98 research outputs found
On holography for (pseudo-)conformal cosmology
We propose a holographic dual for (pseudo-)conformal cosmological scenario,
with a scalar field that forms a moving domain wall in adS_5. The domain wall
separates two vacua with unequal energy densities. Unlike in the existing
construction, the 5d solution is regular in the relevant space-time domain.Comment: 6 pages, 1 figur
Comment on `Strong coupling in extended Horava-Lifshitz gravity'
We show that, contrary to the claim made in arXiv:0911.1299, the extended
Horava gravity model proposed in arXiv:0909.3525 does not suffer from a strong
coupling problem. By studying the observational constraints on the model we
determine the bounds on the scale of the ultraviolet modification for which the
proposal yields a phenomenologically viable, renormalizable and weakly coupled
model of quantum gravity.Comment: A footnote discussing the absence of fine-tuning is adde
Gravity waves from inflating brane or Mirrors moving in adS
We study tensor perturbations in a model with inflating Randall--Sundrum-type
brane embedded in five-dimensional anti-de Sitter (adS) bulk. In this
model, a natural {\it in}-vacuum of gravitons is the vacuum defined in static
adS frame. We show that this vacuum is, in fact, the same as the {\it
in}-vacuum defined in the frame with de Sitter (dS) slicing, in which the
brane is at rest. Thus, during inflation, gravitons on and off the brane remain
in their vacuum state. We then study graviton creation by the brane on which
inflation terminates at some moment of time. We mostly consider gravitons whose
wavelengths at the end of inflation exceed both the horizon size and the
adS radius. Creation of these gravitons is dominated by (zero mode)--(zero
mode) Bogoliubov coefficients and, apart from an overall factor, the spectrum
of produced gravitons is the same as in four-dimensional theory.
``Kaluza--Klein'' gravitons are also produced, but this effect is subdominant.
Graviton spectra at somewhat higher momenta are also presented for
completeness.Comment: 33 pages, 1 figur
Cosmological constraints on Lorentz violating dark energy
The role of Lorentz invariance as a fundamental symmetry of nature has been
lately reconsidered in different approaches to quantum gravity. It is thus
natural to study whether other puzzles of physics may be solved within these
proposals. This may be the case for the cosmological constant problem. Indeed,
it has been shown that breaking Lorentz invariance provides Lagrangians that
can drive the current acceleration of the universe without experiencing large
corrections from ultraviolet physics. In this work, we focus on the simplest
model of this type, called ThetaCDM, and study its cosmological implications in
detail. At the background level, this model cannot be distinguished from
LambdaCDM. The differences appear at the level of perturbations. We show that
in ThetaCDM, the spectrum of CMB anisotropies and matter fluctuations may be
affected by a rescaling of the gravitational constant in the Poisson equation,
by the presence of extra contributions to the anisotropic stress, and finally
by the existence of extra clustering degrees of freedom. To explore these
modifications accurately, we modify the Boltzmann code CLASS. We then use the
parameter inference code Monte Python to confront ThetaCDM with data from
WMAP-7, SPT and WiggleZ. We obtain strong bounds on the parameters accounting
for deviations from LambdaCDM. In particular, we find that the discrepancy
between the gravitational constants appearing in the Poisson and Friedmann
equations is constrained at the level 1.8%.Comment: 17 pages, 5 figure
Higgs inflation: consistency and generalisations
We analyse the self-consistency of inflation in the Standard Model, where the
Higgs field has a large non-minimal coupling to gravity. We determine the
domain of energies in which this model represents a valid effective field
theory as a function of the background Higgs field. This domain is bounded
above by the cutoff scale which is found to be higher than the relevant
dynamical scales throughout the whole history of the Universe, including the
inflationary epoch and reheating. We present a systematic scheme to take into
account quantum loop corrections to the inflationary calculations within the
framework of effective field theory. We discuss the additional assumptions that
must be satisfied by the ultra-violet completion of the theory to allow
connection between the parameters of the inflationary effective theory and
those describing the low-energy physics relevant for the collider experiments.
A class of generalisations of inflationary theories with similar properties is
constructed.Comment: 25 pages, 1 figur
Gauge theory solitons on noncommutative cylinder
We generalize to noncommutative cylinder the solution generation technique,
originally suggested for gauge theories on noncommutative plane. For this
purpose we construct partial isometry operators and complete set of orthogonal
projectors in the algebra of the cylinder, and an isomorphism between the free
module and its direct sum with the Fock module on the cylinder. We construct
explicitly the gauge theory soliton and evaluate the spectrum of perturbations
about this soliton.Comment: References added; to appear in Theor.Math.Phy
Unstable Semiclassical Trajectories in Tunneling
Some tunneling phenomena are described, in the semiclassical approximation,
by unstable complex trajectories. We develop a systematic procedure to
stabilize the trajectories and to calculate the tunneling probability,
including both the suppression exponent and prefactor. We find that the
instability of tunneling solutions modifies the power-law dependence of the
prefactor on h as compared to the case of stable solutions.Comment: Journal version; 4 pages, 2 figure
Cosmological constraints on deviations from Lorentz invariance in gravity and dark matter
We consider a scenario where local Lorentz invariance is violated by the
existence of a preferred time direction at every space-time point. This
scenario can arise in the context of quantum gravity and its description at low
energies contains a unit time-like vector field which parameterizes the
preferred direction. The particle physics tests of Lorentz invariance preclude
a direct coupling of this vector to the fields of the Standard Model, but do
not bear implications for dark matter. We discuss how the presence of this
vector and its possible coupling to dark matter affect the evolution of the
Universe. At the level of homogeneous cosmology the only effect of Lorentz
invariance violation is a rescaling of the expansion rate. The physics is
richer at the level of perturbations. We identify three effects crucial for
observations: the rescaling of the matter contribution to the Poisson equation,
the appearance of an extra contribution to the anisotropic stress and the
scale-dependent enhancement of dark matter clustering. These effects result in
distinctive features in the power spectra of the CMB and density fluctuations.
Making use of the data from Planck and WiggleZ we obtain the most stringent
cosmological constraints to date on departures from Lorentz symmetry. Our
analysis provides the first direct bounds on deviations from Lorentz invariance
in the dark matter sector.Comment: 10 pages, 3 figures, revtex; footnote on isocurvature modes added,
discussion on the decoupling of the Standard Model fields from the aether
extended, a reference added; version to be published in JCA
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