Gravitational waves from gauge fields during cosmic inflation

Sammanfattning

Detecting primordial gravitational waves from the early universe would provide strong evidence that an exponential spatial growth occurred fractions of a second after the Big Bang. If these gravitational waves are found to be polarized, a theoretical model describing their production mechanism is required. In this thesis, we study how gauge fields can produce chiral primordial gravitational waves in the early universe. After presenting the theoretical background and briefly reproducing a chromo-natural inflation (CNI) model, we investigate the Gauge-flation model by first analyzing the background evolution, then continuing with a perturbative analysis and the analysis of the production of chiral gravitational waves. For the specific parameter values considered in this thesis, production of chiral gravitational waves was found for mQ ≳ 1.6 with maximal chirality occurring for mQ ≳ 2.4. The tensor-to-scalar ratio obtained in the regime of chiral gravitational wave production was found to be r ≳ 0.07. These results demonstrate that gauge fields provide a natural mechanism for generating chiral primordial gravitational waves. However, for the Gauge-flation model, the parameter regime that yields strong chirality is incompatible with current observational constraints on the tensor-to-scalar ratio. Instead, models in which gauge fields are complemented by additional fields, such as scalar fields, could yield observables consistent with current observations while preserving the mechanism that generates strong chiral primordial gravitational waves.