1,772 research outputs found
Scalar perturbations in regular two-component bouncing cosmologies
We consider a two-component regular cosmology bouncing from contraction to
expansion, where, in order to include both scalar fields and perfect fluids as
particular cases, the dominant component is allowed to have an intrinsic
isocurvature mode. We show that the spectrum of the growing mode of the Bardeen
potential in the pre-bounce is never transferred to the dominant mode of the
post-bounce. The latter acquires at most a dominant isocurvature component,
depending on the relative properties of the two fluids. Our results imply that
several claims in the literature need substantial revision.Comment: 10 pages, 1 figur
Regular two-component bouncing cosmologies and perturbations therein
We present a full investigation of scalar perturbations in a rather generic
model for a regular bouncing universe, where the bounce is triggered by an
effective perfect fluid with negative energy density. Long before and after the
bounce the universe is dominated by a source with positive energy density,
which may be a perfect fluid, a scalar field, or any other source with an
intrinsic isocurvature perturbation. Within this framework, we present an
analytical method to accurately estimate the spectrum of large-scale scalar
perturbations until their reentry, long after the bounce. We also propose a
simple way to identify non-singular gauge-invariant variables through the
bounce and present the results of extensive numerical tests in several possible
realizations of the scenario. In no case do we find that the spectrum of the
pre-bounce growing mode of the Bardeen potential can be transferred to a
post-bounce constant mode.Comment: 19 pages, 9 figure
Relativistic iron lines in accretion disks: the contribution of higher order images in the strong deflection limit
The shapes of relativistic iron lines observed in spectra of candidate black
holes carry the signatures of the strong gravitational fields in which the
accretion disks lie. These lines result from the sum of the contributions of
all images of the disk created by gravitational lensing, with the direct and
first-order images largely dominating the overall shapes. Higher order images
created by photons tightly winding around the black holes are often neglected
in the modeling of these lines, since they require a substantially higher
computational effort. With the help of the strong deflection limit, we present
the most accurate semi-analytical calculation of these higher order
contributions to the iron lines for Schwarzschild black holes. We show that two
regimes exist depending on the inclination of the disk with respect to the line
of sight. Many useful analytical formulae can be also derived in this
framework.Comment: 23 pages, 13 figure
Scalar fluctuations in dilatonic brane-worlds
We derive and solve the full set of scalar perturbation equations for a class
of five-dimensional brane--world solutions, with a dilaton scalar field coupled
to the bulk cosmological constant and to a 3-brane. The spectrum contains one
localized massless scalar mode, to be interpreted as an effective dilaton on
the brane, inducing long--range scalar interactions. Two massive scalar modes
yield corrections to Newton's law at short distances, which persist even in the
limit of vanishing dilaton (namely, in the standard Randall--Sundrum
configuration).Comment: 10 pages. Talk presented by V. Bozza at COSMO-01 conference,
Rovaniemi, 200
Neutrino Oscillations in Caianiello's Quantum Geometry Model
Neutrino flavor oscillations are analyzed in the framework of Quantum
Geometry model proposed by Caianiello. In particular, we analyze the
consequences of the model for accelerated neutrino particles which experience
an effective Schwarzschild geometry modified by the existence of an upper limit
on the acceleration, which implies a violation of the equivalence principle. We
find a shift of quantum mechanical phase of neutrino oscillations, which
depends on the energy of neutrinos as E^3. Implications on atmospheric and
solar neutrinos are discussed.Comment: 11 page
Maximal Acceleration Effects in Kerr Space
We consider a model in which accelerated particles experience line--elements
with maximal acceleration corrections that are introduced by means of
successive approximations. It is shown that approximations higher than the
first need not be considered. The method is then applied to the Kerr metric.
The effective field experienced by accelerated test particles contains
corrections that vanish in the limit , but otherwise affect the
behaviour of matter greatly. The corrections generate potential barriers that
are external to the horizon and are impervious to classical particles.Comment: 16 pages, 10 figures, to appear on Phys. Lett.
Assisting pre-big bang phenomenology through short-lived axions
We present the results of a detailed study of how isocurvature axion
fluctuations are converted into adiabatic metric perturbations through axion
decay, and discuss the constraints on the parameters of pre-big bang cosmology
needed for consistency with present CMB-anisotropy data. The large-scale
normalization of temperature fluctuations has a non-trivial dependence both on
the mass and on the initial value of the axion. In the simplest, minimal models
of pre-big bang inflation, consistency with the COBE normalization requires a
slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum
requires mild modifications of the minimal backgrounds at large curvature
and/or string coupling.Comment: 14 pages, latex, 1 figure included using epsfig. A few typos
corrected, two references added, the figure slightly improved. To appear in
Phys. Lett.
Constraints on pre-big bang parameter space from CMBR anisotropies
The so-called curvaton mechanism --a way to convert isocurvature
perturbations into adiabatic ones-- is investigated both analytically and
numerically in a pre-big bang scenario where the role of the curvaton is played
by a sufficiently massive Kalb--Ramond axion of superstring theory. When
combined with observations of CMBR anisotropies at large and moderate angular
scales, the present analysis allows us to constrain quite considerably the
parameter space of the model: in particular, the initial displacement of the
axion from the minimum of its potential and the rate of evolution of the
compactification volume during pre-big bang inflation. The combination of
theoretical and experimental constraints favours a slightly blue spectrum of
scalar perturbations, and/or a value of the string scale in the vicinity of the
SUSY-GUT scale.Comment: 63 pages in Latex style with 14 figures include
Analytic Kerr black hole lensing for equatorial observers in the strong deflection limit
In this paper we present an analytical treatment of gravitational lensing by
Kerr black holes in the limit of very large deflection angles, restricting to
observers in the equatorial plane. We accomplish our objective starting from
the Schwarzschild black hole and adding corrections up to second order in the
black hole spin. This is sufficient to provide a full description of all
caustics and the inversion of lens mapping for sources near them. On the basis
of these formulae we argue that relativistic images of Low Mass X-ray Binaries
around Sgr A* are very likely to be seen by future X-ray interferometry
missions.Comment: 19 pages, 9 figures, published on Phys. Rev.
Radiation Bursts from Particles in the Field of Compact, Impenetrable, Astrophysical Objects
The radiation emitted by charged, scalar particles in a Schwarzschild field
with maximal acceleration corrections is calculated classically and in the tree
approximation of quantum field theory. In both instances the particles emit
radiation that has characteristics similar to those of gamma-ray bursters.Comment: 11 pages, three figure
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