Constraints on particle physics from cosmology

This dissertation looks at ways in which one can use observations of the cosmos, nearby and far into the past, to constrain parameters of models of cosmology and particle physics. The constraints presented in each chapter come from distinct epochs in the evolution of the Universe.; In chapter 1, primordial nucleosynthesis of light elements (deuterium, helium and lithium) when the Universe is at temperatures of 1 to 0.1 MeV is used to constrain a certain class of models which are advocated to resolve an outstanding problem in physics, that of the smallness of the cosmological constant. In this class of models proposed to make the vacuum and matter energy densities naturally comparable today, the Universe maintains a uniform power-law expansion. Using the observed primordial abundances of the light elements in the Universe and other observations pertaining to the age of the Universe, we concluded that power-law cosmologies are not viable. The underlying physics for the failure of such models is very simple: to get the right abundances for the light elements in these power-law cosmologies, the Universe has to be younger than what the present observations allow it to be.; In chapter 2, the physics of photons decoupling from electrons or last scattering (which occurs at a temperature of about 0.2 eV) is used to investigate if a possible variation in the fine-structure constant could be constrained by observations of the cosmic microwave background (CMB) anisotropies. The conclusion is that the upcoming CMB anisotropy experiments (MAP, PLANCK) could reveal a variation in the fine-structure constant at the level of a percent or better. Apart from the constraints from nucleosynthesis (which are model dependent), presently this is the best way to probe a variation in the fine-structure constant when the Universe was less than a million years old.; In chapter 3, certain aspects of the role of massive decaying particles in the evolution of the Universe are investigated. The relativistic products of massive particles decaying after last scattering up to the present age can lead to significant changes in the anisotropies of the CMB present at the last scattering surface. Constraints from observations of the anisotropies in CMB can place stringent constraints on the mass and lifetime of such particles. This important issue has been investigated carefully and it is found that the masses of particles which decay with lifetimes greater than the age of the Universe at last scattering, but smaller than the present age, are very well constrained.; In chapter 4, implications of the background of relic neutrinos from supernovae is discussed. One of the goals of Super-Kamiokande collaboration is to observe the relic neutrinos from all supernovae that have occurred in our Universe. The feasibility of this project was investigated using simple physical arguments and relying on observations pertaining to the metal enrichment history of the Universe at redshifts less than one; the metal enrichment rate at any epoch is a direct tracer of the star formation (and hence supernova) rate at that time in our Universe. The results were reasonable, but low, upper bounds to the Supernova relic neutrino flux at Super-Kamiokande leading to the conclusion that it is very unlikely Super-Kamiokande could observe these relic neutrinos.