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 \zeta is 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.Comment: 8 pages, 4 colour figures. Version to appear in Phys. Rev. D. Contains substantial new analysis of the ranges of parameter space for which large changes to the inflation-produced power spectrum are expecte

A new twist to preheating

Metric perturbations typically strengthen field resonances during preheating. In contrast we present a model in which the super-Hubble field resonances are completely {\em suppressed} when metric perturbations are included. The model is the nonminimal Fakir-Unruh scenario which is exactly solvable in the long-wavelength limit when metric perturbations are included, but exhibits exponential growth of super-Hubble modes in their absence. This gravitationally enhanced integrability is exceptional, both for its rarity and for the power with which it illustrates the importance of including metric perturbations in consistent studies of preheating. We conjecture a no-go result - there exists no {\em single-field} model with growth of cosmologically-relevant metric perturbations during preheating.Comment: 6 pages, 3 figures, Version to appear in Physical Review

Post-Inflationary Reheating

We study a model for reheating that has been much investigated for parametric resonance, having a quartic interaction of the scalar inflaton with another scalar field. Attention is particularly on the quantum excitations of the inflaton field and the metric perturbation with a smooth transition from quantum to classical stochastic states, followed through from a specific inflation model to a state including a relativistic fluid. The scalar fields enter non-perturbatively but the metric enters perturbatively, and the validity of this latter is assessed. In this model our work seems to point the large scale curvature parameter changing.Comment: 25 pages, 6 figures. Coding error(misprint) corrected:figures and some conclusions change

Classification of Multiwavelength Transients with Machine Learning

With the advent of powerful telescopes such as the Square Kilometer Array and the Vera C. Rubin Observatory, we are entering an era of multiwavelength transient astronomy that will lead to a dramatic increase in data volume. Machine learning techniques are well suited to address this data challenge and rapidly classify newly detected transients. We present a multiwavelength classification algorithm consisting of three steps: (1) interpolation and augmentation of the data using Gaussian processes; (2) feature extraction using wavelets; and (3) classification with random forests. Augmentation provides improved performance at test time by balancing the classes and adding diversity into the training set. In the first application of machine learning to the classification of real radio transient data, we apply our technique to the Green Bank Interferometer and other radio light curves. We find we are able to accurately classify most of the 11 classes of radio variables and transients after just eight hours of observations, achieving an overall test accuracy of 78 percent. We fully investigate the impact of the small sample size of 82 publicly available light curves and use data augmentation techniques to mitigate the effect. We also show that on a significantly larger simulated representative training set that the algorithm achieves an overall accuracy of 97 percent, illustrating that the method is likely to provide excellent performance on future surveys. Finally, we demonstrate the effectiveness of simultaneous multiwavelength observations by showing how incorporating just one optical data point into the analysis improves the accuracy of the worst performing class by 19 percent.Comment: 16 pages, 12 figure

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.Comment: 4 pages, 5 ps figures - 1 colour, Revtex. Further (colour & 3-D) figures available from http://www.sissa.it/~bassett/reheating/ . Matched to version to appear in Phys. Rev. let

Preheating of the nonminimally coupled inflaton field

We investigate preheating of an inflaton field $\phi$ coupled nonminimally to a spacetime curvature. In the case of a self-coupling inflaton potential $V(\phi)=\lambda \phi^4/4$, the dynamics of preheating changes by the effect of the negative $\xi$. We find that the nonminimal coupling works in two ways. First, since the initial value of inflaton field for reheating becomes smaller with the increase of $|\xi|$, the evolution of the inflaton quanta is delayed for fixed $\lambda$. Second, the oscillation of the inflaton field is modified and the nonadiabatic change around $\phi=0$ occurs significantly. That makes the resonant band of the fluctuation field wider. Especially for strong coupling regimes $|\xi| \gg 1$, the growth of the inflaton flutuation is dominated by the resonance due to the nonminimal coupling, which leads to the significant enhancement of low momentum modes. Although the final variance of the inflaton fluctuation does notchange significantly compared with the minimally coupled case, we have found that the energy transfer from the homogeneous inflaton to created particles efficiently occurs for $\xi<-60$.Comment: 13pages, 11figure

Are black holes over-produced during preheating?

We provide a simple but robust argument that primordial black hole (PBH) production generically does {\em not} exceed astrophysical bounds during the resonant preheating phase after inflation. This conclusion is supported by fully nonlinear lattice simulations of various models in two and three dimensions which include rescattering but neglect metric perturbations. We examine the degree to which preheating amplifies density perturbations at the Hubble scale and show that at the end of the parametric resonance, power spectra are universal, with no memory of the power spectrum at the end of inflation. In addition we show how the probability distribution of density perturbations changes from exponential on very small scales to Gaussian when smoothed over the Hubble scale -- the crucial length for studies of primordial black hole formation -- hence justifying the standard assumption of Gaussianity.Comment: 12 pages, 8 figures, revtex, added references for section

Inflationary Reheating Classes via Spectral Methods

Inflationary reheating is almost completely controlled by the Floquet indices, $\mu_k$. Using spectral theory we demonstrate that the stability bands (where $\mu_k = 0$) of the Mathieu and Lam\'e equations are destroyed even in Minkowski spacetime, leaving a fractal Cantor set or a measure zero set of stable modes in the cases where the inflaton evolves in an almost-periodic or stochastic manner respectively. These two types of potential model the expected multi-field and quantum backreaction effects during reheating.Comment: 5 pages, 2 ps figures, Revtex. Version to appear in Phys. Rev. D (Rapid Communication, July 15

Dynamical Dark Energy or Simply Cosmic Curvature?

We show that the assumption of a flat universe induces critically large errors in reconstructing the dark energy equation of state at z>~0.9 even if the true cosmic curvature is very small, O(1%) or less. The spuriously reconstructed w(z) shows a range of unusual behaviour, including crossing of the phantom divide and mimicking of standard tracking quintessence models. For 1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching (50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble rate measurements have opposite trends due to the asymmetric influence of the curved geodesics. These results show that including curvature as a free parameter is imperative in any future analyses attempting to pin down the dynamics of dark energy, especially at moderate or high redshifts.Comment: 5 pages, 2 figures. To appear in JCA

Variable-Speed-of-Light Cosmology and Second Law of Thermodynamics

We examine whether the cosmologies with varying speed of light (VSL) are compatible with the second law of thermodynamics. We find that the VSL cosmology with varying fundamental constant is severely constrained by the second law of thermodynamics, whereas the bimetric cosmological models are less constrained.Comment: 15 pages, LaTeX, Revised version with minor corrections to appear in Phys. Rev.