Giulia Cusin

Gravitational waves in bigravity cosmology

In this paper we study gravitational wave perturbations in a cosmological setting of bigravity which can reproduce the ΛCDM background and large scale structure. We show that in general gravitational wave perturbations are unstable and only for very fine tuned initial conditions such a cosmology is viable. We quantify this fine tuning. We argue that similar fine tuning is also required in the scalar sector in order to prevent the tensor instability to be induced by second order scalar perturbations. Finally, we show that due to this power law instability, models of bigravity can lead to a large tensor to scalar ratio even for low scale inflation.

Non-local formulation of ghost-free bigravity theory

A bstractWe study the ghost-free bimetric theory of Hassan and Rosen, with parameters βi such that a flat Minkowski solution exists for both metrics. We show that, expanding around this solution and eliminating one of the two metrics with its own equation of motion, the remaining metric is governed by the Einstein-Hilbert action plus a non-local term proportional to Wμνρσ (□ − m2)−1Wμνρσ, where Wμνρσ is the Weyl tensor. The result is valid to quadratic order in the metric perturbation and to all orders in the derivative expansion. This example shows, in a simple setting, how such non-local extensions of GR can emerge from an underlying consistent theory, at the purely classical level.

Nonlocal gravity with a Weyl-square term

Recent work has shown that modifications of General Relativity based on the addition to the action of a non-local term

Conformal symmetry and nonlinear extensions of nonlocal gravity

R\,\Box^{-2}R

Inflationary perturbations in bimetric gravity

, or on the addition to the equations of motion of a term involving

A general mass term for bigravity

(g_{\mu\nu}\Box^{-1} R)

Anisotropy of the astrophysical gravitational wave background: Analytic expression of the angular power spectrum and correlation with cosmological observations

, produce dynamical models of dark energy which are cosmologically viable both at the background level and at the level of cosmological perturbations. We explore a more general class of models based on the addition to the action of terms proportional to

First Predictions of the Angular Power Spectrum of the Astrophysical Gravitational Wave Background

R_{\mu\nu}\,\Box^{-2}R^{\mu\nu}

Nonlinear effective theory of dark energy

and

Dark energy and modified gravity in the Effective Field Theory of Large-Scale Structure

C_{\mu\nu\rho\sigma}\, \Box^{-2}C^{\mu\nu\rho\sigma}