96 research outputs found
On scale-free extensions of massive (bi-)gravity
We discuss a scale-free model of bigravity, in which the mass parameter of
the standard bigravity potential is promoted to a dynamical scalar field. This
modification retains the ghost-free bigravity structure, in particular it
remains free of the Boulware-Deser ghost. We investigate the theory's
interaction structure, focusing on its consistent scaling limits and strong
coupling scales. Furthermore we explore the model's quadratic action, both
around generic background configurations and paying special attention to
cosmological backgrounds and to the associated background evolution. Finally we
consider the possibility of realizing a phase of late-time acceleration as well
as a quasi-de Sitter inflationary stage at early times, when the promoted "mass
scalar" becomes the inflaton.Comment: 36 pages; v2 clarifying comments added, references updated, results
unchange
Non-local formulation of ghost-free bigravity theory
We study the ghost-free bimetric theory of Hassan and Rosen, with parameters
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
, where
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.Comment: 16 page
Anisotropy of the astrophysical gravitational wave background: analytic expression of the angular power spectrum and correlation with cosmological observations
Unresolved and resolved sources of gravitational waves are at the origin of a
stochastic gravitational wave background. While the computation of its mean
density as a function of frequency in a homogeneous and isotropic universe is
standard lore, the computation of its anisotropies requires to understand the
coarse graining from local systems, to galactic scales and then to cosmology.
An expression of the gravitational wave energy density valid in any general
spacetime is derived. It is then specialized to a perturbed
Friedmann-Lema\^itre spacetime in order to determine the angular power spectrum
of this stochastic background as well as its correlation with other
cosmological probes, such as the galaxy number counts and weak lensing. Our
result for the angular power spectrum also provides an expression for the
variance of the gravitational wave background.Comment: 22 pages, 2 figure
Conformal symmetry and nonlinear extensions of nonlocal gravity
We study two nonlinear extensions of the nonlocal gravity
theory. We extend this theory in two different ways suggested by conformal
symmetry, either replacing with , which is the
operator that enters the action for a conformally-coupled scalar field, or
replacing with the inverse of the Paneitz operator, which is a
four-derivative operator that enters in the effective action induced by the
conformal anomaly. We show that the former modification gives an interesting
and viable cosmological model, with a dark energy equation of state today
, which very closely mimics CDM and evolves
asymptotically into a de Sitter solution. The model based on the Paneitz
operator seems instead excluded by the comparison with observations. We also
review some issues about the causality of nonlocal theories, and we point out
that these nonlocal models can be modified so to nicely interpolate between
Starobinski inflation in the primordial universe and accelerated expansion in
the recent epoch.Comment: 27 pages, 4 figure
Non-local 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 , or on the addition
to the equations of motion of a term involving ,
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 and , where is the Weyl
tensor. We find that the term does not give a
viable background evolution. The non-local Weyl-square term, in contrast, does
not contribute to the background evolution but we find that, at the level of
cosmological perturbations, it gives instabilities in the tensor sector. Thus,
only non-local terms which depend just on the Ricci scalar appear to be
cosmologically viable. We discuss how these results can provide a hint for the
mechanism that might generate these effective non-local terms from a
fundamental local theory.Comment: 25 pages, 6 figures. v2: the version to appear in PR
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