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 adS5_5

We study tensor perturbations in a model with inflating Randall--Sundrum-type brane embedded in five-dimensional anti-de Sitter (adS$_5$) bulk. In this model, a natural {\it in}-vacuum of gravitons is the vacuum defined in static adS$_5$ frame. We show that this vacuum is, in fact, the same as the {\it in}-vacuum defined in the frame with de Sitter (dS$_4$) 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$_5$ 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

Experimental assessment of the speed of light perturbation in free-fall absolute gravimeters

Precision absolute gravity measurements are growing in importance, especially in the context of the new definition of the kilogram. For the case of free-fall absolute gravimeters with a Michelson-type interferometer tracking the position of a free falling body, one of the effects that needs to be taken into account is the speed of light perturbation due to the finite speed of propagation of light. This effect has been extensively discussed in the past, and there is at present a disagreement between different studies. In this work, we present the analysis of new data and confirm the result expected from the theoretical analysis applied nowadays in free-fall gravimeters. We also review the standard derivations of this effect (by using phase shift or Doppler effect arguments) and show their equivalence