2,721 research outputs found
Cosmologies with Two-Dimensional Inhomogeneity
We present a new generating algorithm to construct exact non static solutions
of the Einstein field equations with two-dimensional inhomogeneity. Infinite
dimensional families of inhomogeneous solutions with a self interacting
scalar field, or alternatively with perfect fluid, can be constructed using
this algorithm. Some families of solutions and the applications of the
algorithm are discussed.Comment: 9 pages, one postscript figur
On the Entropy and the Density Matrix of Cosmological Perturbations
We look at the transition to the semiclassical behaviour and the decoherence
process for the inhomogeneous perturbations in the inflationary universe. Two
different decoherence mechanisms appear: one dynamical, accompanied with a
negligible, if at all, entropy gain, and the other, effectively irreversible
dephasing, due to a rapid variation in time of the off-diagonal density matrix
elements in the post-inflationary epoch. We thus settle the discrepancies in
the entropy content of perturbations evaluated by different authors.Comment: LaTeX2e with the epsf packag
Penrose Limits, the Colliding Plane Wave Problem and the Classical String Backgrounds
We show how the Szekeres form of the line element is naturally adapted to
study Penrose limits in classical string backgrounds. Relating the "old"
colliding wave problem to the Penrose limiting procedure as employed in string
theory we discuss how two orthogonal Penrose limits uniquely determine the
underlying target space when certain symmetry is imposed. We construct a
conformally deformed background with two distinct, yet exactly solvable in
terms of the string theory on R-R backgrounds, Penrose limits. Exploiting
further the similarities between the two problems we find that the Penrose
limit of the gauged WZW Nappi-Witten universe is itself a gauged WZW plane wave
solution of Sfetsos and Tseytlin. Finally, we discuss some issues related to
singularity, show the existence of a large class of non-Hausdorff solutions
with Killing Cauchy Horizons and indicate a possible resolution of the problem
of the definition of quantum vacuum in string theory on these time-dependent
backgrounds.Comment: Some misprints corrected. Matches the version in print. To appear in
Classical & Quantum Gravit
Two-flux Colliding Plane Waves in String Theory
We construct the two-flux colliding plane wave solutions in higher
dimensional gravity theory with dilaton, and two complementary fluxes. Two
kinds of solutions has been obtained: Bell-Szekeres(BS) type and homogeneous
type. After imposing the junction condition, we find that only Bell-Szekeres
type solution is physically well-defined. Furthermore, we show that the future
curvature singularity is always developed for our solutions.Comment: 16 pages, Latex; typoes corrected; references added, minor
modification
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
Gravitational Optics: Self-phase modulation and harmonic cascades
Nonlinear wave interaction of low amplitude gravitational waves in flat
space-time is considered. Analogy with optics is established. It is shown that
the flat metric space-time is equivalent to a centro-symmetric optical medium,
with no second order susceptibility. The lowest order nonlinear effects are
those due to the third order nonlinearity and include self-phase modulation and
high harmonic generation. These processes lead to an efficient energy dilution
of the gravitational wave energy over an increasingly large spectral range.Comment: 12 pages, REVTEX
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
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.
Cognitive Defusion Versus Thought Distraction: A Clinical Rationale, Training, and Experiential Exercise in Altering Psychological Impacts of Negative Self-Referential Thoughts.
Using two modes of intervention delivery, the present study compared the effects of a cognitive defusion strategy with a thought distraction strategy on the emotional discomfort and believability of negative self-referential thoughts. One mode of intervention delivery consisted of a clinical rationale and training (i.e., Partial condition). The other mode contained a condition-specific experiential exercise with the negative self-referential thought in addition to the clinical rationale and training (i.e., Full condition). Non-clinical undergraduates were randomly assigned to one of five protocols: Partial-Defusion, Full-Defusion, Partial-Distraction, Full-Distraction, and a distraction-based experimental control task. The Full-Defusion condition reduced the emotional discomfort and believability of negative self-referential thoughts significantly greater than other comparison conditions. The positive results of the Full-Defusion condition were also found among participants with elevated depressive symptoms
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
- …