Grand Unified Theories and the origin of the baryon asymmetry

In this article we review the great progress made over the past few years in relating the baryon asymmetry to reactions predicted by GUTs to have taken place about 10-34s after the bang. Since no one complete and unified model has emerged as the Grand Unified Theory, it is not possible to make an exact prediction for the magnitude and/or sign of the baryon asymmetry. Therefore, we concentrate on the common features that different models have in the context of baryogenesis. In the remainder of this section we discuss microphysics in the standard cosmology. In Section 2 we briefly review the observational evidence for the existence of a baryon asymmetry. The GUTs scenario for baryogenesis is presented qualitatively in Section 3, and the three key ingredients--baryon number violation, C and CP violation, and departure from thermal equilibrium--are discussed further in Sections 4,5, and 6. In Section 7 the SU(5) and SO(10) models are considered in more detail, and in Section 8 some simple variants of the standard scenario are discussed. Section 9 contains a brief summary and some concluding remarks