Chaotic dynamics in preheating after inflation

We study chaotic dynamics in preheating after inflation in which an inflaton $\phi$ is coupled to another scalar field $\chi$ through an interaction $(1/2)g^2\phi^2\chi^2$. We first estimate the size of the quasi-homogeneous field $\chi$ at the beginning of reheating for large-field inflaton potentials $V(\phi)=V_0\phi^n$ by evaluating the amplitude of the $\chi$ fluctuations on scales larger than the Hubble radius at the end of inflation. Parametric excitations of the field $\chi$ during preheating can give rise to chaos between two dynamical scalar fields. For the quartic potential ($n=4$, $V_0=\lambda/4$) chaos actually occurs for $g^2/\lambda <{\cal O}(10)$ in a linear regime before which the backreaction of created particles becomes important. This analysis is supported by several different criteria for the existence of chaos. For the quadratic potential ($n=2$) the signature of chaos is not found by the time at which the backreaction begins to work, similar to the case of the quartic potential with $g^2/\lambda \gg 1$.Comment: 12 pages, 10 figures, Version to appear in Classical and Quantum Gravit

Properties of singularities in (phantom) dark energy universe

The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an equation of state $w<-1$ and numerically confirm that this is actually a late-time attractor in the phantom-dominated universe. We also construct a phantom dark energy scenario coupled to dark matter that reproduces singular behaviors of the Big Rip type for the energy density and the curvature of the universe. The effect of quantum corrections coming from conformal anomaly can be important when the curvature grows large, which typically moderates the finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in Physical Review

Testing for double inflation with WMAP

With the WMAP data we can now begin to test realistic models of inflation involving multiple scalar fields. These naturally lead to correlated adiabatic and isocurvature (entropy) perturbations with a running spectral index. We present the first full (9 parameter) likelihood analysis of double inflation with WMAP data and find that despite the extra freedom, supersymmetric hybrid potentials are strongly constrained with less than 7% correlated isocurvature component allowed when standard priors are imposed on the cosomological parameters. As a result we also find that Akaike & Bayesian model selection criteria rather strongly prefer single-field inflation, just as equivalent analysis prefers a cosmological constant over dynamical dark energy in the late universe. It appears that simplicity is the best guide to our universe.Comment: 7 pages, 6 figure

Reconstruction of general scalar-field dark energy models

The reconstruction of scalar-field dark energy models is studied for a general Lagrangian density $p(\phi, X)$, where $X$ is a kinematic term of a scalar field $\phi$. We implement the coupling $Q$ between dark energy and dark matter and express reconstruction equations using two observables: the Hubble parameter $H$ and the matter density perturbation $\delta_m$. This allows us to determine the structure of corresponding theoretical Lagrangian together with the coupling $Q$ from observations. We apply our formula to several forms of Lagrangian and present concrete examples of reconstruction by using the recent Gold dataset of supernovae measurements. This analysis includes a generalized ghost condensate model as a way to cross a cosmological-constant boundary even for a single-field case.Comment: 8 pages, 2 figure

New constraints on multi-field inflation with nonminimal coupling

We study the dynamics and perturbations during inflation and reheating in a multi-field model where a second scalar field $\chi$ is nonminimally coupled to the scalar curvature $(\frac12 \xi R\chi^2$). When $\xi$ is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the $\chi$ fluctuation in sub-Hubble modes can be amplified during preheating for large $\xi$. For negative values of $\xi$, however, long-wave modes of the $\chi$ fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when $|\xi|$ is less than unity. This is because the effective $\chi$ mass becomes negative during inflation. We constrain the strength of $\xi$ and the initial $\chi$ by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the $\chi$ mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.Comment: 20 pages, 8 figures, submitted to Physical Review

Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory

We discuss the realization of inflation and resulting cosmological perturbations in the low-energy effective string theory. In order to obtain nearly scale-invariant spectra of density perturbations and a suppressed tensor-to-scalar ratio, it is generally necessary that the dilaton field $\phi$ is effectively decoupled from gravity together with the existence of a slowly varying dilaton potential. We also study the effect of second-order corrections to the tree-level action which are the sum of a Gauss-Bonnet term coupled to $\phi$ and a kinetic term $(\nabla \phi)^4$. We find that it is possible to realize observationally supported spectra of scalar and tensor perturbations provided that the correction is dominated by the $(\nabla \phi)^4$ term even in the absence of the dilaton potential. When the Gauss-Bonnet term is dominant, tensor perturbations exhibit violent negative instabilities on small-scales about a de Sitter background in spite of the fact that scale-invariant scalar perturbations can be achieved.Comment: 13 pages; v2: minor corrections, refs. added, version to appear in PR

Cosmological constraints from Gauss-Bonnet braneworld with large-field potentials

We calculate the spectral index and tensor-to-scalar ratio for patch inflation defined by $H^2\approx \beta^2_q V^q$ and $\dot{\phi}\approx -V'/3H$, using the slow-roll expansion. The patch cosmology arisen from the Gauss-Bonnet braneworld consists of Gauss-Bonnet (GB), Randall-Sundrum (RS), and 4D general relativistic (GR) cosmological models. In this work, we choose large-field potentials of $V=V_0\phi^p$ to compare with the observational data. Since second-order corrections are rather small in the slow-roll limit, the leading-order calculation is sufficient to compare with the data. Finally, we show that it is easier to discriminate between quadratic potential and quartic potential in the GB cosmological model rather than the GR or RS cosmological models.Comment: 13 pages, title changed, version to appear in JCA

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