A general proof of the equivalence between the \delta N and covariant formalisms

Recently, the equivalence between the \delta N and covariant formalisms has been shown (Suyama et al. 2012), but they essentially assumed Einstein gravity in their proof. They showed that the evolution equation of the curvature covector in the covariant formalism on uniform energy density slicings coincides with that of the curvature perturbation in the \delta N formalism assuming the coincidence of uniform energy and uniform expansion (Hubble) slicings, which is the case on superhorizon scales in Einstein gravity. In this short note, we explicitly show the equivalence between the \delta N and covariant formalisms without specifying the slicing condition and the associated slicing coincidence, in other words, regardless of the gravity theory.Comment: 7 pages,a reference added, to be published in EP

Collision of Domain Walls and Reheating of the Brane Universe

We study a particle production at the collision of two domain walls in 5-dimensional Minkowski spacetime. This may provide the reheating mechanism of an ekpyrotic (or cyclic) brane universe, in which two BPS branes collide and evolve into a hot big bang universe. We evaluate a production rate of particles confined to the domain wall. The energy density of created particles is given as $\rho \approx 20 \bar{g}^4 N_b ~m_\eta^4$ where $\bar{g}$ is a coupling constant of particles to a domain-wall scalar field, $N_b$ is the number of bounces at the collision and $m_\eta$ is a fundamental mass scale of the domain wall. It does not depend on the width $d$ of the domain wall, although the typical energy scale of created particles is given by $\omega\sim 1/d$. The reheating temperature is evaluated as $T_{\rm R}\approx 0.88 ~ \bar{g} ~ N_b^{1/4}$. In order to have the baryogenesis at the electro-weak energy scale, the fundamental mass scale is constrained as m_\eta \gsim 1.1\times 10^7 GeV for $\bar{g}\sim 10^{-5}$.Comment: 10 pages, 12 figure

Conservation of the nonlinear curvature perturbation in generic single-field inflation

It is known that the curvature perturbation on uniform energy density (or comoving or uniform Hubble) slices on superhorizon scales is conserved to full nonlinear order if the pressure is only a function of the energy density (ie, if the perturbation is purely adiabatic), independent of the gravitational theory. Here we explicitly show that the same conservation holds for a universe dominated by a single scalar field provided that the field is in an attractor regime, for a very general class of scalar field theories. However, we also show that if the scalar field equation contains a second time derivative of the metric, as in the case of the Galileon theory, one has to invoke the gravitational field equations to show the conservation.Comment: 6 pages, minor revisions made but conclusion unchanged, references added, to be published in CQG as a fast track communicatio

Hybrid compactifications and brane gravity in six dimensions

We consider a six-dimensional axisymmetric Einstein-Maxwell model of warped braneworlds. The bulk is bounded by two branes, one of which is a conical 3-brane and the other is a 4-brane wrapped around the axis of symmetry. The latter brane is assumed to be our universe. If the tension of the 3-brane is fine-tuned, it folds the internal two-dimensional space in a narrow cone, making sufficiently small the Kaluza-Klein circle of the 4-brane. An arbitrary energy-momentum tensor can be accommodated on this ring-like 4-brane. We study linear perturbations sourced by matter on the brane, and show that weak gravity is apparently described by a four-dimensional scalar-tensor theory. The extra scalar degree of freedom can be interpreted as the fluctuation of the internal space volume (or that of the circumference of the ring), the effect of which turns out to be suppressed at long distances. Consequently, four-dimensional Einstein gravity is reproduced on the brane. We point out that as in the Randall-Sundrum model, the brane bending mode is crucial for recovering the four-dimensional tensor structure in this setup.Comment: 15 pages, 2 figures; v2: references added; v3: accepted for publication in Class. Quant. Gra

Low energy effective theory on a regularized brane in 6D gauged chiral supergravity

We derive the low energy effective theory on a brane in six-dimensional chiral supergravity. The conical 3-brane singularities are resolved by introducing cylindrical codimension one 4-branes whose interiors are capped by a regular spacetime. The effective theory is described by the Brans-Dicke (BD) theory with the BD parameter given by $\omega_{\rm BD}=1/2$. The BD field is originated from a modulus which is associated with the scaling symmetry of the system. If the dilaton potentials on the branes preserve the scaling symmetry, the scalar field has an exponential potential in the Einstein frame. We show that the time dependent solutions driven by the modulus in the four-dimensional effective theory can be lifted up to the six-dimensional exact solutions found in the literature. Based on the effective theory, we discuss a possible way to stabilize the modulus to recover standard cosmology and also study the implication for the cosmological constant problem.Comment: 12 pages, 1 figur

Inflation and late time acceleration in braneworld cosmological models with varying brane tension

Braneworld models with variable brane tension $\lambda$ introduce a new degree of freedom that allows for evolving gravitational and cosmological constants, the latter being a natural candidate for dark energy. We consider a thermodynamic interpretation of the varying brane tension models, by showing that the field equations with variable $\lambda$ can be interpreted as describing matter creation in a cosmological framework. The particle creation rate is determined by the variation rate of the brane tension, as well as by the brane-bulk energy-matter transfer rate. We investigate the effect of a variable brane tension on the cosmological evolution of the Universe, in the framework of a particular model in which the brane tension is an exponentially dependent function of the scale factor. The resulting cosmology shows the presence of an initial inflationary expansion, followed by a decelerating phase, and by a smooth transition towards a late accelerated de Sitter type expansion. The varying brane tension is also responsible for the generation of the matter in the Universe (reheating period). The physical constraints on the model parameters, resulted from the observational cosmological data, are also investigated.Comment: 25 pages, 8 figures, accepted for publication in European Physical Journal

Beyond δN formalism

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