2,651 research outputs found

    Modulated Perturbations from Instant Preheating after new Ekpyrosis

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    We present a mechanism to transfer the spectrum of perturbations in a scalar isocurvature field ξ\xi onto the matter content in the radiation era via modulated, instant preheating after ekpyrosis. In this setup, ξ\xi determines the coupling constant relevant for the decay of a preheat matter field into Fermions. The resulting power spectrum is scale invariant if ξ\xi remains close to a scaling solution in new ekpyrotic models of the universe; by construction the spectrum is independent of the detailed physics near the bounce. The process differs from the curvaton mechanism, which has been used recently to revive the ekpyrotic scenario, in that no peculiar behavior of ξ\xi shortly before or during the bounce is needed. In addition, a concrete and efficient realization of reheating after ekpyrosis is provided; this mechanism is not tied to ekpyrotic models, but could equally well be used in other setups, for instance inflationary ones. We estimate non-Gaussianities and find no additional contributions in the most simple realizations, in contrast to models using the curvaton mechanism.Comment: 21 pages; v2 references added, minor clarification

    Nonlinear superhorizon perturbations in Horava-Lifshitz gravity

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    We perform a fully nonlinear analysis of superhorizon perturbation in Ho\v{r}ava-Lifshitz gravity, based on the gradient expansion method. We present a concrete expression for the solution of gravity equations up to the second order in the gradient expansion, and prove that the solution can be extended to any order. The result provides yet another example for analogue of the Vainshtein effect: the nonlinear solution is regular in the limit λ1\lambda\to 1 and recovers general relativity coupled to dark matter at low energy. Finally, we propose a definition of nonlinear curvature perturbation R{\cal R} in Ho\v{r}ava-Lifshitz gravity and show that it is conserved up to the first order in the gradient expansion.Comment: 11 page

    Transport equations for the inflationary spectral index

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    We present a simple and efficient method to compute the superhorizon evolution of the spectral index in multifield inflationary models, using transport equation techniques. We illustrate the evolution of n(s) with time for various interesting potentials

    Quasinormal resonances of a massive scalar field in a near-extremal Kerr black hole spacetime

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    The fundamental resonances of near-extremal Kerr black holes due to massive scalar perturbations are derived {\it analytically}. We show that there exists a critical mass parameter, μc\mu_c, below which increasing the mass μ\mu of the field increases the oscillation frequency (ω)\Re(\omega) of the resonance. On the other hand, above the critical field mass increasing the mass μ\mu increases the damping rate (ω)\Im(\omega) of the mode. We confirm our analytical results by numerical computations.Comment: 6 page

    Effects of particle production during inflation

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    The impact of particle production during inflation on the primordial curvature perturbation spectrum is investigated both analytically and numerically. We obtain an oscillatory behavior on small scales, while on large scales the spectrum is unaffected. The amplitude of the oscillations is proportional to the number of coupled fields, their mass, and the square of the coupling constant. The oscillations are due a discontinuity in the second time derivative of the inflaton, arising from a temporary violation of the slow-roll conditions. A similar effect on the power spectrum should be produced also in other inflationary models where the slow-roll conditions are temporarily violated.Comment: 7 pages, 5 figure

    Primordial perturbation with a modified dispersion relation

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    In this paper we study the generation of primordial perturbation with a modified dispersion relation in various cosmological evolutions. We stress that the formation of the power spectrum is strongly dependent on the background. Working in a bounce model with a matter-like contracting phase, we obtain a red tilt spectrum due to the modified dispersion relation.Comment: 6 pages, 2 figures, 1 tabl

    A note on the resonant frequencies of rapidly rotating black holes

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    I discuss the range of validity of Detweiler's formula for the resonant frequencies of rapidly rotating Kerr black holes. While his formula is correct for extremal black holes, it has also been commonly accepted that it describes very well the resonant frequencies of near extremal black holes, and that therefore there is a large number of modes clustering on the real axis as the black hole becomes extremal. I will show that this last statement is not only incorrect, but that it also does not follow from Detweiler's formula, provided it is handled with due care. It turns out that only the first n << -log{(r_+-r_-)/r_+} modes are well described by that formula, which translates, for any astrophysical black hole, into one or two modes only. All existing numerical data gives further support to this claim. I also discuss some implications of this result for recent investigations on the late-time dynamics of rapidly rotating black holes.Comment: 5 pages, ReVTeX