219 research outputs found
On the accuracy of slow-roll inflation given current observational constraints
We investigate the accuracy of slow-roll inflation in light of current
observational constraints, which do not allow for a large deviation from scale
invariance. We investigate the applicability of the first and second order
slow-roll approximations for inflationary models, including those with large
running of the scalar spectral index. We compare the full numerical solutions
with those given by the first and second order slow-roll formulae. We find that
even first order slow-roll is generally accurate; the largest deviations arise
in models with large running where the error in the power spectrum can be at
the level of 1-2%. Most of this error comes from inaccuracy in the calculation
of the slope and not of the running or higher order terms. Second order
slow-roll does not improve the accuracy over first order. We also argue that in
the basis , ,
introduced by Schwarz et al. (2001), slow-roll does not require all of the
parameters to be small. For example, even a divergent leads to
finite solutions which are accurately described by a slow-roll approximation.
Finally, we argue that power spectrum parametrization recently introduced by
Abazaijan, Kadota and Stewart does not work for models where spectral index
changes from red to blue, while the usual Taylor expansion remains a good
approximation.Comment: Matches the published PRD version; added comments on Abazaijan,
Kadota and Stewart (2005) parameterizatio
Hawking-Moss Tunneling with a Dirac-Born-Infeld Action
The Hawking-Moss tunneling rate for a field described by the
Dirac-Born-Infeld action is calculated using a stochastic approach. We find
that the effect of the non-trivial kinetic term is to enhance the tunneling
rate, which can be exponentially significant. This result should be compared to
the DBI enhancement found in the Coleman-de Luccia case.Comment: 4 pages, version accepted in Phys. Rev. D., additional references and
example applicatio
The cosmological gravitational wave background from primordial density perturbations
We discuss the gravitational wave background generated by primordial density
perturbations evolving during the radiation era. At second-order in a
perturbative expansion, density fluctuations produce gravitational waves. We
calculate the power spectra of gravitational waves from this mechanism, and
show that, in principle, future gravitational wave detectors could be used to
constrain the primordial power spectrum on scales vastly different from those
currently being probed by large-scale structure. As examples we compute the
gravitational wave background generated by both a power-law spectrum on all
scales, and a delta-function power spectrum on a single scale.Comment: 8 Page
Greybody factors in a rotating black-hole background-II : fermions and gauge bosons
We study the emission of fermion and gauge boson degrees of freedom on the
brane by a rotating higher-dimensional black hole. Using matching techniques,
for the near-horizon and far-field regime solutions, we solve analytically the
corresponding field equations of motion. From this, we derive analytical
results for the absorption probabilities and Hawking radiation emission rates,
in the low-energy and low-rotation case, for both species of fields. We produce
plots of these, comparing them to existing exact numerical results with very
good agreement. We also study the total absorption cross-section and
demonstrate that, as in the non-rotating case, it has a different behaviour for
fermions and gauge bosons in the low-energy limit, while it follows a universal
behaviour -- reaching a constant, spin-independent, asymptotic value -- in the
high-energy regime.Comment: 22 pages, 8 figures, added reference
Challenging the weak cosmic censorship conjecture with charged quantum particles
Motivated by the recent attempts to violate the weak cosmic censorship
conjecture for near-extreme black-holes, we consider the possibility of
overcharging a near-extreme Reissner-Nordstr\"om black hole by the quantum
tunneling of charged particles. We consider the scattering of spin-0 and
spin-1/2 particles by the black hole in a unified framework and obtain
analytically, for the first time, the pertinent reflection and transmission
coefficients without any small charge approximation. Based on these results, we
propose some gedanken experiments that could lead to the violation of the weak
cosmic censorship conjecture due to the (classically forbidden) absorption of
small energy charged particles by the black hole. As for the case of scattering
in Kerr spacetimes, our results demonstrate explicitly that scalar fields are
subject to (electrical) superradiance phenomenon, while spin-1/2 fields are
not. Superradiance impose some limitations on the gedanken experiments
involving spin-0 fields, favoring, in this way, the mechanisms for creation of
a naked singularity by the quantum tunneling of spin-1/2 charged fermions. We
also discuss the implications that vacuum polarization effects and quantum
statistics might have on these gedanken experiments. In particular, we show
that they are not enough to prevent the absorption of incident small energy
particles and, consequently, the formation of a naked singularity.Comment: 9 pages; Final version to appear in PR
Gravitational wave generation in hybrid quintessential inflationary models
We investigate the generation of gravitational waves in the hybrid
quintessential inflationary model. The full gravitational-wave energy spectrum
is calculated using the method of continuous Bogoliubov coefficients. The
post-inflationary kination period, characteristic of quintessential
inflationary models, leaves a clear signature on the spectrum, namely, a peak
at high frequencies. The maximum of the peak is firmly located at the MHz-GHz
region of the spectrum and corresponds to . This
peak is substantially smaller than the one appearing in the gravitational-wave
energy spectrum of the original quintessential inflationary model, therefore
avoiding any conflict with the nucleosynthesis constraint on
\Omega_\Omega_{GW}.Comment: 10 pages, 11 figures, one reference adde
The effect of a scanning flat fold mirror on a CMB B-mode experiment
We investigate the possibility of using a flat-fold beam steering mirror for
a CMB B-mode experiment. An aluminium flat-fold mirror is found to add
0.075% polarization, which varies in a scan synchronous way. Time-domain
simulations of a realistic scanning pattern are performed, and the effect on
the power-spectrum illustrated and a possible method of correction applied.Comment: 8 pages, 5 figures. Accepted for publication in Rev Sci Ins
Emission of Massive Scalar Fields by a Higher-Dimensional Rotating Black-Hole
We perform a comprehensive study of the emission of massive scalar fields by
a higher-dimensional, simply rotating black hole both in the bulk and on the
brane. We derive approximate, analytic results as well as exact numerical ones
for the absorption probability, and demonstrate that the two sets agree very
well in the low and intermediate-energy regime for scalar fields with mass
m_\Phi < 1 TeV in the bulk and m_\Phi < 0.5 TeV on the brane. The numerical
values of the absorption probability are then used to derive the Hawking
radiation power emission spectra in terms of the number of extra dimensions,
angular-momentum of the black hole and mass of the emitted field. We compute
the total emissivities in the bulk and on the brane, and demonstrate that,
although the brane channel remains the dominant one, the bulk-over-brane energy
ratio is considerably increased (up to 33%) when the mass of the emitted field
is taken into account.Comment: 28 pages, 18 figure
WMAP-normalized Inflationary Model Predictions and the Search for Primordial Gravitational Waves with Direct Detection Experiments
In addition to density perturbations, inflationary models of the early
universe generally predict a stochastic background of gravitational waves or
tensor fluctuations. By making use of the inflationary flow approach for single
field models and fitting the models with Monte-Carlo techniques to cosmic
microwave background (CMB) data from the {\it Wilkinson Microwave Anisotropy
Probe} (WMAP), we discuss the expected properties of the gravitational wave
background from inflation at scales corresponding to direct detection
experiments with laser interferometers in space. We complement the Monte-Carlo
numerical calculations by including predictions expected under several classes
of analytical inflationary models. We find that an improved version of {\it Big
Bang Observer} (BBO-grand) can be used to detect a gravitational wave
background at 0.1 Hz with a corresponding CMB tensor-to-scalar ratio above
10. Even if the CMB tensor-to-scalar ratio were to be above 10,
we suggest that BBO-grand will be useful to study inflationary models as the
standard version of BBO, with a sensitivity to a stochastic gravitational wave
background , will only allow a marginal
detection of the amplitude while leaving the tensor spectral index at 0.1 Hz
unconstrained. We also discuss the extent to which CMB measurements can be used
to predict the gravitational wave background amplitude in a direct detection
experiment and how any measurement of the amplitude and the spectral tilt of
the gravitational wave background at direct detection frequencies together with
the CMB tensor-to-scalar ratio can be used to establish slow-roll inflation.Comment: 18 pages, 12 figures. Submitted to PRD. Low resolution figures
submitted here. A copy with high resolution figures and software to generate
numerical models can be obtained at http://www.cooray.org/inflation.htm
Non-Gaussianity and large-scale structure in a two-field inflationary model
Single field inflationary models predict nearly Gaussian initial conditions
and hence a detection of non-Gaussianity would be a signature of the more
complex inflationary scenarios. In this paper we study the effect on the cosmic
microwave background and on large scale structure from primordial
non-Gaussianity in a two-field inflationary model in which both the inflaton
and curvaton contribute to the density perturbations. We show that in addition
to the previously described enhancement of the galaxy bias on large scales,
this setup results in large-scale stochasticity. We provide joint constraints
on the local non-Gaussianity parameter and the ratio
of the amplitude of primordial perturbations due to the inflaton and curvaton
using WMAP and SDSS data
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