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

Brane preheating

We study brane-world preheating in massive chaotic inflationary scenario where scalar fields are confined on the 3-brane. Assuming that quadratic contribution in energy densities dominates the Hubble expansion rate during preheating, the amplitude of inflaton decreases slowly relative to the standard dust-dominated case. This leads to an efficient production of $\chi$ particles via nonperturbative decay of inflaton even if its coupling is of order $g=10^{-5}$. We also discuss massive particle creation heavier than inflaton, which may play important roles for the baryo- and lepto-genesis scenarios.Comment: 6 pages, 2 figures, submitted to Physical Review

Enhancing the tensor-to-scalar ratio in simple inflation

We show that in theories with a nontrivial kinetic term the contribution of the gravitational waves to the CMB fluctuations can be substantially larger than that is naively expected in simple inflationary models. This increase of the tensor-to-scalar perturbation ratio leads to a larger B-component of the CMB polarization, thus making the prospects for future detection much more promising. The other important consequence of the considered model is a higher energy scale of inflation and hence higher reheating temperature compared to a simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly extended, published versio

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.Comment: 11 pages, RevTex; minor changes to text; conclusions unchanged; version to appear in JCA

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 the 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 multiparameter numerical search for correlations in two concrete models. The correlation is found to be particularly strong in a supersymmetric scenario due to the 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

Multiple-field inflation and the CMB

In this paper, we investigate some consequences of multiple-field inflation for the cosmic microwave background radiation (CMB). We derive expressions for the amplitudes, the spectral indices and the derivatives of the indices of the CMB power spectrum in the context of a very general multiple-field theory of slow-roll inflation, where the field metric can be non-trivial. Both scalar (adiabatic, isocurvature and mixing) and tensor perturbations are treated and the differences with single-field inflation are discussed. From these expressions, several relations are derived that can be used to determine the importance of multiple-field effects observationally from the CMB. We also study the evolution of the total entropy perturbation during radiation and matter domination and the influence of this on the isocurvature spectral quantities.Comment: 24 pages. References added, some very minor textual changes, matches version to be published in CQ

A Quintessentially Geometric Model

We consider string inspired cosmology on a solitary $D3$-brane moving in the background of a ring of branes located on a circle of radius $R$. The motion of the $D3$-brane transverse to the plane of the ring gives rise to a radion field which can be mapped to a massive non-BPS Born-Infeld type field with a cosh potential. For certain bounds of the brane tension we find an inflationary phase is possible, with the string scale relatively close to the Planck scale. The relevant perturbations and spectral indices are all well within the expected observational bounds. The evolution of the universe eventually comes to be dominated by dark energy, which we show is a late time attractor of the model. However we also find that the equation of state is time dependent, and will lead to late time Quintessence.Comment: 11 pages, 3 figures. References and comments adde

Stochastic Gravitational Wave Production After Inflation

In many models of inflation, the period of accelerated expansion ends with preheating, a highly non-thermal phase of evolution during which the inflaton pumps energy into a specific set of momentum modes of field(s) to which it is coupled. This necessarily induces large, transient density inhomogeneities which can source a significant spectrum of gravitational waves. In this paper, we consider the generic properties of gravitational waves produced during preheating, perform detailed calculations of the spectrum for several specific inflationary models, and identify problems that require further study. In particular, we argue that if these gravitational waves exist they will necessarily fall within the frequency range that is feasible for direct detection experiments -- from laboratory through to solar system scales. We extract the gravitational wave spectrum from numerical simulations of preheating after $\lambda \phi^4$ and $m_{\phi}^2 \phi^2$ inflation, and find that they lead to a gravitational wave amplitude of around $\Omega_{gw}h^2\sim 10^{-10}$. This is considerably higher than the amplitude of the primordial gravitational waves produced during inflation. However, the typical wavelength of these gravitational waves is considerably shorter than LIGO scales, although in extreme cases they may be visible at scales accessible to the proposed BBO mission. We survey possible experimental approaches to detecting any gravitational wave background generated during preheating.Comment: 11 pages. Updated references. Minor clarification

f(R) theories

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in Relativity, Published version, Comments are welcom

Inflation models and observation

We consider small-field models, which invoke the usual framework for the effective field theory, and large-field models, which go beyond this. Present and future possibilities for discriminating between the models are assessed, on the assumption that the primordial curvature perturbation is generated during inflation. With PLANCK data, the theoretical and observational uncertainties on the spectral index will be comparable, providing useful discrimination between small-field models. Further discrimination between models may come later through the tensor fraction, the running of the spectral index and non-Gaussianity