2,121 research outputs found
Initial Conditions and the Structure of the Singularity in Pre-Big-Bang Cosmology
We propose a picture, within the pre-big-bang approach, in which the universe
emerges from a bath of plane gravitational and dilatonic waves. The waves
interact gravitationally breaking the exact plane symmetry and lead generically
to gravitational collapse resulting in a singularity with the Kasner-like
structure. The analytic relations between the Kasner exponents and the initial
data are explicitly evaluated and it is shown that pre-big-bang inflation may
occur within a dense set of initial data. Finally, we argue that plane waves
carry zero gravitational entropy and thus are, from a thermodynamical point of
view, good candidates for the universe to emerge from.Comment: 18 pages, LaTeX, epsfig. 3 figures included. Minor changes; paragraph
added in the introduction, references added and typos corrected. Final
version published in Classical and Quantum Gravit
O(d,d)-invariance in inhomogeneous string cosmologies with perfect fluid
In the first part of the present paper, we show that O(d,d)-invariance
usually known in a homogeneous cosmological background written in terms of
proper time can be extended to backgrounds depending on one or several
coordinates (which may be any space-like or time-like coordinate(s)). In all
cases, the presence of a perfect fluid is taken into account and the equivalent
duality transformation in Einstein frame is explicitly given. In the second
part, we present several concrete applications to some four-dimensional
metrics, including inhomogeneous ones, which illustrate the different duality
transformations discussed in the first part. Note that most of the dual
solutions given here do not seem to be known in the literature.Comment: 25 pages, no figures, Latex. Accepted for publication in General
Relativity and Gravitatio
Inhomogeneous Einstein-Rosen String Cosmology
Families of anisotropic and inhomogeneous string cosmologies containing
non-trivial dilaton and axion fields are derived by applying the global
symmetries of the string effective action to a generalized Einstein-Rosen
metric. The models exhibit a two-dimensional group of Abelian isometries. In
particular, two classes of exact solutions are found that represent
inhomogeneous generalizations of the Bianchi type VI_h cosmology. The
asymptotic behaviour of the solutions is investigated and further applications
are briefly discussed.Comment: Minor extension of concluding section; 18 pages, to appear in
Phys.Rev.
Exact non-singular waves in the anti-de Sitter universe
A class of radiative solutions of Einstein's field equations with a negative
cosmological constant and a pure radiation is investigated. The space-times,
which generalize the Defrise solution, represent exact gravitational waves
which interact with null matter and propagate in the anti-de Sitter universe.
Interestingly, these solutions have homogeneous and non-singular wave-fronts
for all freely moving observers. We also study properties of sandwich and
impulsive waves which can be constructed in this class of space-times.Comment: 15 pages, 3 figures, To appear in Gen. Rel. Gra
Colliding Plane Waves in Einstein-Maxwell-Dilaton Fields
Within the metric structure endowed with two orthogonal space-like Killing
vectors a class of solutions of the Einstein-Maxwell-Dilaton field equations is
presented. Two explicitly given sub-classes of solutions bear an interpretation
as colliding plane waves in the low-energy limit of the heterotic string
theory.Comment: 14 pages, LaTex; To appear in Phys. Rev.
Conceptual Inadequacy of the Shannon Information in Quantum Measurements
In a classical measurement the Shannon information is a natural measure of
our ignorance about properties of a system. There, observation removes that
ignorance in revealing properties of the system which can be considered to
preexist prior to and independent of observation. Because of the completely
different root of a quantum measurement as compared to a classical measurement
conceptual difficulties arise when we try to define the information gain in a
quantum measurement using the notion of Shannon information. The reason is
that, in contrast to classical measurement, quantum measurement, with very few
exceptions, cannot be claimed to reveal a property of the individual quantum
system existing before the measurement is performed.Comment: 11 pages, 5 figures, important Ref. [6] is now cited in all
appropriate place
The highly polarized open cluster Trumpler 27
We have carried out multicolor linear polarimetry (UBVRI) of the brightest
stars in the area of the open cluster Trumpler 27. Our data show a high level
of polarization in the stellar light with a considerable dispersion, from to . The polarization vectors of the cluster members appear to be
aligned. Foreground polarization was estimated from the data of some non-member
objects, for which two different components were resolved: the first one
associated with a dust cloud close to the Sun producing
and degrees, and a second component, the main source of
polarization for the cluster members, originated in another dust cloud, which
polarizes the light in the direction of degrees. From a detailed
analysis, we found that the two components have associated values for the first one, and for the other. Due the
difference in the orientation of both polarization vectors, almost 90 degrees
(180 degrees at the Stokes representation), the first cloud (
degrees) depolarize the light strongly polarized by the second one ( degrees).Comment: 12 Pages, 6 Figures, 2 tables (9 Pages), accepted for publication in
A
Numerical Study of Inhomogeneous Pre-Big-Bang Inflationary Cosmology
We study numerically the inhomogeneous pre-big-bang inflation in a
spherically symmetric space-time. We find that a large initial inhomogeneity
suppresses the onset of the pre-big-bang inflation. We also find that even if
the pre-big-bang inflationary stage is realized, the initial inhomogeneities
are not homogenized. Namely, during the pre-big-bang inflation
``hairs''(irregularities) do not fall, in sharp contrast to the usual
(potential energy dominated) inflation where initial inhomogeneity and
anisotropy are damped and thus the resulting universe is less sensitive to
initial conditions.Comment: 12 pages + 14 figures, to be published in Phys.Rev.
G1 Cosmologies with Gravitational and Scalar Waves
I present here a new algorithm to generate families of inhomogeneous massless
scalar field cosmologies. New spacetimes, having a single isometry, are
generated by breaking the homogeneity of massless scalar field models
along one direction. As an illustration of the technique I construct
cosmological models which in their late time limit represent perturbations in
the form of gravitational and scalar waves propagating on a non-static
inhomogeneous background. Several features of the obtained metrics are
discussed, such as their early and late time limits, structure of singularities
and physical interpretation.Comment: 24 pages, 2 figure
On the Asymptotic Behaviour of Cosmological Models in Scalar-Tensor Theories of Gravity
We study the qualitative properties of cosmological models in scalar-tensor
theories of gravity by exploiting the formal equivalence of these theories with
general relativity minimally coupled to a scalar field under a conformal
transformation and field redefinition. In particular, we investigate the
asymptotic behaviour of spatially homogeneous cosmological models in a class of
scalar-tensor theories which are conformally equivalent to general relativistic
Bianchi cosmologies with a scalar field and an exponential potential whose
qualitative features have been studied previously. Particular attention is
focussed on those scalar-tensor theory cosmological models, which are shown to
be self-similar, that correspond to general relativistic models that play an
important r\^{o}le in describing the asymptotic behaviour of more general
models (e.g., those cosmological models that act as early-time and late-time
attractors).Comment: 22 pages, submitted to Phys Rev
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