231 research outputs found
The separate universe and the back reaction of long wavelength fluctuations
We investigate the backreaction of cosmological long wavelength perturbations
on the evolution of the Universe. By applying the renormalization group method
to a Friedmann-Robertson-Walker universe with long wavelength fluctuations, we
demonstrate that the renormalized solution with the backreaction effect is
equivalent to that of the separate universe. Then, using the effective
Friedmann equation, we show that only non-adiabatic mode of long wavelength
fluctuations affects the expansion law of the spatially averaged universe.Comment: 10 pages. to be published in Phys. Rev.
Gravitational Wave Background from Phantom Superinflation
Recently, the early superinflation driven by phantom field has been proposed
and studied. The detection of primordial gravitational wave is an important
means to know the state of very early universe. In this brief report we discuss
in detail the gravitational wave background excited during the phantom
superinflation.Comment: 3 pages, 2 eps figures, to be published in PRD, revised with
published version, refs. adde
Quantum corrections to gravity and their implications for cosmology and astrophysics
The quantum contributions to the gravitational action are relatively easy to
calculate in the higher derivative sector of the theory. However, the
applications to the post-inflationary cosmology and astrophysics require the
corrections to the Einstein-Hilbert action and to the cosmological constant,
and those we can not derive yet in a consistent and safe way. At the same time,
if we assume that these quantum terms are covariant and that they have relevant
magnitude, their functional form can be defined up to a single free parameter,
which can be defined on the phenomenological basis. It turns out that the
quantum correction may lead, in principle, to surprisingly strong and
interesting effects in astrophysics and cosmology.Comment: 15 pages, LaTeX, WS style, contribution to the Proceedings of the
QFEXT-2011 conference in the Centro de Ciencias de Benasque Pedro Pasqual,
Spai
Quantum effects and superquintessence in the new age of precision cosmology
Recent observations of Type Ia supernova at high redshifts establish that the
dark energy component of the universe has (a probably constant) ratio between
pressure and energy density . The
conventional quintessence models for dark energy are restricted to the range
, with the cosmological constant corresponding to .
Conformally coupled quintessence models are the simplest ones compatible with
the marginally allowed superaccelerated regime (). However, they are
known to be plagued with anisotropic singularities.
We argue here that the extension of the classical approach to the
semiclassical one, with the inclusion of quantum counterterms necessary to
ensure the renormalization, can eliminate the anisotropic singularities
preserving the isotropic behavior of conformally coupled superquintessence
models. Hence, besides of having other interesting properties, they are
consistent candidates to describe the superaccelerated phases of the universe
compatible with the present experimental data.Comment: 7 pages. Essay selected for "Honorable Mention" in the 2004 Awards
for Essays on Gravitation, Gravity Research Foundatio
Quantum Gravity Corrections to the One Loop Scalar Self-Mass during Inflation
We compute the one loop corrections from quantum gravity to the
self-mass-squared of a massless, minimally coupled scalar on a locally de
Sitter background. The calculation was done using dimensional regularization
and renormalized by subtracting fourth order BPHZ counterterms. Our result
should determine whether quantum gravitational loop corrections can
significantly alter the dynamics of a scalar inflaton.Comment: 47 pages, 3 figures, 20 tables, uses LaTeX 2 epsilon, version 2
revised for publication in Physical Review
On the Asymptotic Stability of De-Sitter Spacetime: a non-linear perturbative approach
We derive evolution and constraint equations for second order perturbations
of flat dust homogeneous and isotropic solutions to the Einstein field
equations using all scalar, vector and tensor perturbation modes. We show that
the perturbations decay asymptotically in time and that the solutions converge
to the De-Sitter solution. By induction, this result is valid for perturbations
of arbitrary order. This is in agreement with the cosmic no-hair conjecture of
Gibbons and Hawking.Comment: 11 pages, 2 figure
Reflection coefficient for superresonant scattering
We investigate superresonant scattering of acoustic disturbances from a
rotating acoustic black hole in the low frequency range. We derive an
expression for the reflection coefficient, exhibiting its frequency dependence
in this regime.Comment: 7 page
The graceful exit from the anomaly-induced inflation: Supersymmetry as a key
The stable version of the anomaly-induced inflation does not need a fine
tuning and leads to sufficient expansion of the Universe. The non-stable
version (Starobinsky model) provides the graceful exit to the FRW phase. We
indicate the possibility of the inflation which is stable at the beginning and
unstable at the end. The effect is due to the soft supersymmetry breaking and
the decoupling of the massive sparticles at low energy.Comment: 10 pages, 2 figures using axodraw. Modified version. Discussion
concerning the gravitational scale modified, the effect of massive particles
in the last stage of inflation taken into accoun
Superradiance from BEC vortices: a numerical study
The scattering of sound wave perturbations from vortex excitations of
Bose-Einstein condensates(BEC) is investigated by numerical integration of the
associated Klein-Gordon equation. It is found that, at sufficiently high
angular speeds, sound wave-packets can extract a sizeable fraction of the
vortex energy through a mechanism of superradiant scattering. It is conjectured
that this superradiant regime may be detectable in BEC experiments.Comment: 4 pages, 4 figure
Tensor perturbations in high-curvature string backgrounds
We derive a generalized equation for the evolution of tensor perturbations in
a cosmological background, taking into account higher-curvature contributions
and a tree-level coupling to the dilaton in the string frame. The equation is
obtained by perturbing the gravi-dilaton string effective action, expanded up
to first order in . The corrections can modify the
low-energy perturbation spectrum, but the modifications are shown to be small
when the background curvature keeps constant in the string frame.Comment: 9 pages, REVTEX, three figures included using EPSFIG. An updated
collection of papers on the pre-big bang scenario in string cosmology is a
available at http://www.to.infn.it/teorici/gasperin
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