216 research outputs found
Adiabatic regularization of the graviton stress-energy tensor in de Sitter space-time
We study the renormalized energy-momentum tensor of gravitons in a de Sitter
space-time. After canonically quantizing only the physical degrees of freedom,
we adopt the standard adiabatic subtraction used for massless minimally coupled
scalar fields as a regularization procedure and find that the energy density of
gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the
Hubble parameter, but with a positive sign. According to this result the scalar
expansion rate, which is gauge invariant in de Sitter space-time, is increased
by the fluctuations. This implies that gravitons may then add to conformally
coupled matter in driving the Starobinsky model of inflation.Comment: 5 pages, revtex, final version accepted for publication in PR
Primordial black holes formation in a early matter dominated era from the pre-big bang scenario
We discuss the production of primordial black holes in an early matter
dominated era, which typically takes place in string inspired early universe
cosmological models. In particular, we consider a pre-big bang scenario
(extending previous results regarding formation in the radiation dominated era)
where the enhancement of curvature perturbations is induced by a variation of
the sound-speed parameter c_s during the string phase of high-curvature
inflation. After imposing all relevant observational constraints, we find that
the considered class of models is compatible with the production of a large
amount of primordial black holes, in the mass range relevant to dark matter,
only for a small range of the parameters space. On the other hand, we find that
a huge production of light primordial black holes may occur both in such matter
dominated era and in the radiation dominated one.Comment: 7 pages, 3 figure
Isotropic Observers and the Inflationary Backreaction Problem
In an inflationary regime driven by a free massive inflaton we derive within
a genuinely gauge invariant approach the backreaction effects due to long
wavelength scalar fluctuations on the effective Hubble factor and equation of
state with respect to a class of observers which sees an inhomogeneous and
isotropic Universe. We find that, for such so-called isotropic observers,
contrary to what happens for the observables defined by free-falling observers,
there is an effect to leading order in the slow-roll parameter in the direction
of slowing down the measured rate of expansion and of having an effective
equation of state less de Sitter like. From a general point of view the
isotropic observers result has to be considered complementary to other cases
(observers) in helping to characterize the physical properties of the models
under investigation.Comment: 16 pages, latex. Comments added, errors and typos corrected, main
results unchanged. Version accepted for publication in Class. Quantum Gra
Stochastic growth of quantum fluctuations during slow-roll inflation
We compute the growth of the mean square of quantum fluctuations of test
fields with small effective mass during a slowly changing, nearly de Sitter
stage which took place in different inflationary models. We consider a
minimally coupled scalar with a small mass, a modulus with an effective mass (with as the Hubble parameter) and a massless non-minimally
coupled scalar in the test field approximation and compare the growth of their
relative mean square with the one of gauge-invariant inflaton fluctuations. We
find that in most of the single field inflationary models the mean square gauge
invariant inflaton fluctuation grows {\em faster} than any test field with a
non-negative effective mass. Hybrid inflationary models can be an exception:
the mean square of a test field can dominate over the gauge invariant inflaton
fluctuation one on suitably choosing parameters. We also compute the stochastic
growth of quantum fluctuation of a second field, relaxing the assumption of its
zero homogeneous value, in a generic inflationary model; as a main result, we
obtain that the equation of motion of a gauge invariant variable associated,
order by order, with a generic quantum scalar fluctuation during inflation can
be obtained only if we use the number of e-folds as the time variable in the
corresponding Langevin and Fokker-Planck equations for the stochastic approach.
We employ this approach to derive some bounds in the case of a model with two
massive fields.Comment: 9 pages, 4 figures. Added references, minor changes, matches the
version to be published in Phys. Rev.
Primordial Black Holes from Pre-Big Bang inflation
We discuss the possibility of producing a significant fraction of dark matter
in the form of primordial black holes in the context of the pre-big bang
inflationary scenario. We take into account, to this purpose, the enhancement
of curvature perturbations possibly induced by a variation of the sound-speed
parameter during the string phase of high-curvature inflation. After
imposing all relevant observational constraints, we find that the considered
class of models is compatible with the production of a large amount of
primordial black holes in the mass range relevant to dark matter, provided the
sound-speed parameter is confined in a rather narrow range of values, .Comment: 26 pages, two figures. Many new references and a few comments added.
Version accepted for publication in JCA
Primordial black holes from pre-big bang inflation
We discuss the possibility of producing a significant fraction of dark matter in the form of primordial black holes in the context of the pre-big bang inflationary scenario. We take into account, to this purpose, the enhancement of curvature perturbations possibly induced by a variation of the sound-speed parameter cs during the string phase of high-curvature inflation. After imposing all relevant observational constraints, we find that the considered class of models is compatible with the production of a large amount of primordial black holes in the mass range relevant to dark matter, provided the sound-speed parameter is confined in a rather narrow range of values, 0.003 cs 0.01
Second Order Gauge-Invariant Perturbations during Inflation
The evolution of gauge invariant second-order scalar perturbations in a
general single field inflationary scenario are presented. Different second
order gauge invariant expressions for the curvature are considered. We evaluate
perturbatively one of these second order curvature fluctuations and a second
order gauge invariant scalar field fluctuation during the slow-roll stage of a
massive chaotic inflationary scenario, taking into account the deviation from a
pure de Sitter evolution and considering only the contribution of super-Hubble
perturbations in mode-mode coupling. The spectra resulting from their
contribution to the second order quantum correlation function are nearly
scale-invariant, with additional logarithmic corrections to the first order
spectrum. For all scales of interest the amplitude of these spectra depend on
the total number of e-folds. We find, on comparing first and second order
perturbation results, an upper limit to the total number of e-folds beyond
which the two orders are comparable.Comment: 17 pages, 6 figures. Final version to appear in Phys. Rev.
Backreaction during inflation: a physical gauge invariant formulation
Within a genuinely gauge invariant approach recently developed for the
computation of the cosmological backreaction, we study, in a cosmological
inflationary context and with respect to various observers, the impact of
scalar fluctuations on the space-time dynamics in the long wavelength limit. We
stress that such a quantum backreaction effect is evaluated in a truly gauge
independent way using a set of effective equations which describe the dynamics
of the averaged geometry. In particular we show under what conditions the free
falling (geodetic) observers do not experience any scalar-induced backreaction
in the effective Hubble rate and fluid equation of state.Comment: 4 pages. Typos corrected, comment added, version accepted for
publication in Phys. Rev. Let
Generation of fluctuations during inflation: comparison of stochastic and field-theoretic approaches
We prove that the stochastic and standard field-theoretical approaches
produce exactly the same results for the amount of light massive scalar field
fluctuations generated during inflation in the leading order of the slow-roll
approximation. This is true both in the case for which this field is a test one
and inflation is driven by another field, and the case for which the field
plays the role of inflaton itself. In the latter case, in order to calculate
the average of the mean square of the gauge-invariant inflaton fluctuation, the
logarithm of the scale factor has to be used as the time variable in the
Fokker-Planck equation in the stochastic approach. The implications of particle
production during inflation for the second stage of inflation and for the
moduli problem are also discussed. The case of a massless self-interacting test
scalar field in a de Sitter background with a zero initial renormalized mean
square is also considered in order to show how the stochastic approach can
easily produce results corresponding to diagrams with an arbitrary number of
scalar field loops in the field-theoretical approach (explicit results up to 4
loops inclusive are presented).Comment: Discussion expanded, references added, conclusions unchanged, matches
the version to be published in Phys. Rev.
Light-cone averaging in cosmology: formalism and applications
We present a general gauge invariant formalism for defining cosmological
averages that are relevant for observations based on light-like signals. Such
averages involve either null hypersurfaces corresponding to a family of past
light-cones or compact surfaces given by their intersection with timelike
hypersurfaces. Generalized Buchert-Ehlers commutation rules for derivatives of
these light-cone averages are given. After introducing some adapted "geodesic
light-cone" coordinates, we give explicit expressions for averaging the
redshift to luminosity-distance relation and the so-called "redshift drift" in
a generic inhomogeneous Universe.Comment: 20 pages, 2 figures. Comments and references added, typos corrected.
Version accepted for publication in JCA
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