Hunting modifications of gravity: from the lab to cosmology via compact objects

TL;DR

This paper explores how modified gravity theories, especially Horndeski and Galileon models, can be tested across cosmological, astrophysical, and laboratory scales, focusing on screening mechanisms and observational constraints.

Contribution

It provides a comprehensive analysis of the observational predictions of Horndeski and Galileon gravity models at multiple scales, including numerical simulations and astrophysical implications.

Findings

Chameleon screening tested with atom interferometry.

Variation of gravitational coupling inside compact stars analyzed.

Potential viability of the Fab Four model for inflation discussed.

Abstract

Modifications of gravity have been considered to model the primordial inflation and the late-time cosmic acceleration. Provided that modified gravity models do not suffer from theoretical instabilities, they must be confronted with observations, not only at the cosmological scales, but also with the local tests of gravity, in the lab and in the Solar System, as well as at the astrophysical scales. Considering in particular sub-classes of the Horndeski gravity, we study their observational predictions at different scales. In order to pass the local tests of gravity while allowing for long-range interactions in cosmology, Horndeski gravity exhibits screening mechanisms, among them the chameleon. The chameleon screening mechanism has been tested recently using atom interferometry in a vacuum chamber. Numerical simulations are provided in this thesis in order to refine the analytical predictions. At the astrophysical scale, Horndeski gravity predicts a variation of the gravitational coupling inside compact stars. Focusing on Higgs inflation, we discuss to what extent the Higgs vacuum expectation value varies inside stars and conclude whether the effect is detectable in gravitational and nuclear physics. Finally, the covariant Galileon model exhibits non-linearities in the scalar field kinetic term such that it might pass the local tests of gravity thanks to the Vainshtein screening mechanism. We discuss if a sub-class of the covariant Galileon theory dubbed the Fab Four model leads to a viable inflationary phase and provide combined analysis with neutron stars and Solar System observables.