On 18 August at 14:00 Ulbossyn Ualikhanova will defend her doctoral thesis “Gravity theories based on torsion: theoretical and observational constraints” for obtaining the degree of Doctor of Philosophy (in Physics).
Assoc. Prof. Manuel Hohmann, University of Tartu
Assoc. Prof. Laur Järv, University of Tartu
Dr. Alexey Golovnev, British University in Egypt, Cairo (Egypt)
Dr. Jose Beltrán Jiménez, University of Salamanca (Spain)
Although General Relativity (GR) has been very successful in describing a wide range of phenomena, by itself does not provide any explanation for a number of observations in modern cosmology: the homogeneity of the cosmic microwave background, the accelerating expansion of the universe, the origin of structure and the motion of galaxies. These observations can be explained by introducing inflation, dark energy and dark matter, but the precise nature of these has remained unknown. These reasons have motivated the study of a large number of possible modifications of GR. However, in this thesis we do not study the modifications of GR itself, but a number of theories that differ from it by the underlying geometric description. Instead of curvature, these teleparallel theories employ torsion or nonmetricity as the mediator of gravity. The aim of the thesis is to check the viability of these theories. An important theoretical issue in the mentioned geometries is local Lorentz invariance. In torsion-based theories, this issue is addressed in their covariant formulation, which restores the Lorenz invariance by introducing a corresponding gauge field. In particular, we present the covariant formulation of scalartorsion gravity. Then we study the gravitational wave (GW) propagation in two general families of teleparallel gravity theories, which are based on different geometric interpretations of gravity. Our results show that all GW modes propagate at the speed of light and there are up to 6 possible polarizations. We also derive the post-Newtonian limit of a general class of teleparallel gravity theories. Our results show that the class of theories we consider is fully conservative, which means that there are no violation of local Lorentz invariance, local position invariance or total energy-momentum conservation. Finally, we use the dynamical systems to describe a wide range of phenomena in cosmology, like acceleration, phantom dark energy and finite time singularities in f(T) theories.