arXiv subjects

David Wands

Publications and source records attributed to David Wands.

At least 127 records · Page 7Linked to original sources

arXiv2007

Multiple field inflation

Inflation offers a simple model for very early evolution of our Universe and the origin of primordial perturbations on large scales. Over the last 25 years we have become familiar with the predictions of single-field models, but inflation with more than one light scalar field can alter preconceptions about the inflationary dynamics and our predictions for the primordial perturbations. I will discuss how future observational data could distinguish between inflation driven by one field, or many fields. As an example, I briefly review the curvaton as an alternative to the inflaton scenario for the origin of structure.

astro-ph↗

arXiv2007

Primordial perturbations from slow-roll inflation on a brane

In this paper we quantise scalar perturbations in a Randall-Sundrum-type model of inflation where the inflaton field is confined to a single brane embedded in five-dimensional anti-de Sitter space-time. In the high energy regime, small-scale inflaton fluctuations are strongly coupled to metric perturbations in the bulk and gravitational back-reaction has a dramatic effect on the behaviour of inflaton perturbations on sub-horizon scales. This is in contrast to the standard four-dimensional result where gravitational back-reaction can be neglected on small scales. Nevertheless, this does not give rise to significant particle production, and the correction to the power spectrum of the curvature perturbations on super-horizon scales is shown to be suppressed by a slow-roll parameter. We calculate the complete first order slow-roll corrections to the spectrum of primordial curvature perturbations.

hep-th↗

arXiv2006

Primordial trispectrum from inflation

We use the delta N-formalism to describe the leading order contributions to the primordial power spectrum, bispectrum and trispectrum in multiple-field models of inflation at leading order in a perturbative expansion. In slow-roll models where the initial field fluctuations at Hubble-exit are nearly Gaussian, any detectable non-Gaussianity is expected to come from super-Hubble evolution. We show that the contribution to the primordial trispectrum can be described by two non-linearity parameters, tau_{NL} and g_{NL}, which are dependent upon the second and third derivatives of the local expansion with respect to the field values during inflation.

astro-ph↗

arXiv2006

The cosmological gravitational wave background from primordial density perturbations

We discuss the gravitational wave background generated by primordial density perturbations evolving during the radiation era. At second-order in a perturbative expansion, density fluctuations produce gravitational waves. We calculate the power spectra of gravitational waves from this mechanism, and show that, in principle, future gravitational wave detectors could be used to constrain the primordial power spectrum on scales vastly different from those currently being probed by large-scale structure. As examples we compute the gravitational wave background generated by both a power-law spectrum on all scales, and a delta-function power spectrum on a single scale.

gr-qc↗

arXiv2006

Non-Gaussianity of the primordial perturbation in the curvaton model

We use the delta N -formalism to investigate the non-Gaussianity of the primordial curvature perturbation in the curvaton scenario for the origin of structure. We numerically calculate the full probability distribution function allowing for the non-instantaneous decay of the curvaton and compare this with analytic results derived in the sudden-decay approximation. We also present results for the leading-order contribution to the primordial bispectrum and trispectrum. In the sudden-decay approximation we derive a fully non-linear expression relating the primordial perturbation to the initial curvaton perturbation. As an example of how non-Gaussianity provides additional constraints on model parameters, we show how the primordial bispectrum on CMB scales can be used to constrain variance on much smaller scales in the curvaton field. Our analytical and numerical results allow for multiple tests of primordial non-Gaussianity, and thus they can offer consistency tests of the curvaton scenario.

astro-ph↗

arXiv2006

Non-Gaussianity and constraints for the variance of perturbations in the curvaton model

Recently, the primordial non-Gaussianity in the curvaton model has been predicted assuming sudden decay of the curvaton. We extend the calculation to non-instantaneous decay by employing delta N -formalism. The difference between the sudden-decay approximation and our numerical result is larger than 1% only if the non-linearity parameter is small, -1.16 < f_NL < 60. Thus it is safe to use the sudden-decay approximation when deriving constraints for the curvaton model from WMAP3 (f_NL < 114), but with the Planck forecast |f_NL| <5 one should employ the fully numerical result. Often, the curvaton perturbations $δσ$ have been assumed to be small compared to the background value of the curvaton field $σ_0$. Consequently, the variance $Δ^2 = <δσ^2> / σ_0^2$ has been assumed to be negligible. However, the measurements of CMB or large-scale structure perturbation amplitude do not constrain the variance if the main contribution to it comes from the ultraviolet (UV) scales, i.e., from smaller than observable scales. We discuss how, even in this case, observational constraints on non-Gaussianity set an upper bound to the small scale variance, Delta^2_UV < 90.

astro-ph↗

arXiv2006

Cosmological matching conditions

We investigate the evolution of scalar metric perturbations across a sudden cosmological transition, allowing for an inhomogeneous surface stress at the transition leading to a discontinuity in the local expansion rate, such as might be expected in a big crunch/big bang event. We assume that the transition occurs when some function of local matter variables reaches a critical value, and that the surface stress is also a function of local matter variables. In particular we consider the case of a single scalar field and show that a necessary condition for the surface stress tensor to be perturbed at the transition is the presence of a non-zero intrinsic entropy perturbation of the scalar field. We present the matching conditions in terms of gauge-invariant variables assuming a sudden transition to a fluid-dominated universe with barotropic equation of state. For adiabatic perturbations the comoving curvature perturbation is continuous at the transition, while the Newtonian potential may be discontinuous if there is a discontinuity in the background Hubble expansion.

hep-th↗

arXiv2006

Non-Gaussianities in two-field inflation

We study the bispectrum of the curvature perturbation on uniform energy density hypersurfaces in models of inflation with two scalar fields evolving simultaneously. In the case of a separable potential, it is possible to compute the curvature perturbation up to second order in the perturbations, generated on large scales due to the presence of non-adiabatic perturbations, by employing the $δN$-formalism, in the slow-roll approximation. In this case, we provide an analytic formula for the nonlinear parameter $f_{NL}$. We apply this formula to double inflation with two massive fields, showing that it does not generate significant non-Gaussianity; the nonlinear parameter at the end of inflation is slow-roll suppressed. Finally, we develop a numerical method for generic two-field models of inflation, which allows us to go beyond the slow-roll approximation and confirms our analytic results for double inflation.

astro-ph↗

arXiv2006

Enhanced gravitational scattering from large extra dimensions

We investigate whether enhanced gravitational scattering on small scales (< 0.1mm), which becomes possible in models with large extra dimensions, can establish statistical equilibrium between different particle species in the early Universe. We calculate the classical relativistic energy transfer rate for two species with a large ratio between their masses for a general elastic scattering cross section. Although the classical calculation suggests that ultra-light WIMPs (e.g., axions) can be thermalized by gravitational scattering, such interactions are considerably less efficient once quantum effects are taken into account on scales below the Compton wavelength. However the energy transfer rate in models with several extra dimensions may still be sensitive to trans-Planckian physics.

hep-th↗

arXiv2006

Inflation Dynamics and Reheating

We review the theory of inflation with single and multiple fields paying particular attention to the dynamics of adiabatic and entropy/isocurvature perturbations which provide the primary means of testing inflationary models. We review the theory and phenomenology of reheating and preheating after inflation providing a unified discussion of both the gravitational and nongravitational features of multi-field inflation. In addition we cover inflation in theories with extra dimensions and models such as the curvaton scenario and modulated reheating which provide alternative ways of generating large-scale density perturbations. Finally we discuss the interesting observational implications that can result from adiabatic-isocurvature correlations and non-Gaussianity.

astro-ph↗

arXiv2006

Brane-world cosmology

Brane-world models, where observers are restricted to a brane in a higher-dimensional spacetime, offer a novel perspective on cosmology. I discuss some approaches to cosmology in extra dimensions and some interesting aspects of gravity and cosmology in brane-world models.

gr-qc↗

arXiv2005

Non-Gaussian perturbations from multi-field inflation

We show how the primordial bispectrum of density perturbations from inflation may be characterised in terms of manifestly gauge-invariant cosmological perturbations at second order. The primordial metric perturbation, zeta, describing the perturbed expansion of uniform-density hypersurfaces on large scales is related to scalar field perturbations on unperturbed (spatially-flat) hypersurfaces at first- and second-order. The bispectrum of the metric perturbation is thus composed of (i) a local contribution due to the second-order gauge-transformation, and (ii) the instrinsic bispectrum of the field perturbations on spatially flat hypersurfaces. We generalise previous results to allow for scale-dependence of the scalar field power spectra and correlations that can develop between fields on super-Hubble scales.

astro-ph↗

arXiv2005

Scale-invariant Perturbations from Chaotic Inflation

Vacuum fluctuations in the inflaton field driving chaotic inflation with a quadratic potential give a red spectrum of primordial density perturbations, n=0.97. However angular fluctuations in an O(N)-symmetric quadratic potential have a very nearly scale-invariant spectrum, n=0.9998. We investigate the possibility that these isocurvature field perturbations could give the dominant contribution to the primordial density perturbation after inflation.

astro-ph↗

arXiv2005

Braneworld Flux Inflation

We propose a geometrical model of brane inflation where inflation is driven by the flux generated by opposing brane charges and terminated by the collision of the branes, with charge annihilation. We assume the collision process is completely inelastic and the kinetic energy is transformed into the thermal energy after collision. Thereafter the two branes coalesce together and behave as a single brane universe with zero effective cosmological constant. In the Einstein frame, the 4-dimensional effective theory changes abruptly at the collision point. Therefore, our inflationary model is necessarily 5-dimensional in nature. As the collision process has no singularity in 5-dimensional gravity, we can follow the evolution of fluctuations during the whole history of the universe. It turns out that the radion field fluctuations have a steeply tilted, red spectrum, while the primordial gravitational waves have a flat spectrum. Instead, primordial density perturbations could be generated by a curvaton mechanism.

hep-th↗

arXiv2005

Slow-roll corrections to inflaton fluctuations on a brane

Quantum fluctuations of an inflaton field, slow-rolling during inflation are coupled to metric fluctuations. In conventional four dimensional cosmology one can calculate the effect of scalar metric perturbations as slow-roll corrections to the evolution of a massless free field in de Sitter spacetime. This gives the well-known first-order corrections to the field perturbations after horizon-exit. If inflaton fluctuations on a four dimensional brane embedded in a five dimensional bulk spacetime are studied to first-order in slow-roll then we recover the usual conserved curvature perturbation on super-horizon scales. But on small scales, at high energies, we find that the coupling to the bulk metric perturbations cannot be neglected, leading to a modified amplitude of vacuum oscillations on small scales. This is a large effect which casts doubt on the reliability of the usual calculation of inflaton fluctuations on the brane neglecting their gravitational coupling.

hep-th↗

arXiv2005

The Amplitude of Dark Energy Perturbations

We propose a model which produces dark energy perturbations large enough to explain the lack of power seen at the quadrupole scale in the cosmic microwave background. If the dark energy is frozen from horizon exit during inflation until dark energy domination, then it is not possible to have perturbations in the dark energy which are large enough. We propose using a tachyonic amplification mechanism to overcome this. The dark energy is taken to be a complex scalar field, where the radial field has a Mexican hat potential. During inflation, the radial component is trapped near the maximum of its potential. At the end of inflation, it rolls down to the minimum. The dark energy today is taken to be a pseudo-Nambu-Goldstone boson. The perturbations generated during inflation are amplified by the rolling of the radial field. We also examine the use of the variable decay mechanism in order to generate an anti-correlation between the dark energy perturbations and the curvature perturbation. We show that using this mechanism then constrains the properties of the dark energy and its evolution from redshift one until today.

astro-ph↗

arXiv2005

Coupled boundary and bulk fields in anti-de Sitter

We investigate the dynamics of a boundary field coupled to a bulk field with a linear coupling in an anti-de Sitter bulk spacetime bounded by a Minkowski (Randall-Sundrum) brane. An instability criterion for the coupled boundary and bulk system is found. There exists a tachyonic bound state when the coupling is above a critical value, determined by the masses of the brane and bulk fields and AdS curvature scale. This bound state is normalizable and localised near the brane, and leads to a tachonic instability of the system on large scales. Below the critical coupling, there is no tachyonic state and no bound state. Instead, we find quasi-normal modes which describe stable oscillations, but with a finite decay time. Only if the coupling is tuned to the critical value does there exist a massless stable bound state, as in the case of zero coupling for massless fields. We discuss the relation to gravitational perturbations in the Randall-Sundrum brane-world.

hep-th↗

arXiv2005

Adiabatic and entropy perturbations with interacting fluids and fields

We develop a gauge-invariant formalism for the study of density perturbations in a Friedmann-Robertson-Walker universe with multiple interacting fluids and/or scalar fields. We show how N scalar fields may be described by N kinetic fluids (with maximally stiff equation of state) interacting with a non-dynamical potential (with vacuum equation of state). We split generic perturbations into adiabatic and entropic parts, and give the coupled first-order evolution equations on all scales, including energy and momentum exchange. We identify the non-adiabatic effects on large scales, and define adiabatic initial conditions in the presence of multiple fluids and fields.

astro-ph↗