31 research outputs found
Generalising the matter coupling in massive gravity: a search for new interactions
Massive gravity theory introduced by de Rham, Gabadadze, Tolley (dRGT) is
restricted by several uniqueness theorems that protect the form of the
potential and kinetic terms, as well as the matter coupling. These restrictions
arise from the requirement that the degrees of freedom match the expectation
from Poincar\'e representations of a spin--2 field. Any modification beyond the
dRGT form is known to invalidate a constraint that the theory enjoys and revive
a dangerous sixth mode. One loophole is to exploit the effective nature of the
theory by pushing the sixth mode beyond the strong coupling scale without
completely removing it. In this paper, we search for modifications to dRGT
action by coupling the matter sector to an arbitrary metric constructed out of
the already existing degrees of freedom in the dRGT action. We formulate the
conditions that such an extension should satisfy in order to prevent the sixth
mode from contaminating the effective theory. Our approach provides a new
perspective for the "composite coupling" which emerges as the unique extension
up to four-point interactions.Comment: 19 pages; v2: new references added, accepted for publication in PR
Horava-Lifshitz gravity with
In the framework of the power-counting renormalizable theory of gravitation,
recently proposed by Ho\v{r}ava, we study the limit , which
is arguably the most natural candidate for the ultraviolet fixed point of the
renormalization group flow. In the projectable version with dynamical critical
exponent , we analyze the Friedmann-Robertson-Walker background driven by
the so-called "dark matter as integration constant" and perturbations around
it. We show that amplitudes of quantum fluctuations for both scalar and tensor
gravitons remain finite in the limit and that the theory is weakly coupled
under a certain condition
Stable cosmology in ghost-free quasidilaton theory
We present a novel cosmological solution in the framework of extended
quasidilaton theory which underwent scrutiny recently. We only consider terms
that do not generate the Boulware-Deser degree of freedom, hence the
"ghost-free" quasidilaton theory, and show three new branches of cosmological
evolution therein. One of the solutions passes the perturbative stability
tests. This new solution exhibits a late time self-acceleration and all
graviton polarizations acquire masses that converge to a constant in the
asymptotic future. Moreover, all modes propagate at the speed of light. We
propose that this solution can be used as a benchmark model for future
phenomenological studies.Comment: 12 page
Role of matter in extended quasidilaton massive gravity
The extended quasidilaton theory is one of the simplest Lorentz-invariant
massive gravity theories which can accommodate a stable self-accelerating
vacuum solution. In this paper we revisit this theory and study the effect of
matter fields. For a matter sector that couples minimally to the physical
metric, we find hints of a Jeans type instability in the IR. In the analogue
k-essence field set-up, this instability manifests itself as an IR ghost for
the scalar field perturbation, but this can be interpreted as a classical
instability that becomes relevant below some momentum scale in terms of matter
density perturbations. We also consider the effect of the background evolution
influenced by matter on the stability of the gravity sector perturbations. In
particular, we address the previous claims of ghost instability in the IR
around the late time attractor. We show that, although the matter-induced
modification of the evolution potentially brings tension to the stability
conditions, one goes beyond the regime of validity of the effective theory well
before the solutions become unstable. We also draw attention to the fact that
the IR stability conditions are also enforced by the existence requirements of
consistent background solutions.Comment: 17 pages, accepted for publication in PR
On the viability of bigravity cosmology
We revisit the question of viability of bigravity cosmology as a candidate
for dark energy. In the context of the low energy limit model, where matter
couples to a single metric, we study linear perturbations around homogeneous
and isotropic backgrounds to derive the Poisson's equation for the Newtonian
potential. Extending to second order perturbations, we identify the Vainshtein
radius below which non-linear scalar self interactions conspire to reproduce GR
on local scales. We combine all of these results to determine the parameter
space that allows a late time de-Sitter attractor compatible with observations
and a successful Vainsthein mechanism. We find that the requirement on having a
successful Vainsthein mechanism is not compatible with the existence of
cosmological solutions at early times.Comment: Accepted for publication in PR
Massive gravity: nonlinear instability of the homogeneous and isotropic universe
We argue that all homogeneous and isotropic solutions in nonlinear massive
gravity are unstable. For this purpose, we study the propagating modes on a
Bianchi type--I manifold. We analyze their kinetic terms and dispersion
relations as the background manifold approaches the homogeneous and isotropic
limit. We show that in this limit, at least one ghost always exists and that
its frequency tends to vanish for large scales, meaning that it cannot be
integrated out from the low energy effective theory. This ghost mode is
interpreted as a leading nonlinear perturbation around a homogeneous and
isotropic background.Comment: 4 pages, uses REVTeX4.1; v2: minor update to match the published
versio
Cosmology in bimetric theory with an effective composite coupling to matter
We study the cosmology of bimetric theory with a composite matter coupling.
We find two possible branches of background evolution. We investigate the
question of stability of cosmological perturbations. For the tensor and vector
perturbations, we derive conditions on the absence of ghost and gradient
instabilities. For the scalar modes, we obtain conditions for avoiding ghost
degrees. In the first branch, we find that one of the scalar modes becomes a
ghost at the late stages of the evolution. Conversely, this problem can be
avoided in the second branch. However, we also find that the constraint for the
second branch prevents the doubly coupled matter fields from being the standard
ingredients of cosmology. We thus conclude that a realistic and stable
cosmological model requires additional minimally coupled matter fields.Comment: 22 page