Lectures / | cosmograv4

Lecture 1. Jutta Kunz

Lectures slides available here: Coming soon

Compact astrophysical objects such as black holes and neutron stars are perfect probes to study strong gravitational fields and thus to test Einstein's theory of general relativity and its contenders by comparing their predictions with observations. In particular, alternative theories of gravity may lead to distinctivefeatures of black holes and neutron stars, like various types of hair or spontaneous scalarization, and they will modify the spectrum of gravitational waves emitted by these compact objects when colliding. Whereas the analysis of the properties of black holes may allow for a direct conclusion concerning different gravity theories, the analysis of the properties of neutron stars involves as an intermediate step the use of universal relations, in order to obtain (to a large extent) independence of the still unknown neutron star equation of state. In these lectures I will first discuss neutron stars and then black holes, recalling some of their properties in general relativity and highlighting next some of the new features in various metric theories of gravity. These features include their multipoles, quasi-normal modes and black hole shadows.

Lecture 2. Jackson Said Levi

Lectures slides available here: Coming soon

Email: jsaid01@um.edu.mt

Standard gravity expresses gravitational effects by curving spacetime about matter. It does this through the Levi-Civita connection which is what produces the curvature feature. However, this is not the only choice, there also exists the teleparallel connection through which gravitation exhibits a torsional behaviour. Teleparallel gravity can be constructed to be dynamically equivalent to regular general relativity (GR) (that is, produce equal field equations), known as the teleparallel equivalent to general relativity (TEGR). As in standard GR, one can modify the gravitational theory and consider beyond GR theories. While GR and TEGR are equivalent, their modifications are not for the most part. In this way, modifications of TEGR are novel and have shown to produce behaviour different to the traditional modifications of gravity. In cosmology, these proposals have shown to produce models beyond GR that can compete with GR in a number of important ways that range from their gravitational wave polarization signatures to their ability to produce the correct expansion dynamics across the cosmological history of the Universe, among others. In this presentation, we will cover the salient features of teleparallel gravity and its key observational predictions in the cosmological regime. We will discuss the confrontation of teleparallel models with the most recent observational surveys and the corresponding constraints on the corresponding beyond GR models. We will close with a discussion on areas to expect the theory to be further developed in and where it can become even more competitive with GR in the coming years.

Further reference: Teleparallel Theories of Gravity: Illuminating a Fully Invariant Approach