Introduction to big bang cosmology (Q2776585)

The standard cosmological model assumes that the universe is spatially homogeneous and isotropic on large scales, an assumption originally dignified as the cosmological principle. Empirical justification for this assumption was provided initially by counts of galaxies and was supplemented subsequently by observations of relic radiation backgrounds. Recent astronomical observations have required a renaissance of this question with the discovery of cosmic structures on very large spatial scales. The present paper is concerned with the physics of the standard cosmological evolution during the radiation-dominated era. This is the furthest range one can reliably proceed on the basis of observational data. The reconstruction of the thermal history of the universe back to this epoch constitutes the best constraint or boundary condition that can be imposed on the speculative ideas that abound concerning the early universe. The overall conclusion at present is that the universe is indeed homogeneous when averaged on scales comparable to its present size. At the same time, the working picture of the early universe before the nucleosynthesis era is undergoing a radical revision as particle theorists explore new possibilities for the mechanisms for generation of the baryon asymmetry, dark matter, and the primordial density perturbation. The so far little explored cosmology of brane-world may turn out to be just as radical a departure from present ideas as the inflationary universe was in its time.NEWLINENEWLINEFor the entire collection see [Zbl 0976.00040].