8,467 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
Quasi Non-linear Evolution of Stochastic Bias
It is generally believed that the spatial distribution of galaxies does not
trace that of the total mass. The understanding of the bias effect is therefore
necessary to determine the cosmological parameters and the primordial density
fluctuation spectrum from the galaxy survey. The deterministic description of
bias may not be appropriate because of the various stochasticity of galaxy
formation process. In nature, the biasing is epoch dependent and recent deep
survey of the galaxy shows the large biasing at high redshift. Hence, we
investigate quasi non-linear evolution of the stochastic bias by using the tree
level perturbation method. Especially, the influence of the initial cross
correlation on the evolution of the skewness and the bi-spectrum is examined in
detail. We find that the non-linear bias can be generated dynamically. The
small value of the initial cross correlation can bend the \dg-\dm relation
effectively and easily lead to the negative curvature (). We also
propose a method to predict the bias, cross correlation and skewness at high
redshift. As an illustration, the possibility of the large biasing at high
redshift is discussed. Provided the present bias parameter as and
, we predict the large scale bias as at by fitting
the bi-spectrum to the Lick catalog data. Our results will be important for the
future deep sky survey.Comment: 20 pages, 5 Encapsulated Postscript figures, aastex, final version to
appear in Ap
Scalar perturbations in braneworld cosmology
We study the behaviour of scalar perturbations in the radiation-dominated era
of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk
and brane master wave equations. We find that density perturbations with
wavelengths less than a critical value (set by the bulk curvature length) are
amplified during horizon re-entry. This means that the radiation era matter
power spectrum will be at least an order of magnitude larger than the
predictions of general relativity (GR) on small scales. Conversely, we
explicitly confirm from simulations that the spectrum is identical to GR on
large scales. Although this magnification is not relevant for the cosmic
microwave background or measurements of large scale structure, it will have
some bearing on the formation of primordial black holes in Randall-Sundrum
models.Comment: 17 pages, 7 figure
Assessing non-linear models for galaxy clustering III: Theoretical accuracy for Stage IV surveys
We provide in depth MCMC comparisons of two different models for the halo
redshift space power spectrum, namely a variant of the commonly applied
Taruya-Nishimichi-Saito (TNS) model and an effective field theory of large
scale structure (EFTofLSS) inspired model. Using many simulation realisations
and Stage IV survey-like specifications for the covariance matrix, we check
each model's range of validity by testing for bias in the recovery of the
fiducial growth rate of structure formation. The robustness of the determined
range of validity is then tested by performing additional MCMC analyses using
higher order multipoles, a larger survey volume and a more highly biased tracer
catalogue. We find that under all tests, the TNS model's range of validity
remains robust and is found to be much higher than previous estimates. The
EFTofLSS model fails to capture the spectra for highly biased tracers as well
as becoming biased at higher wavenumbers when considering a very large survey
volume. Further, we find that the marginalised constraints on for all
analyses are stronger when using the TNS model.Comment: 25 pages, 19 figures. Accepted version for publication in JCA
Quantum-mechanical generation of gravitational waves in braneworld
We study the quantum-mechanical generation of gravitational waves during
inflation on a brane embedded in a five-dimensional anti-de Sitter bulk. To
make the problem well-posed, we consider the setup in which both initial and
final phases are given by a de Sitter brane with different values of the Hubble
expansion rate. Assuming that the quantum state is in a de Sitter invariant
vacuum in the initial de Sitter phase, we numerically evaluate the amplitude of
quantum fluctuations of the growing solution of the zero mode in the final de
Sitter phase. We find that the vacuum fluctuations of the initial Kaluza-Klein
gravitons as well as of the zero mode gravitons contribute to the final
amplitude of the zero mode on small scales, and the power spectrum is quite
well approximated by what we call the rescaled spectrum, which is obtained by
rescaling the standard four-dimensional calculation following a simple mapping
rule. Our results confirm the speculation raised in Ref.
\cite{Kobayashi:2003cn} before.Comment: 11 pages, 11 figure
Inflaton perturbations in brane-world cosmology with induced gravity
We study cosmological perturbations in the brane models with an induced
Einstein-Hilbert term on a brane. We consider an inflaton confined to a de
Sitter brane in a five-dimensional Minkowski spacetime. Inflaton fluctuations
excite Kaluza-Klein modes of bulk metric perturbations with mass and where is an
integer. There are two branches ( branches) of solutions for the
background spacetime. In the branch, which includes the self-accelerating
universe, a resonance appears for a mode with due to a spin-0
perturbation with . The self-accelerating universe has a distinct
feature because there is also a helicity-0 mode of spin-2 perturbations with
. In the branch, which can be thought as the Randall-Sundrum
type brane-world with the high energy quantum corrections, there is no
resonance. At high energies, we analytically confirm that four-dimensional
Einstein gravity is recovered, which is related to the disappearance of van
Dam-Veltman-Zakharov discontinuity in de Sitter spacetime.
On sufficiently small scales, we confirm that the lineariaed gravity on the
brane is well described by the Brans-Dicke theory with in
branch and in branch, respectively, which confirms the
existence of the ghost in branch. We also study large scale perturbations.
In branch, the resonance induces a non-trivial anisotropic stress on the
brane via the projection of Weyl tensor in the bulk, but no instability is
shown to exist on the brane.Comment: 20 pages, 4 figure
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
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