| This thesis is based on the study of vector fields during cosmic inflation. Cosmic inflation has proven to be an accurate description of the very early universe, not only because of its success in resolving the classical problems of big bang cosmology, but also for introducing a natural mechanism for the generation of primordial fluctuations which give rise to the structure (galaxies and cluster of galaxies) in the universe. For simplicity, most inflationary scenarios assume that the expansion is driven by a scalar field. However, due to the fact that the underlying particle physics model of inflation is unknown, and due to some features emerged in some studies of the cosmic microwave background data, there have recently been considerable interest in vector field driven models of inflation. In this thesis, I present a complete stability analysis of some of the compelling models where vector fields are assumed to play an important role during inflation. The stability analysis is performed by studying all possible fluctuations around the background solution of these models. It is explicitly proven that for models where the gauge invariance of the vector field is broken, the background solution is unstable. The proof is performed both (1) by studying the quadratic action for the fluctuations, and showing that ghost instabilities are present in the model; and (2) by studying the linearized Einstein equations and showing that the solutions diverge close to horizon crossing. For models that are free of instabilities, relevant power spectra are computed and the resulting phenomenology is discussed. |
