Gravitational waves from compact objects – a tool for testing strong gravity and nuclear matter at extreme densities

11/09/2019 to 10/09/2025

Gravity plays a central role in almost every grant challenge in astrophysics, cosmology and fundamental physics making it a subject to a strong interdisciplinarity. Recently there was a major breakthrough in the efforts to test the predictions of general relativity, namely the direct detection of gravitational waves. In the following years, an unprecedented view of previously invisible parts of the Universe will be opened through gravitational wave observations of mergers of the most compact objects known to exist in the universe - the neutron stars and the black holes.

On another front, the accelerated expansion of the Universe, that is one of the most important open problems in physics nowadays, can be attributed to our lack of understanding of gravity on astrophysical and cosmological scales where the pure general relativity breaks down giving rise to a more complete theory of gravity. Modifications of Einstein's theory are predicted as well from the theories trying to unify all the interaction or the quantum corrections in the strong gravity regime. Moreover, general relativity is indeed tested with remarkable accuracy but only in the weak field regime, while the strong field regime is poorly explored and constrained.

The aim of the current project is to perform a systematic effort in exploring the gravitational wave emission from merging neutron stars and black holes in generalized theories of gravity covering almost all stages of the merger: the inspiral, the post-merger phase and in some cases - the gravitational wave afterglow. We will cover a wide class of alternative theories of gravity that are theoretically well motivated and in agreement with the observations. This would be the first study of this magnitude since up to now mainly isolated results of some alternative theories of gravity or of certain mechanisms of gravitational wave emission were obtained.

Using the above results as a basis, the second extremely important goal would be to break the degeneracy between equation of state uncertainties and uncertainties in the underlying gravitational theory for the gravitational wave observations of merging neutron stars. Most of the observational constraints on the equation of state, though, implicitly assume pure general relativity as the underlying theory of gravity. This poses serious difficulties in testing the alternative theories of gravity. Since one of the main goals of the future gravitational wave observations of merging neutron stars is to impose constraints on the behavior of matter at extreme densities, developing a methodology that is independent of the underlying theory of gravity is of great importance.