Nuevos modelos asociados a sistemas gravitacionales extremos

Résumé

A large number of classes of modified theories of gravity have been developed for a long time. They have attracted much attention from physicists, since they show different aspects concerning gravitational interaction. In fact, these aspects may extend the role of gravity not only at large scales but at microscopic regimes, so that they have been systematically related to fundamental issues such as the occurrence of space-time singularities, the loss of renormalizability or the origin of the accelerated expansion of the universe, among others. Despite the successful predictions and the highly tested accuracy of General Relativity (GR) in describing the gravitational phenomena, the absence of an appropriate explanation for these issues has stimulated the investigation of new alternative models of gravitation. The extension of the conventional approach can be addressed by the introduction into the gravitational action of higher order corrections depending on the metric tensor alone. Such a procedure preserves the geometric structure of the space-time and potentially yields new propagating degrees of freedom related to metric, which means that not only the phenomenological compatibility with GR must be considered by the new framework but also the viability of its stability conditions. On the other hand, it is also possible to define a more complex geometry by the modification of the affine connection. Namely, the Levi-Civita connection of GR is subject to the fulfillment of two independent constraints: the conservation of the metric tensor under parallel transport and the vanishing of its antisymmetric component. Hence, in this case there is an increase in the number of degrees of freedom contained in the connection, which can involve new geometrical and dynamical effects in the space-time. From a theoretical point of view, the resulting post-Riemannian geometry can be related to the existence of a new fundamental symmetry in nature by applying the gauge principles. This scheme leads to the appearance of new theories of gravitation, such as the Metric-Affine or the Poincaré Gauge theory. In the present thesis, we use these notions to investigate the nature and the implications of the space-time torsion in the framework of the Poincaré Gauge theory, providing new bases and methodologies to describe and measure its possible existence in the universe. Since this quantity appears to be directly connected to the intrinsic angular momentum of elementary particles, it is expected to generate negligible effects at macroscopic scales. Therefore, the focusing on extreme gravitational systems that may intensify such effects is especially relevant to overcome these observational issues.