arXiv subjects

Bruce A. Bassett

Publications and source records attributed to Bruce A. Bassett.

At least 91 records · Page 5Linked to original sources

arXiv2003

Correlation-consistency cartography of the double inflation landscape

We show explicitly some exciting features of double-inflation: (i) it can often lead to strongly correlated adiabatic and entropy (isocurvature) power spectra. (ii) The two-field slow-roll consistency relations can be violated when the correlation is large at Hubble crossing. (iii) The spectra of adiabatic and entropy perturbations can be strongly scale-dependent and tilted toward either the red or blue. These effects are typically due to a light or time-dependent entropy mass and a non-negligible angular velocity in field space during inflation. They are illustrated via a multi-parameter numerical search for correlations in two concrete models. The correlation is found to be particularly strong in a supersymmetric scenario due to rapid growth of entropy perturbations in the tachyonic region separating the two inflationary stages. Our analysis suggests that realistic double-inflation models will provide a rich and fruitful arena for the application of future cosmic data sets and new approximation schemes which go beyond slow-roll.

astro-ph↗

arXiv2003

Aspects of String-Gas Cosmology at Finite Temperature

We study string-gas cosmology in dilaton gravity, inspired by the fact that it naturally arises in a string theory context. Our main interest is the thermodynamical treatment of the string-gas and the resulting implications for the cosmology. Within an adiabatic approximation, thermodynamical equilibrium and a small, toroidal universe as initial conditions, we numerically solve the corresponding equations of motions in two different regimes describing the string-gas thermodynamics: (i) the Hagedorn regime, with a single scale factor, and (ii) an almost-radiation dominated regime, which includes the leading corrections due to the lightest Kaluza Klein and winding modes, with two scale factors. The scale factor in the Hagedorn regime exhibits very slow time evolution with nearly constant energy and negligible pressure. By contrast, in case (ii) we find interesting cosmological solutions where the large dimensions continue to expand and the small ones are kept undetectably small.

hep-th↗

arXiv2002

Did the dark energy always have a large Compton wavelength?

Until now all well-studied dark energy models proposed have very large Compton wavelengths, $λ_c > 100 Mpc$ at all time and hence show no clustering on small scales. Evidence against this would disfavour both $Λ$CDM and quintessence. We show however, that large $λ_c$ is actually favoured by current CMB, LSS and SN1a data by comparing quintessence with a condensate scenario with the same background dynamics. The condensate scenario has a very small $λ_c$ before making a transition to negative pressure and large $λ_c$ and therefore exhibits a rich CMB phenomenology which roughly interpolates between $Λ$CDM and flat CDM models.

astro-ph↗

arXiv2002

When can preheating affect the CMB?

We discuss the principles governing the selection of inflationary models for which preheating can affect the CMB. This is a (fairly small) subset of those models which have non-negligible entropy/isocurvature perturbations on large scales during inflation. We study new models which belong to this class - two-field inflation with negative nonminimal coupling and hybrid/double/supernatural inflation models where the tachyonic growth of entropy perturbations can lead to the variation of the curvature perturbation, ${\cal R}$, on super-Hubble scales. Finally we present evidence against recent claims for the variation of ${\cal R}$ in the absence of substantial super-Hubble entropy perturbations.

astro-ph↗

arXiv2002

A late-time transition in the cosmic dark energy?

We study constraints from the latest CMB, large scale structure (2dF, Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in their equation of state, w(z). Such a transition is motivated by models like vacuum metamorphosis where non-perturbative quantum effects are important at late times. We allow the transition to occur at a specific redshift, z_t, to a final negative pressure -1 < w_f < -1/3. We find that the CMB and supernovae data, in particular, prefer a late-time transition due to the associated delay in cosmic acceleration. The best fits (with 1 sigma errors) to all the data are z_t = 2.0^{+2.2}_{-0.76}, Ω_Q = 0.73^{+0.02}_{-0.04} and w_f = -1^{+0.2}.

astro-ph↗

arXiv2001

Fermion production from preheating-amplified metric perturbations

We study gravitational creation of light fermions in the presence of classical scalar metric perturbations about a flat Friedmann-Lemaitre- Robertson-Walker (FLRW) background. These perturbations can be large during preheating, breaking the conformal flatness of the background spacetime. We compute numerically the total number of particles generated by the modes of the metric perturbations which have grown sufficiently to become classical. In the absence of inhomogeneities massless fermions are not gravitationally produced, and then this effect may be relevant for abundance estimates of light gravitational relics.

hep-ph↗

arXiv2001

Preheating - cosmic magnetic dynamo ?

We study the amplification of large-scale magnetic fields during preheating and inflation in several different models. Preheating can resonantly amplify seed fields on cosmological scales. In the presence of conductivity, however, the effect of resonance is typically weakened while the amplitude of produced magnetic fields depends sensitively on the evolution of the conductivity during the preheating and thermalisation phases. In addition we discuss geometric magnetisation, where amplification of magnetic fields occurs through coupling to curvature invariants. This can be efficient during inflation due to a negative coupling instability. Finally we discuss the breaking of the conformal flatness of the background metric whereby magnetic fields can be stimulated through the growth of scalar metric perturbations during metric preheating.

astro-ph↗

arXiv2001

Inflationary preheating and primordial black holes

Preheating after inflation may over-produce primordial black holes (PBH's) in many regions of parameter space. As an example we study two-field models with a massless self-interacting inflaton, taking into account second order field and metric backreaction effects as spatial averages. We find that a complex quilt of parameter regions above the Gaussian PBH over-production threshold emerges due to the enhancement of curvature perturbations on all scales. It should be possible to constrain realistic models of inflation through PBH over-production although many issues, such as rescattering and non-Gaussianity, remain unsolved or unexplored.

hep-ph↗

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

Geometrodynamics of Variable-Speed-of-Light Cosmologies

This paper is dedicated to the memory of Dennis Sciama. Variable-Speed-of-Light (VSL) cosmologies are currently attracting interest as an alternative to inflation. We investigate the fundamental geometrodynamic aspects of VSL cosmologies and provide several implementations which do not explicitly break Lorentz invariance (no "hard" breaking). These "soft" implementations of Lorentz symmetry breaking provide particularly clean answers to the question "VSL with respect to what?". The class of VSL cosmologies we consider are compatible with both classical Einstein gravity and low-energy particle physics. These models solve the "kinematic" puzzles of cosmology as well as inflation does, but cannot by themselves solve the flatness problem, since in their purest form no violation of the strong energy condition occurs. We also consider a heterotic model (VSL plus inflation) which provides a number of observational implications for the low-redshift universe if chi contributes to the "dark energy" either as CDM or quintessence. These implications include modified gravitational lensing, birefringence, variation of fundamental constants and rotation of the plane of polarization of light from distant sources.

astro-ph↗

arXiv2000

Multi-field fermionic preheating

Fermion creation during preheating in the presence of multiple scalar fields exhibits a range of interesting behaviour relevant to estimating post-inflation gravitino abundances. We present non-perturbative analysis of this phenomenon over a 6-dimensional parameter space in an expanding background paying particular attention to the interplay between instant and direct fermion preheating. In the broad resonance regime we find that instant fermion production is sensitive to suppression of the long wavelength scalar field modes during inflation. Further, the standard scenario of resonant fermionic preheating through inflaton decay can be significantly modified by instant preheating, and may even lead to a decrease in the number of fermions produced. We explicitly include the effects of metric perturbations and demonstrate that they are important at small coupling but not at strong coupling, due to the rapid saturation of the Pauli bound.

hep-ph↗

arXiv2000

Detection of gravitational waves with quantum encryption technology

We propose a new technique for detecting gravitational waves using Quantum Entangled STate (QUEST) technology. Gravitational waves reduce the non-locality of correlated quanta controlled by Bell's inequalities, distorting quantum encryption key statistics away from a pure white noise. Gravitational waves therefore act as shadow eavesdroppers. The resulting colour distortions can, at least in principle, be separated from noise and can differentiate both deterministic and stochastic sources.

gr-qc↗

arXiv2000

Massless Metric Preheating

Can super-Hubble metric perturbations be amplified exponentially during preheating ? Yes. An analytical existence proof is provided by exploiting the conformal properties of massless inflationary models. The traditional conserved quantity ζis non-conserved in many regions of parameter space. We include backreaction through the homogeneous parts of the inflaton and preheating fields and discuss the role of initial conditions on the post-preheating power-spectrum. Maximum field variances are strongly underestimated if metric perturbations are ignored. We illustrate this in the case of strong self-interaction of the decay products. Without metric perturbations, preheating in this case is very inefficient. However, metric perturbations increase the maximum field variances and give alternative channels for the resonance to proceed. This implies that metric perturbations can have a large impact on calculations of relic abundances of particles produced during preheating.

hep-ph↗

arXiv2000

Restoring the sting to metric preheating

The relative growth of field and metric perturbations during preheating is sensitive to initial conditions set in the preceding inflationary phase. Recent work suggests this may protect super-Hubble metric perturbations from resonant amplification during preheating. We show that this possibility is fragile and sensitive to the specific form of the interactions between the inflaton and other fields. The suppression is naturally absent in two classes of preheating in which either (1) the vacua of the non-inflaton fields during inflation are deformed away from the origin, or (2) the effective masses of non-inflaton fields during inflation are small but during preheating are large. Unlike the simple toy model of a $g^2 ϕ^2 χ^2$ coupling, most realistic particle physics models contain these other features. Moreover, they generically lead to both adiabatic and isocurvature modes and non-Gaussian scars on super-Hubble scales. Large-scale coherent magnetic fields may also appear naturally.

hep-ph↗

arXiv2000

Chi-Variable-Speed-of-Light Cosmologies

Variable-Speed-of-Light (VSL) cosmologies are currently attracting much interest as a possible alternative to cosmological inflation. We discuss the fundamental geometrodynamic aspects of VSL cosmologies, and provide several alternative implementations. These implementations provide a large class of VSL cosmologies that pass the zeroth-order consistency tests of being compatible with both classical Einstein gravity and low-energy particle physics. While they solve the ``kinematic'' puzzles as well as inflation does, VSL cosmologies typically do not solve the flatness problem since in their purest form no violation of the strong energy condition occurs. Nevertheless, these models are easy to unify with inflation.

astro-ph↗

arXiv1999

Metric preheating and limitations of linearized gravity

Recently it has become clear that the resonant amplification of quantum field fluctuations at preheating must be accompanied by resonant amplification of scalar metric perturbations, since the two are united by Einstein's equations. Furthermore, this "metric preheating" enhances particle production and leads to gravitational rescattering effects even at linear order. In multi-field models with strong preheating (q \gg 1), metric perturbations are driven nonlinear, with the strongest amplification typically on super-Hubble scales (k \to 0). This amplification is causal, being due to the super- Hubble coherence of the inflaton condensate, and is accompanied by resonant growth of entropy perturbations. The amplification invalidates the use of the linearized Einstein field equations, irrespective of the amount of fine-tuning of the initial conditions. This has serious implications at all scales - from the large-angle cosmic microwave background (CMB) anisotropies to primordial black holes. We investigate the (q,k) parameter space in a two-field model, and introduce the time to nonlinearity, t_{nl}, as the timescale for the breakdown of the linearized Einstein equations. Backreaction effects are expected to shut down the linear resonances, but cannot remove the existing amplification, which threatens the viability of strong preheating when confronted with the CMB. We discuss ways to escape the above conclusions, including secondary phases of inflation and preheating solely to fermions. Finally we rank known classes of inflation from strongest (chaotic and strongly coupled hybrid inflation) to weakest (hidden sector, warm inflation) in terms of the distortion of the primordial spectrum due to these resonances in preheating.

hep-ph↗

arXiv1999

General Relativistic effects in preheating

General relativistic effects in the form of metric perturbations are usually neglected in the preheating era that follows inflation. We argue that in realistic multi-field models these effects are in fact crucial, and the fully coupled system of metric and quantum field fluctuations needs to be considered. Metric perturbations are resonantly amplified, breaking the scale-invariance of the primordial spectrum, and in turn stimulate scalar field resonances via gravitational rescattering. This non-gravitationally dominated nonlinear growth of gravitational fluctuations may have significant effects on the Doppler peaks in the cosmic background radiation, primordial black hole formation, gravitational waves and nonthermal symmetry restoration.

hep-ph↗

arXiv1998

Inflationary Reheating in Grand Unified Theories

Grand unified theories may display multiply interacting fields with strong coupling dynamics. This poses two new problems: (1) What is the nature of chaotic reheating after inflation, and (2) How is reheating sensitive to the mass spectrum of these theories ? We answer these questions in two interesting limiting cases and demonstrate an increased efficiency of reheating which strongly enhances non-thermal topological defect formation, including monopoles and domain walls. Nevertheless, the large fluctuations may resolve this monopole problem via a modified Dvali-Liu-Vachaspati mechanism in which non-thermal destabilsation of discrete symmetries occurs at reheating.

hep-ph↗