge theories of weak, electromagnetic and strong interactions. Until that time, we had no idea of properties of matter at densities much greater than nuclear density ∼ 1014 g/cm3, and everybody thought that the main thing we need to know about the early universe is the equation of state of superdense matter. In the beginning of the 1970s we learned that not only the size and the temperature of our universe, but also the properties of elementary particles in the early universe were quite different from what we see now. According to the theory of the cosmological phase transitions, during the first 10−10 seconds after the big bang there was not much difference between weak and electromagnetic interactions. The discovery of the asymptotic freedom for the first time allowed us to investigate the properties of matter even closer to the big bang, at densities almost 80 orders of magnitude higher than the nuclear density.