Hunting modifications of gravity

: from the lab to cosmology via compact objects

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 con-
fronted 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.

Date of Award	7 Oct 2016
Original language	English
Awarding Institution	University of Namur
Supervisor	Andre Fuzfa (Supervisor), Christophe Ringeval (Co-Supervisor), Anne Lemaitre (President), Ruth Dürrer (Jury) & David F. Mota (Jury)