The failure of cosmological perturbation theory in the new ekpyrotic and cyclic ekpyrotic scenarios

Cosmological perturbation theory fails in the new ekpyrotic and cyclic ekpyrotic scenarios, before the scale factor of the Universe reaches zero. As a result, a recently-proposed recipe for evolving the curvature perturbation through the bounce in these scenarios cannot be justified.Comment: 10 pages. v2: Logic of the demonstration of non-linearity stated more explicitely. Remarks on non-singular bounces removed for inclusion in a later paper. v3: As accepted by Phys Lett B v4: Comment on the paper of Banks and Fischler removed following its withdrawal; more precise abstrac

Late-time creation of gravitinos from the vacuum

Starting with the vacuum fluctuation, it is known that gravitinos will be created just after inflation, with number density $\sim 10^{-2}M^3$ where $M$ is the mass of the inflaton. Here, we argue that creation may be expected to continue, maintaining about the same number density, until a usually much later epoch. This epoch is either the `intermediate epoch' when Hubble parameter falls below the gravitino mass, or the reheat epoch if that is earlier. We verify that such late-time creation indeed occurs if only a single chiral superfield is relevant, using the description of the helicity 1/2 gravitino provided recently by Kallosh et. al. (hep-th/9907124) and Giudice et. al. (hep-ph/9907510). Arguments are presented in favor of late-time creation in the general case. For the usual inflation models, $M$ is rather large and gravitinos from late-time creation are so abundant that a subsequent era of thermal inflation is needed to dilute them.Comment: 9 pages. As it will appear in PLB. Small typos corrected, a sign discrepancy cleared up, nothing significan

Generation of perturbation after multi-field inflation

We explore a new possibility that some inflaton fields in multi-field inflation models satisfy the observed value of the spectral index so that the curvature perturbation generated by them through post-inflationary dynamics may be relevant for the current observations. We illustrate this point using a simple and reasonable multi-field chaotic inflation model motivated from string theory. Possible non-Gaussianity and the tensor-to-scalar ratio are briefly addressed.Comment: 8 pages, 1 figure; (v2) 9 pages, discussions expanded, to appear in Physics Letters

The gravitino abundance in supersymmetric `new' inflation models

We consider the abundance of gravitinos created from the vacuum fluctuation, in a class of `new' inflation models for which global supersymmetry is a good approximation. Immediately after inflation, gravitinos are produced, with number density determined by equations recently presented by Kallosh et. al. (hep-th/9907124) and Giudice et. al. (hep-ph/9907510). Unless reheating intervenes, creation may continue, maintaining about the same number density, until the Hubble parameter falls below the gravitino mass. In any case, the abundance of gravitinos created from the vacuum fluctuation exceeds the abundance from thermal collisions in a significant regime of parameter space, leading to tighter cosmological constraints.Comment: 9 pages latex. Same results but argument for late-time creation now appears in hep-ph/991231

Constraints on TeV-scale hybrid inflation and comments on non-hybrid alternatives

During hybrid inflation, the slowly-rolling inflaton field has a significant coupling to the trigger field which is responsible for most of the potential. Barring a fine-tuned accidental cancellation, this coupling induces a minimal one-loop contribution to the inflaton potential. The requirement that this contribution be not too large constrains a wide class of hybrid inflation models. Assuming that the inflaton perturbation generates structure in the Universe, the inflaton field and/or the trigger field after inflation have to be bigger than 10^9\GeV. This and other results make hybrid inflation at or below the TeV scale problematical. (There is no problem with hybrid inflation at the high energy scales normally considered.) `New' and thermal inflation seem to be viable alternatives for inflation at or below the TeV scale, including the case that quantum gravity is at the TeV scale. In any case, supersymmetry is needed required during inflation, in order to protect a scalar mass.Comment: 15 pages, one ref added in V

Models of inflation and their predictions

Taking field theory seriously, inflation model-building is difficult but not impossible. The observed value of the spectral index of the adiabatic density perturbation is starting to discriminate between models, and may well pick out a unique one in the forseeable future.Comment: Latex 5 pages. To appear in Particle Physics and the Early Universe (COSMO99), AIP (1999