Modified gravity : cosmological tests

Abstract

The formulation of General Relativity revolutionised the principles to understand the mechanics of gravitation. Under this viewpoint, gravity was no longer a force as postulated by Newton but a manifestation of curvature of the unification of space and time, spacetime. Despite its success, General Relativity started to be incompatible with observations as it required the introduction of dark matter and dark energy. Furthermore, General Relativity is a classical theory and is not renormalisable. Modifications to General Relativity have therefore been proposed. In this work, an alternative proposal, known as Teleparallel gravity, is investigated. This formulation reintroduces gravity as a force by replacing curvature of spacetime through torsion. Furthermore, the theory can reproduce the field equations of General Relativity, referred to as Teleparallel Equivalent of General Relativity, and hence can be deemed as an alternative description of gravitation. Throughout this work, various teleparallel gravity models are studied under different topics: (i) stability of the Friedmann-Lemaître-Robertson-Walker geometry through homogeneous and isotropic perturbations, (ii) reconstruction of the gravitational Lagrangian, (iii) gravitational waves, and (iv) large scale structure. It is shown that this formulation is capable of accounting for the accelerating universe without invoking dark energy, while also being stable under homogeneous perturbations, hosting various expansion histories, agreeing with the observed predictions of gravitational waves, while also being able to correctly generate the observed large scale structure.