String-inspired cosmologies with Torsion

This thesis investigates two gravitational models with torsion: the Einstein-Cartan-Sciamma-Kibble (ECSK) theory and a low-energy effective string-inspired approach.In the context of the latter, this assignment explores the effects of torsion contribution in the cosmological evolution of the universe.Starting with the ECSK theory, torsion emerges as a geometrical property of spacetime, where it couples to matter spin.Quantum anomaly corrections to torsion conservation induce an axion-like degree of freedom, and spinspin interactions become significant at high spin densities, replacing the Big Bang singularity with a Big Bounce scenario.In the string-inspired model, which is the primary focus of this thesis, torsion arises as the field strength of the KalbRamond (KR) antisymmetric tensor.Through the cancellation of gravitational anomalies in the early universe, an axion-like pseudoscalar field is naturally introduced, coupling to gravity via a ChernSimons term.This coupling modifies the Einstein field equations by introducing a Cotton tensor, leading to parity-violating corrections in the propagation of gravitational waves in a perturbed FLRW universe.These perturbations are connected to primordial gravitational waves in this model (rising in the context of a first hill-top inflationary scenario).Finally, the quantum expectation value of the anomaly-induced condensate contributes a term analogous to H 4 during the stiff-matter era, when the kinetic term of the axion-like field dominates, leading to an inflationary phase similar to the Running Vacuum Model (RVM).During inflation, the anomaly condensate remains approximately constant, resulting in a quaside Sitterlike expansion.This effective energy density can be described by a nearly linear potential arising from shift-symmetry breaking and can be interpreted as a positive cosmological constant driving the accelerated expansion.