64 research outputs found
Gravity Waves Signatures from Anisotropic pre-Inflation
We show that expanding or contracting Kasner universes are unstable due to
the amplification of gravitational waves (GW). As an application of this
general relativity effect, we consider a pre-inflationary anisotropic geometry
characterized by a Kasner-like expansion, which is driven dynamically towards
inflation by a scalar field. We investigate the evolution of linear metric
fluctuations around this background, and calculate the amplification of the
long-wavelength GW of a certain polarization during the anisotropic expansion
(this effect is absent for another GW polarization, and for scalar
fluctuations). These GW are superimposed to the usual tensor modes of quantum
origin from inflation, and are potentially observable if the total number of
inflationary e-folds exceeds the minimum required to homogenize the observable
universe only by a small margin. Their contribution to the temperature
anisotropy angular power spectrum decreases with the multipole l as l^(-p),
where p depends on the slope of the initial GW power-spectrum. Constraints on
the long-wavelength GW can be translated into limits on the total duration of
inflation and the initial GW amplitude. The instability of classical GW (and
zero-vacuum fluctuations of gravitons) during Kasner-like expansion (or
contraction) may have other interesting applications. In particular, if GW
become non-linear, they can significantly alter the geometry before the onset
of inflation
Anisotropic Inflation with Non-Abelian Gauge Kinetic Function
We study an anisotropic inflation model with a gauge kinetic function for a
non-abelian gauge field. We find that, in contrast to abelian models, the
anisotropy can be either a prolate or an oblate type, which could lead to a
different prediction from abelian models for the statistical anisotropy in the
power spectrum of cosmological fluctuations. During a reheating phase, we find
chaotic behaviour of the non-abelian gauge field which is caused by the
nonlinear self-coupling of the gauge field. We compute a Lyapunov exponent of
the chaos which turns out to be uncorrelated with the anisotropy.Comment: 16 pages, 4 figure
New symmetries in Fierz-Pauli massive gravity
We expose a new symmetry for linear perturbations around a solution of
non-linear Fierz-Pauli massive gravity plus a bare cosmological constant. The
cosmological constant is chosen such that the background metric is flat while
the Stuckelberg fields have a non-trivial profile. Around this background, at
linear order the new symmetry reduces the propagating degrees of freedom to
those of General Relativity, namely the massless helicity 2 modes only. We
discuss the physical consequences and possible applications of these findings.Comment: 9 pages, no figure
Gravitational Field Equations on and off a 3-Brane World
The effective gravitational field equations on and off a 3-brane world
possessing a Z_{2} mirror symmetry and embedded in a five-dimensional bulk
spacetime with cosmological constant were derived by Shiromizu, Maeda and
Sasaki (SMS) in the framework of the Gauss-Codazzi projective approach with the
subsequent specialization to the Gaussian normal coordinates in the
neighborhood of the brane. However, the Gaussian normal coordinates imply a
very special slicing of spacetime and clearly, the consistent analysis of the
brane dynamics would benefit from complete freedom in the slicing of spacetime,
pushing the layer surfaces in the fifth dimension at any rates of evolution and
in arbitrary positions. We generalize the SMS effective field equations on and
off a 3-brane to the case where there is an arbitrary energy-momentum tensor in
the bulk. We use a more general setting to allow for acceleration of the
normals to the brane surface through the lapse function and the shift vector in
the spirit of Arnowitt, Deser and Misner. We show that the gravitational
influence of the bulk spacetime on the brane may be described by a traceless
second-rank tensor W_{ij}, constructed from the "electric" part of the bulk
Riemann tensor. We also present the evolution equations for the tensor W_{ij},
as well as for the corresponding "magnetic" part of the bulk curvature. These
equations involve the terms determined by both the nonvanishing acceleration of
normals in the nongeodesic slicing of spacetime and the presence of other
fields in the bulk.Comment: 22 pages, REVTEX
Cosmological perturbations in Massive Gravity and the Higuchi bound
In de Sitter spacetime there exists an absolute minimum for the mass of a
spin-2 field set by the Higuchi bound m^2 \geq 2H^2. We generalize this bound
to arbitrary spatially flat FRW geometries in the context of the recently
proposed ghost-free models of Massive Gravity with an FRW reference metric, by
performing a Hamiltonian analysis for cosmological perturbations. We find that
the bound generically indicates that spatially flat FRW solutions in FRW
massive gravity, which exhibit a Vainshtein mechanism in the background as
required by consistency with observations, imply that the helicity zero mode is
a ghost. In contradistinction to previous works, the tension between the
Higuchi bound and the Vainshtein mechanism is equally strong regardless of the
equation of state for matter.Comment: 24 pages, typos and conventions correcte
Bounce and cyclic cosmology in extended nonlinear massive gravity
We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA
Cosmological Solutions in Bimetric Gravity and their Observational Tests
We obtain the general cosmological evolution equations for a classically
consistent theory of bimetric gravity. Their analytic solutions are
demonstrated to generically allow for a cosmic evolution starting out from a
matter dominated FLRW universe while relaxing towards a de Sitter (anti-de
Sitter) phase at late cosmic time. In particular, we examine a subclass of
models which contain solutions that are able to reproduce the expansion history
of the cosmic concordance model inspite of the nonlinear couplings of the two
metrics. This is demonstrated explicitly by fitting these models to
observational data from Type Ia supernovae, Cosmic Microwave Background and
Baryon Acoustic Oscillations.Comment: Latex, 26 pages. References added and minor revision of introduction
and appendix
Non-detection of a statistically anisotropic power spectrum in large-scale structure
We search a sample of photometric luminous red galaxies (LRGs) measured by
the Sloan Digital Sky Survey (SDSS) for a quadrupolar anisotropy in the
primordial power spectrum, in which P(\vec{k}) is an isotropic power spectrum
P(k) multiplied by a quadrupolar modulation pattern. We first place limits on
the 5 coefficients of a general quadrupole anisotropy. We also consider
axisymmetric quadrupoles of the form P(\vec{k}) = P(k){1 +
g_*[(\hat{k}\cdot\hat{n})^2-1/3]} where \hat{n} is the axis of the anisotropy.
When we force the symmetry axis \hat{n} to be in the direction (l,b)=(94
degrees,26 degrees) identified in the recent Groeneboom et al. analysis of the
cosmic microwave background, we find g_*=0.006+/-0.036 (1 sigma). With uniform
priors on \hat{n} and g_* we find that -0.41<g_*<+0.38 with 95% probability,
with the wide range due mainly to the large uncertainty of asymmetries aligned
with the Galactic Plane. In none of these three analyses do we detect evidence
for quadrupolar power anisotropy in large scale structure.Comment: 23 pages; 10 figures; 3 tables; replaced with version published in
JCAP (added discussion of scale-varying quadrupolar anisotropy
Creating Statistically Anisotropic and Inhomogeneous Perturbations
In almost all structure formation models, primordial perturbations are
created within a homogeneous and isotropic universe, like the one we observe.
Because their ensemble averages inherit the symmetries of the spacetime in
which they are seeded, cosmological perturbations then happen to be
statistically isotropic and homogeneous. Certain anomalies in the cosmic
microwave background on the other hand suggest that perturbations do not
satisfy these statistical properties, thereby challenging perhaps our
understanding of structure formation. In this article we relax this tension. We
show that if the universe contains an appropriate triad of scalar fields with
spatially constant but non-zero gradients, it is possible to generate
statistically anisotropic and inhomogeneous primordial perturbations, even
though the energy momentum tensor of the triad itself is invariant under
translations and rotations.Comment: 20 pages, 1 figure. Uses RevTeX
Cosmology of a Scalar Field Coupled to Matter and an Isotropy-Violating Maxwell Field
Motivated by the couplings of the dilaton in four-dimensional effective
actions, we investigate the cosmological consequences of a scalar field coupled
both to matter and a Maxwell-type vector field. The vector field has a
background isotropy-violating component. New anisotropic scaling solutions
which can be responsible for the matter and dark energy dominated epochs are
identified and explored. For a large parameter region the universe expands
almost isotropically. Using that the CMB quadrupole is extremely sensitive to
shear, we constrain the ratio of the matter coupling to the vector coupling to
be less than 10^(-5). Moreover, we identify a large parameter region,
corresponding to a strong vector coupling regime, yielding exciting and viable
cosmologies close to the LCDM limit.Comment: Refs. added, some clarifications. Published in JHEP10(2012)06
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