M. Crisostomi - Massive Gravity (PhD thesis defence)

by Marco Crisostomi (AQ)

Wednesday 2 Apr 2014, 10:00 → 11:30 Europe/Rome

Pontecorvo room (LNGS)

PhD thesis defense
Marco Crisostomi from University of L'Aquila and Paris 7 (co-tutorship) will defend his PhD thesis in theoretical physics.

Title: Massive Gravity

Abstract
A study of modifications of General Relativity (GR) at large distances is presented . GR is the unique theory of a self interacting massless spin two field, the graviton; our study considers the case where the spin two field is instead massive. For a long time, it was believed that  such kind of theories were inconsistent due to a ghost instability that generically emerges at nonlinear level. Only recently this problem has been overcome and several modifications that evade this instability have been found. The simplest version of massive gravity theory consists in adding to the Einstein-Hilbert action a potential built from non-derivative self-couplings of the metric. As matter of fact such modification needs additional structure besides the metric. We present the canonical analysis for a general massive deformation of GR in a fully non-perturbative and background independent way and show the conditions that a potential has to satisfy in order to avoid such a instability. Among the possible candidates, we find that Lorentz invariance considerably restricts the viable potentials and, when Minkowski space is required as a background, they probably reduce to a single one.
For this potential we analyze the basic phenomenology in the framework of bigravity theories, where the extra metric needed is taken dynamical. For spherically  symmetric solutions, though  a discontinuity in the zero mass limit is present at  linear level, we show that GR  can be recovered close to the source, via the Vainshtein mechanism. Far from the source the static potential has Yukawa behaviour.
The study of cosmological solutions and their linear perturbations is also discussed. In particular, FRW solutions exist and are very close to the ones in GR at early time; at late time the universe flows in a dS phase. Unfortunately, scalar cosmological perturbations show an exponential instability that signals the unreliability of the standard perturbation theory in sharp contrast to GR.


Committee
E. Copeland, University of Nottingham
K. Koyama, University of Porthsmouth
M. Maggiore, University of Geneva

Referees
S. F. Hassan, University of Stockolm
A. Padilla, University of Nottingham

Advisor L. Pilo, University of L'Aquila
co-Advisor C. Charmousis, Laboratoire de Physique Theorique, Orsay