arXiv subjects

David Wands

Publications and source records attributed to David Wands.

At least 163 records · Page 9Linked to original sources

arXiv2002

Cosmological perturbations in the bulk and on the brane

We study cosmological perturbations in a brane-world scenario where the matter fields live on a four-dimensional brane and gravity propagates in the five-dimensional bulk. We present the equations of motion in an arbitrary gauge for metric perturbations in the bulk and matter perturbations on the brane. Gauge-invariant perturbations are then constructed corresponding to perturbations in longitudinal and Gaussian normal gauges. Longitudinal gauge metric perturbations may be directly derived from three master variables (separately describing scalar, vector and tensor metric perturbations) which obey five-dimensional wave-equations. Gaussian normal gauge perturbations are directly related to the induced metric perturbations on the brane with the additional bulk degrees of freedom interpreted as an effective Weyl energy-momentum tensor on the brane. We construct gauge-invariant perturbations describing the effective density, momentum and pressures of this Weyl fluid at the brane and throughout the bulk. We show that there exist gauge-invariant curvature perturbations on the brane and in the bulk that are conserved on large-scales when three-dimensional spatial gradients are negligible.

astro-ph↗

arXiv2001

Enhancement of superhorizon scale inflationary curvature perturbations

We show that there exists a simple mechanism which can enhance the amplitude of curvature perturbations on superhorizon scales during inflation, relative to their amplitude at horizon crossing. The enhancement may occur even in a single-field inflaton model, and occurs if the quantity $a\dotϕ/H$ becomes sufficiently small, as compared to its value at horizon crossing, for some time interval during inflation. We give a criterion for this enhancement in general single-field inflation models.

astro-ph↗

arXiv2001

Cosmic vorticity on the brane

We study vector perturbations about four-dimensional brane-world cosmologies embedded in a five-dimensional vacuum bulk. Even in the absence of matter perturbations, vector perturbations in the bulk metric can support vector metric perturbations on the brane. We show that during de Sitter inflation on the brane vector perturbations in the bulk obey the same wave equation for a massless five-dimensional field as found for tensor perturbations. However, we present the second-order effective action for vector perturbations and find no normalisable zero-mode in the absence of matter sources. The spectrum of normalisable states is a continuum of massive modes that remain in the vacuum state during inflation.

hep-th↗

arXiv2000

Large-scale cosmological perturbations on the brane

In brane-world cosmologies of Randall-Sundrum type, we show that evolution of large-scale curvature perturbations may be determined on the brane, without solving the bulk perturbation equations. The influence of the bulk gravitational field on the brane is felt through a projected Weyl tensor which behaves effectively like an imperfect radiation fluid with anisotropic stress. We define curvature perturbations on uniform density surfaces for both the matter and Weyl fluids, and show that their evolution on large scales follows directly from the energy conservation equations for each fluid. The total curvature perturbation is not necessarily constant for adiabatic matter perturbations, but can change due to the Weyl entropy perturbation. To relate this curvature perturbation to the longitudinal gauge metric potentials requires knowledge of the Weyl anisotropic stress which is not determined by the equations on the brane. We discuss the implications for large-angle anisotropies on the cosmic microwave background sky.

hep-th↗

arXiv2000

Adiabatic and entropy perturbations from inflation

We study adiabatic (curvature) and entropy (isocurvature) perturbations produced during a period of cosmological inflation that is driven by multiple scalar fields with an arbitrary interaction potential. A local rotation in field space is performed to separate out the adiabatic and entropy modes. The resulting field equations show explicitly how on large scales entropy perturbations can source adiabatic perturbations if the background solution follows a curved trajectory in field space, and how adiabatic perturbations cannot source entropy perturbations in the long-wavelength limit. It is the effective mass of the entropy field that determines the amplitude of entropy perturbations during inflation. We present two applications of the equations. First, we show why one in general expects the adiabatic and entropy perturbations to be correlated at the end of inflation, and calculate the cross-correlation in the context of a double inflation model with two non-interacting fields. Second, we consider two-field preheating after inflation, examining conditions under which entropy perturbations can alter the large-scale curvature perturbation and showing how our new formalism has advantages in numerical stability when the background solution follows a non-trivial trajectory in field space.

astro-ph↗

arXiv2000

Dilaton-gravity on the brane

We derive the four-dimensional field equations for the induced metric and scalar field on the world-volume of a 3-brane in a five-dimensional bulk with Einstein gravity plus a self-interacting scalar field. We calculate the effective four-dimensional gravitational constant and cosmological constant for arbitrary forms of the brane tension and self-interaction potential for the scalar field in the bulk. In addition to the canonical energy-momentum tensor for the scalar field and ordinary matter on the brane, the effective four-dimensional Einstein equations include terms due to the scalar field and gravitational waves in the bulk. We present solutions corresponding to static Minkowski brane worlds and also dynamical Friedmann-Robertson-Walker brane world cosmologies. We discuss the induced coupling of the scalar field to ordinary matter on the brane.

hep-th↗

arXiv2000

Gravitational waves from inflation on the brane

We discuss the evolution of gravitational waves in a brane-world cosmology embedded in five-dimensional anti-de Sitter spacetime. We show that during slow-roll inflation, modelled as a period of quasi-de Sitter expansion on the brane, there is a discrete normalizable massless graviton mode. There is a mass gap due to the expansion, above which there is a continuum of massive modes. Only the massless mode is `light' compared with the Hubble scale during inflation, leading to the production of classical perturbations on large scales from vacuum fluctuations on small scales. We calculate the amplitude of these fluctuations at horizon-crossing and show that the standard four-dimensional result is recovered at low energies, but the amplitude of the perturbations is enhanced at high energies.

hep-th↗

arXiv2000

A new approach to the evolution of cosmological perturbations on large scales

We discuss the evolution of linear perturbations about a Friedmann-Robertson-Walker background metric, using only the local conservation of energy-momentum. We show that on sufficiently large scales the curvature perturbation on spatial hypersurfaces of uniform-density is conserved when the non-adiabatic pressure perturbation is negligible. This is the first time that this result has been demonstrated independently of the gravitational field equations. A physical picture of long-wavelength perturbations as being composed of separate Robertson-Walker universes gives a simple understanding of the possible evolution of the curvature perturbation, in particular clarifying the conditions under which super-horizon curvature perturbations may vary.

astro-ph↗

arXiv2000

Cosmological perturbation spectra from SL(4,R)-invariant effective actions

We investigate four-dimensional cosmological vacuum solutions derived from an effective action invariant under global SL(n,R) transformations. We find the general solutions for linear axion field perturbations about homogeneous dilaton-moduli-vacuum solutions for an SL(4,R)-invariant action and find the spectrum of super-horizon perturbations resulting from vacuum fluctuations in a pre big bang scenario. We show that for SL(n,R)-invariant actions with n>3 there exists a regime of parameter space of non-zero measure where all the axion field spectra have positive spectral tilt, as required if light axion fields are to provide a seed for anisotropies in the microwave background and large-scale structure in the universe.

hep-th↗

arXiv2000

Superstring Cosmology

Aspects of superstring cosmology are reviewed with an emphasis on the cosmological implications of duality symmetries in the theory. The string effective actions are summarized and toroidal compactification to four dimensions reviewed. Global symmetries that arise in the compactification are discussed and the duality relationships between the string effective actions are then highlighted. Higher-dimensional Kasner cosmologies are presented and interpreted in both string and Einstein frames, and then given in dimensionally reduced forms. String cosmologies containing both non-trivial Neveu-Schwarz/Neveu-Schwarz and Ramond-Ramond fields are derived by employing the global symmetries of the effective actions. Anisotropic and inhomogeneous cosmologies in four-dimensions are also developed. The review concludes with a detailed analysis of the pre-big bang inflationary scenario. The generation of primordial spectra of cosmological perturbations in such a scenario is discussed. Possible future directions offered in the Horava-Witten theory are outlined.

hep-th↗

arXiv1999

Super-horizon perturbations and preheating

It has recently been claimed by Bassett et al that preheating after inflation may affect the amplitude of curvature perturbations on large scales, undermining the usual inflationary prediction. We analyze the simplest model, and confirm the results of Jedamzik and Sigl and of Ivanov that in linear perturbation theory the effect is negligible. However the dominant effect is second-order in the field perturbation and we show that this too is negligible, and hence conclude that preheating has no significant influence on large-scale perturbations in this model. We briefly discuss the likelihood of an effect in other models.

hep-ph↗

arXiv1999

Self-similar cosmological solutions with a non-minimally coupled scalar field

We present self-similar cosmological solutions for a barotropic fluid plus scalar field with Brans-Dicke-type coupling to the spacetime curvature and an arbitrary power-law potential energy. We identify all the fixed points in the autonomous phase-plane, including a scaling solution where the fluid density scales with the scalar field's kinetic and potential energy. This is related by a conformal transformation to a scaling solution for a scalar field with exponential potential minimally coupled to the spacetime curvature, but non-minimally coupled to the barotropic fluid. Radiation is automatically decoupled from the scalar field, but energy transfer between the field and non-relativistic dark matter can lead to a change to an accelerated expansion at late times in the Einstein frame. The scalar field density can mimic a cosmological constant even for steep potentials in the strong coupling limit.

gr-qc↗

arXiv1999

Duality Invariance of Cosmological Perturbation Spectra

I show that cosmological perturbation spectra produced from quantum fluctuations in massless or self-interacting scalar fields during an inflationary era remain invariant under a two parameter family of transformations of the homogeneous background fields. This relates slow-roll inflation models to solutions which may be far from the usual slow-roll limit. For example, a scale-invariant spectrum of perturbations in a minimally coupled, massless field can be produced by an exponential expansion with $a\propto e^{Ht}$, or by a collapsing universe with $a\propto (-t)^{2/3}$.

gr-qc↗

arXiv1999

Scaling Solutions in Robertson-Walker Spacetimes

We investigate the stability of cosmological scaling solutions describing a barotropic fluid with $p=(γ-1)ρ$ and a non-interacting scalar field $ϕ$ with an exponential potential $V(ϕ)=V_0\e^{-κϕ}$. We study homogeneous and isotropic spacetimes with non-zero spatial curvature and find three possible asymptotic future attractors in an ever-expanding universe. One is the zero-curvature power-law inflation solution where $Ω_ϕ=1$ ($γ<2/3,κ^2<3γ$ and $γ>2/3,κ^2<2$). Another is the zero-curvature scaling solution, first identified by Wetterich, where the energy density of the scalar field is proportional to that of matter with $Ω_ϕ=3γ/κ^2$ ($γ<2/3,κ^2>3γ$). We find that this matter scaling solution is unstable to curvature perturbations for $γ>2/3$. The third possible future asymptotic attractor is a solution with negative spatial curvature where the scalar field energy density remains proportional to the curvature with $Ω_ϕ=2/κ^2$ ($γ>2/3,κ^2>2$). We find that solutions with $Ω_ϕ=0$ are never late-time attractors.

gr-qc↗

arXiv1998

Dynamics of Assisted Inflation

We investigate the dynamics of the recently proposed model of assisted inflation. In this model an arbitrary number of scalar fields with exponential potentials evolve towards an inflationary scaling solution, even if each of the individual potentials is too steep to support inflation on its own. By choosing an appropriate rotation in field space we can write down explicitly the potential for the weighted mean field along the scaling solution and for fields orthogonal to it. This demonstrates that the potential has a global minimum along the scaling solution. We show that the potential close to this attractor in the rotated field space is analogous to a hybrid inflation model, but with the vacuum energy having an exponential dependence upon a dilaton field. We present analytic solutions describing homogeneous and inhomogeneous perturbations about the attractor solution without resorting to slow-roll approximations. We discuss the curvature and isocurvature perturbation spectra produced from vacuum fluctuations during assisted inflation.

astro-ph↗

arXiv1998

Axion perturbation spectra in string cosmologies

We discuss the semi-classical perturbation spectra produced in the massless fields of the low energy string action in a pre big bang type scenario. Axion fields may possess an almost scale-invariant spectrum on large scales dependent upon the evolution of the dilaton and moduli fields to which they are coupled. As an example we calculate the spectra for three axion fields present in a truncated type IIB model and show that they are related with at least one of the fields having a scale-invariant or red perturbation spectrum. In the simplest pre big bang scenario this may be inconsistent with the observed isotropy of the microwave background. More generally, relations between the perturbation spectra in low energy string cosmologies reflect the symmetries of the theory.

hep-th↗

arXiv1998

Exponential potentials and cosmological scaling solutions

We present a phase-plane analysis of cosmologies containing a barotropic fluid with equation of state $p_γ= (γ-1) ρ_γ$, plus a scalar field $ϕ$ with an exponential potential $V \propto \exp(-λκϕ)$ where $κ^2 = 8πG$. In addition to the well-known inflationary solutions for $λ^2 < 2$, there exist scaling solutions when $λ^2 > 3γ$ in which the scalar field energy density tracks that of the barotropic fluid (which for example might be radiation or dust). We show that the scaling solutions are the unique late-time attractors whenever they exist. The fluid-dominated solutions, where $V(ϕ)/ρ_γ\to 0$ at late times, are always unstable (except for the cosmological constant case $γ= 0$). The relative energy density of the fluid and scalar field depends on the steepness of the exponential potential, which is constrained by nucleosynthesis to $λ^2 > 20$. We show that standard inflation models are unable to solve this `relic density' problem.

gr-qc↗

arXiv1998

Symmetries for generating string cosmologies

We discuss the symmetry properties of the low-energy effective action of the type IIB superstring that may be employed to derive four-dimensional solutions. A truncated effective action, compactified on a six-torus, but including both Neveu/Schwarz-Neveu/Schwarz and Ramond-Ramond field strengths, can be expressed as a non-linear sigma model which is invariant under global SL(3,R) transformations. This group contains as a sub-group the SL(2,R) symmetry of the ten-dimensional theory and a discrete Z2 reflection symmetry which leads to a further SL(2,R) sub-group. The symmetries are employed to analyse a general class of spatially homogeneous cosmological solutions with non-trivial Ramond-Ramond fields.

hep-th↗