60 research outputs found
New bulk scalar field solutions in brane worlds
We use nonlinear perturbation theory to obtain new solutions for brane world
models that incorporate a massive bulk scalar field. We then consider tensor
perturbations and show that Newtonian gravity is recovered on the brane for
both a light scalar field and for a bulk field with large negative mass. This
latter result points to the viability of higher-derivative theories of gravity
in the context of bulk extra dimensions.Comment: 4+\epsilon pages, no figure
Cosmological Perturbations in Flux Compactifications
Kaluza-Klein compactifications with four-dimensional inflationary geometry
combine the attractive idea of higher dimensional models with the attempt to
incorporate four-dimensional early-time or late-time cosmology. We analyze the
mass spectrum of cosmological perturbations around such compactifications,
including the scalar, vector, and tensor sector. Whereas scalar perturbations
were discussed before, the spectrum of vector and tensor perturbations is a new
result of this article. Moreover, the complete analysis shows, that possible
instabilities of such compactifications are restricted to the scalar sector.
The mass squares of the vector and tensor perturbations are all non-negative.
We discuss form fields with a non-trivial background flux in the extra space as
matter degrees of freedom. They provide a source of scalar and vector
perturbations in the effective four-dimensional theory. We analyze the
perturbations in Freund-Rubin compactifications. Although it can only be
considered as a toy model, we expect the results to qualitatively generalize to
similar configurations. We find that there are two possible channels of
instabilities in the scalar sector of perturbations, whose stabilization has to
be addressed in any cosmological model that incorporates extra dimensions und
form fields. One of the instabilities is associated with the perturbations of
the form field.Comment: 16 pages, v2 figure and references added, accepted version for JCA
Quantum backreaction of massive fields and self-consistent semiclassical extreme black holes and acceleration horizons
We consider the effect of backreaction of quantized massive fields on the
metric of extreme black holes (EBH). We find the analytical approximate
expression for the stress-energy tensor for a scalar (with an arbitrary
coupling), spinor and vector fields near an event horizon. We show that,
independent of a concrete type of EBH, the energy measured by a freely falling
observer is finite on the horizon, so that quantum backreaction is consistent
with the existence of EBH. For the Reissner-Nordstrom EBH with a total mass
M_{tot} and charge Q we show that for all cases of physical interest M_{tot}<
Q. We also discuss different types of quantum-corrected Bertotti-Robinson
spacetimes, find for them exact self-consistent solutions and consider
situations in which tiny quantum corrections lead to the qualitative change of
the classical geometry and topology. In all cases one should start not from a
classical background with further adding quantum corrections but from the
quantum-corrected self-consistent geometries from the very beginning.Comment: Minor corrections. To appear in Phys. Rev.
Gravity and non-gravity mediated couplings in multiple-field inflation
Mechanisms for the generation of primordial non-Gaussian metric fluctuations
in the context of multiple-field inflation are reviewed. As long as kinetic
terms remain canonical, it appears that nonlinear couplings inducing
non-gaussianities can be split into two types. The extension of the one-field
results to multiple degrees of freedom leads to gravity mediated couplings that
are ubiquitous but generally modest. Multiple-field inflation offers however
the possibility of generating non-gravity mediated coupling in isocurvature
directions that can eventually induce large non-Gaussianities in the metric
fluctuations. The robustness of the predictions of such models is eventually
examined in view of a case study derived from a high-energy physics
construction.Comment: 14 pages, 3 figures, invited review for CQG issue on non-linear
cosmolog
DEFROST: A New Code for Simulating Preheating after Inflation
At the end of inflation, dynamical instability can rapidly deposit the energy
of homogeneous cold inflaton into excitations of other fields. This process,
known as preheating, is rather violent, inhomogeneous and non-linear, and has
to be studied numerically. This paper presents a new code for simulating scalar
field dynamics in expanding universe written for that purpose. Compared to
available alternatives, it significantly improves both the speed and the
accuracy of calculations, and is fully instrumented for 3D visualization. We
reproduce previously published results on preheating in simple chaotic
inflation models, and further investigate non-linear dynamics of the inflaton
decay. Surprisingly, we find that the fields do not want to thermalize quite
the way one would think. Instead of directly reaching equilibrium, the
evolution appears to be stuck in a rather simple but quite inhomogeneous state.
In particular, one-point distribution function of total energy density appears
to be universal among various two-field preheating models, and is exceedingly
well described by a lognormal distribution. It is tempting to attribute this
state to scalar field turbulence.Comment: RevTeX 4.0; 16 pages, 9 figure
Can Inflating Braneworlds be Stabilized?
We investigate scalar perturbations from inflation in braneworld cosmologies
with extra dimensions. For this we calculate scalar metric fluctuations around
five dimensional warped geometry with four dimensional de Sitter slices. The
background metric is determined self-consistently by the (arbitrary) bulk
scalar field potential, supplemented by the boundary conditions at both
orbifold branes. Assuming that the inflating branes are stabilized (by the
brane scalar field potentials), we estimate the lowest eigenvalue of the scalar
fluctuations - the radion mass. In the limit of flat branes, we reproduce well
known estimates of the positive radion mass for stabilized branes.
Surprisingly, however, we found that for de Sitter (inflating) branes the
square of the radion mass is typically negative, which leads to a strong
tachyonic instability. Thus, parameters of stabilized inflating braneworlds
must be constrained to avoid this tachyonic instability. Instability of
"stabilized" de Sitter branes is confirmed by the BraneCode numerical
calculations in the accompanying paper hep-th/0309001. If the model's
parameters are such that the radion mass is smaller than the Hubble parameter,
we encounter a new mechanism of generation of primordial scalar fluctuations,
which have a scale free spectrum and acceptable amplitude.Comment: 7 pages, RevTeX 4.
Inflation and de Sitter Thermodynamics
We consider the quasi-de Sitter geometry of the inflationary universe. We
calculate the energy flux of the slowly rolling background scalar field through
the quasi-de Sitter apparent horizon and set it equal to the change of the
entropy (1/4 of the area) multiplied by the temperature, dE=TdS. Remarkably,
this thermodynamic law reproduces the Friedmann equation for the rolling scalar
field. The flux of the slowly rolling field through the horizon of the quasi-de
Sitter geometry is similar to the accretion of a rolling scalar field onto a
black hole, which we also analyze. Next we add inflaton fluctuations which
generate scalar metric perturbations. Metric perturbations result in a
variation of the area entropy. Again, the equation dE=TdS with fluctuations
reproduces the linearized Einstein equations. In this picture as long as the
Einstein equations hold, holography does not put limits on the quantum field
theory during inflation. Due to the accumulating metric perturbations, the
horizon area during inflation randomly wiggles with dispersion increasing with
time. We discuss this in connection with the stochastic decsription of
inflation. We also address the issue of the instability of inflaton
fluctuations in the ``hot tin can'' picture of de Sitter horizon.Comment: 19 pages, 5 figure
Semiclassical Stability of the Extreme Reissner-Nordstrom Black Hole
The stress-energy tensor of a free quantized scalar field is calculated in
the extreme Reissner-Nordstr\"{o}m black hole spacetime in the zero temperature
vacuum state. The stress-energy appears to be regular on the event horizon,
contrary to the suggestion provided by two-dimensional calculations. An
analytic calculation on the event horizon for a thermal state shows that if the
temperature is nonzero then the stress-energy diverges strongly there.Comment: 10 pages, REVTeX, 4 figures in separate uuencoded compressed fil
Entropy of Quantum Fields for Nonextreme Black Holes in the Extreme Limit
Nonextreme black hole in a cavity within the framework of the canonical or
grand canonical ensemble can approach the extreme limit with a finite
temperature measured on a boundary located at a finite proper distance from the
horizon. In spite of this finite temperature, it is shown that the one-loop
contribution of quantum fields to the thermodynamic entropy due
to equilibrium Hawking radiation vanishes in the limit under consideration. The
same is true for the finite temperature version of the Bertotti-Robinson
spacetime into which a classical Reissner-Nordstr\"{o}m black hole turns in the
extreme limit. The result is attributed to the nature of a horizon
for the Bertotti-Robinson spacetime.Comment: 11 pages, ReVTeX, no figures. New references added, discussion
expanded, presentation and English improved. Accepted for publication in
Phys. Rev.
- …