2,651 research outputs found

### Modulated Perturbations from Instant Preheating after new Ekpyrosis

We present a mechanism to transfer the spectrum of perturbations in a scalar
isocurvature field $\xi$ onto the matter content in the radiation era via
modulated, instant preheating after ekpyrosis. In this setup, $\xi$ determines
the coupling constant relevant for the decay of a preheat matter field into
Fermions. The resulting power spectrum is scale invariant if $\xi$ 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
$\xi$ 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

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 $\lambda\to
1$ and recovers general relativity coupled to dark matter at low energy.
Finally, we propose a definition of nonlinear curvature perturbation ${\cal R}$
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

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

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, $\mu_c$, below which increasing the mass $\mu$ of
the field increases the oscillation frequency $\Re(\omega)$ of the resonance.
On the other hand, above the critical field mass increasing the mass $\mu$
increases the damping rate $\Im(\omega)$ of the mode. We confirm our analytical
results by numerical computations.Comment: 6 page

### Effects of particle production during inflation

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

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

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

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