• cosmology
• Kalb-Ramond axion
• string theory

The aim of this thesis is to study a cosmological model based on String Theory, particularly focusing on the behavior of one of the massless modes of the theory, the Kalb-Ramond Axion.
Current inflationary models address some classical cosmological problems, such as the flatness or the homogeneity problems. Although these models yield results consistent with measurements of the CMB, they fail to resolve the singularity at the Big Bang.
With a string-based model the singularity would be replaced by a more physically reasonable bounce that continuously connects the pre and post-Big Bang periods. A Lagrangian with the massless modes is used and it is developed to all orders in α’, that is a constant related to the fundamental string length. The modes are the Dilaton, the Metric, and the Kalb-Ramond Axion. These models have already been extensively studied, neglecting the Axion. In a recent work, a Routhian formalism is used to consider all α’ orders.
In my thesis, based on the work mentioned above, we implemented the Axion into the study of solutions and examined both isotropic and anisotropic metric cases. Then, we used Wolfram to numerically solve the equation of motion. The study of the theory led to understand that there are valid solutions depending on a wide variety of initial values given. Thus, we have a lot of different solutions that converge to the same state in the distant future, and the anisotropies, as expected, tend to disappear times after the Big Bang.