Dark Matter, Inflation, Cosmic Background Radiation and Superstring Axions.

This dissertation explores a new candidate for dark matter, fine-tuning in models of inflation, three -point correlations in the cosmic microwave background and the decay of superstring axions. We show that a very weakly coupled scalar field with an initial vacuum expectation value V and mass m will provide enough mass to close the universe if V ~eq (1 times 10^9 GeV)(100 GeV/m)^{1/4} . We reanalyze the dependence of density fluctuations on the coupling constant in quartic models of inflation and show that some earlier results, stating that the coupling constant is not constrained to be small in models of inflation with rapid reheating of the universe, are false. We also show that inflationary models predict a definite and model independent angular dependence for the three-point correlation function of Delta T/T for the cosmic microwave background radiation. Finally, we study the astrophysical constraints on the decay of superstring axions. We conclude that the axions have to decay before nucleosynthesis. We also calculate the entropy released by superstring axion decays and find that many models of baryogenesis can not tolerate the subsequent dilution of the baryon asymmetry of the universe.