Constraining cosmological modifications of gravity using compact objects

For more than twenty years, we have known that the expansion of the universe is accelerating. This requires that the majority of the energy of the universe is in some form of matter with negative pressure – vacuum energy according to the ΛCDM model of cosmology. Could it be something else?

One of the candidates for dark energy is a modification of gravity, a deviation from General Relativity. On the largest – super-galactic – scales gravity is not well constrained, so General Relativity might not be a good description of the dynamics there. On the other hand, it is well established that attempting to change the large-scale dynamics, usually ends up significantly changing small-scale physics e.g. around stars.

In particular, the modification of gravity can often be described as medium in which small fluctuations – e.g. gravitational waves – propagate. This medium also provides a boundary condition far from any source which the gravitational field around any object must smoothly connect to and which is not asymptotically flat. For example, black holes are not necessarily Schwarzschild, but have „hair“. Thus a large-scale change seems to lead to a modification of the solutions which should be considered when a star is embedded in cosmology.

This project is aimed at understanding how best to describe such compact object solutions which can be embedded in an evolving cosmology and therefore determine how well observations of gravitational waves can constrain modifications of gravity relevant to cosmology. There is space here both for a student with a preference for theory, as well as one with a preference for numerical explorations and modelling.

References:

[1] I. Saltas , I. Sawicki, L. Amendola, M. Kunz, Anisotropic Stress as a Signature of Nonstandard Propagation of Gravitational Waves, Phys.Rev.Lett. 113 (2014) 19, 191101 https://arxiv.org/abs/1406.7139
[2] E. Babichev and G. Esposito-Farese, Time-Dependent Spherically Symmetric Covariant Galileons, Phys.Rev.D 87 (2013) 044032, https://arxiv.org/abs/1212.1394
[3] T. Sotiriou and S.-Y. Zhou, Black hole hair in generalized scalar-tensor gravity: An explicit example, Phys.Rev.D 90 (2014) 124063, https://arxiv.org/abs/1408.1698