Bulk gravitons from a cosmological brane

We investigate the emission of gravitons by a cosmological brane into an Anti de Sitter five-dimensional bulk spacetime. We focus on the distribution of gravitons in the bulk and the associated production of `dark radiation' in this process. In order to evaluate precisely the amount of dark radiation in the late low-energy regime, corresponding to standard cosmology, we study numerically the emission, propagation and bouncing off the brane of bulk gravitons.Comment: 27 pages, 5 figures, minor corrections. Final versio

Cosmic Microwave Background Dipole induced by double inflation

The observed CMBR dipole is generally interpreted as the consequence of the peculiar motion of the Sun with respect to the reference frame of the CMBR. This article proposes an alternative interpretation in which the observed dipole is the result of isocurvature perturbations on scales larger than the present Hubble radius. These perturbations are produced in the simplest model of double inflation, depending on three parameters. The observed dipole and quadrupole can be explained in this model, while severely constraining its parameters.Comment: Latex, 9 pages, no figure, to appear in Phys. Rev.

Low energy effective gravitational equations on a Gauss-Bonnet brane

We present effective gravitational equations at low energies in a $Z_2$-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based on the geometrical projection approach, and we solve iteratively the bulk geometry using the gradient expansion scheme. Although the original field equations are quite complicated due to the presence of the Gauss-Bonnet term, our final result clearly has the form of the Einstein equations plus correction terms, which is simple enough to handle. As an application, we consider homogeneous and isotropic cosmology on the brane. We also comment on the holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.Comment: 10 pages, v2: minor clarification

Correlated adiabatic and isocurvature perturbations from double inflation

It is shown that double inflation (two minimally coupled massive scalar fields) can produce correlated adiabatic and isocurvature primordial perturbations. Depending on the two relevant parameters of the model, the contributions to the primordial perturbations are computed, with special emphasis on the correlation, which can be quantitatively represented by a correlation spectrum. Finally the primordial spectra are evolved numerically to obtain the CMBR anisotropy multipole expectation values. It turns out that the existence of mixing and correlation can alter very significantly the temperature fluctuation predictions.Comment: 23 pages, 7 figures, RevTex. To appear in Phys. Rev.

Bulk Gravitational Field and Cosmological Perturbations on the Brane

We investigate the effect of the bulk gravitational field on the cosmological perturbations on a brane embedded in the 5D Anti-de Sitter (AdS) spacetime. The effective 4D Einstein equations for the scalar cosmological perturbations on the brane are obtained by solving the perturbations in the bulk. Then the behaviour of the corrections induced by the bulk gravitational field to the conventional 4D Einstein equation are determined. Two types of the corrections are found. First we investigate the corrections which become significant at scales below the AdS curvature scales and in the high energy universe with the energy density larger than the tension of the brane. The evolution equation for the perturbations on the brane is found and solved. Another type of the corrections is induced on the brane if we consider the bulk perturbations which do not contribute to the metric perturbations but do contribute to the matter perturbations. At low energies, they have imaginary mass m^2=-(2/3) \k^2 in the bulk where \k is the 3D comoving wave number of the perturbations. They diverge at the horizon of the AdS spacetime. The induced density perturbations behave as sound waves with sound velocity $1/\sqrt{3}$ in the low energy universe. At large scales, they are homogeneous perturbations that depend only on time and decay like radiation. They can be identified as the perturbations of the dark radiation. They produce isocurvature perturbations in the matter dominated era. Their effects can be observed as the shifts of the location and the height of the acoustic peak in the CMB spectrum.Comment: 35 pages, 1 figur

Isocurvature perturbations in extra radiation

Recent cosmological observations, including measurements of the CMB anisotropy and the primordial helium abundance, indicate the existence of an extra radiation component in the Universe beyond the standard three neutrino species. In this paper we explore the possibility that the extra radiation has isocurvatrue fluctuations. A general formalism to evaluate isocurvature perturbations in the extra radiation is provided in the mixed inflaton-curvaton system, where the extra radiation is produced by the decay of both scalar fields. We also derive constraints on the abundance of the extra radiation and the amount of its isocurvature perturbation. Current observational data favors the existence of an extra radiation component, but does not indicate its having isocurvature perturbation. These constraints are applied to some particle physics motivated models. If future observations detect isocurvature perturbations in the extra radiation, it will give us a hint to the origin of the extra radiation.Comment: 41 pages, 8 figures; version accepted for publication in JCA

Moduli-Space Approximation for BPS Brane-Worlds

We develop the moduli-space approximation for the low energy regime of BPS-branes with a bulk scalar field to obtain an effective four-dimensional action describing the system. An arbitrary BPS potential is used and account is taken of the presence of matter in the branes and small supersymmetry breaking terms. The resulting effective theory is a bi-scalar tensor theory of gravity. In this theory, the scalar degrees of freedom can be stabilized naturally without the introduction of additional mechanisms other than the appropriate BPS potential. We place observational constraints on the shape of the potential and the global configuration of branes.Comment: 10 pages, 1 figur

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

Excited by a quantum field: Does shape matter?

The instantaneous transition rate of an arbitrarily accelerated Unruh-DeWitt particle detector on four-dimensional Minkowski space is ill defined without regularisation. We show that Schlicht's regularisation as the zero-size limit of a Lorentz-function spatial profile yields a manifestly well-defined transition rate with physically reasonable asymptotic properties. In the special case of stationary trajectories, including uniform acceleration, we recover the results that have been previously obtained by a regularisation that relies on the stationarity. Finally, we discuss evidence for the conjecture that the zero-size limit of the transition rate is independent of the detector profile.Comment: 7 pages, uses jpconf. Talk given at NEB XII (Nafplio, Greece, 29 June - 2 July 2006