34 research outputs found
Bianchi-type string cosmology
Bianchi-type string cosmology involves generalizations of the FRW backgrounds
with three transitive spacelike Killing symmetries, but without any a priori
assumption of isotropy in the 3D sections of homogeneity. With emphasis on
those cases with diagonal metrics and vanishing cosmological constant which
which have not been previously examined in the literature, the present findings
allow an overview and the classification of all Bianchi-type backgrounds. These
string solutions (at least to lowest order in alpha prime) offer prototypes for
the study of spatial anisotropy and its impact on the dynamics of the early
universe.Comment: 12 pages, latex, 1 table, no figure
Mixing towards isotropization and gravitational-wave relic in string cosmology
The cosmological problem of {\em mixing}, with its various facets better known individually (as the horizon problem, the isotropy and entropy problems etc) is re-examined in the context of string cosmology and in terms of an open alternative to the mixmaster model. The mixing agent here is a gravitational wave with leading-order isotropization attained whe the first full wavelength is formed in the expanding young horizon. Standing and circularly polarized, this wave is inscribed within Bianchi-type VII_0 spatial sections of homogeneity. The finally emerging asymptotically flat FRW background, is intimately related to the axion scalar, in spite of the fact that the latter has virtually disappeared by that time. Surviving, however, is a relic gravitational wave with data and other observable imrints from the early string dynamics
On certain cosmological relics of the early string dynamics
The tracing of cosmological relics from the early string dynamics may enhance
the theory and provide new perspectives on the major cosmological problems.
This point is illustrated in a leading-order Bianchi-type string
background, wherein spatial isotropy can be claimed as such a relic. A much
finer one, descending from a premordial gravitational wave, could be retrieved
from its imprint on the small-scale structure of the cosmic microwave
background. In spite of the absence of conventional inflation, there is no
horizon problem thanks to the presence of an equally fundamental mixmaster
dynamics. Implications and certain new perspectives which thus arise for the
more general problem of cosmological mixing are briefly discussed.Comment: Latex file, 11 pages, 0 figures. Re-titled revision of "Mixing
towards isotropization..." to appear in Class. Quantum Gravit
Spatially Homogeneous String Cosmologies
We determine the most general form of the antisymmetric -field tensor
derived from a purely time-dependent potential that is admitted by all possible
spatially homogeneous cosmological models in 3+1-dimensional low-energy bosonic
string theory. The maximum number of components of the field that are left
arbitrary is found for each homogeneous cosmology defined by the Bianchi group
classification. The relative generality of these string cosmologies is found by
counting the number of independent pieces of Cauchy data needed to specify the
general solution of Einstein's equations. The hierarchy of generality differs
significantly from that characteristic of vacuum and perfect-fluid cosmologies.
The degree of generality of homogeneous string cosmologies is compared to that
of the generic inhomogenous solutions of the string field equations.Comment: 16 pages, Latex, assumptions clarified, calculations unchanged,
published in Phys. Rev.
Stringy Toda Cosmologies
We discuss a particular stringy modular cosmology with two axion fields in
seven space-time dimensions, decomposable as a time and two flat three-spaces.
The effective equations of motion for the problem are those of the Toda
molecule, and hence are integrable. We write down the solutions, and show that
all of them are singular. They can be thought of as a generalization of the
Pre-Big-Bang cosmology with excited internal degrees of freedom, and still
suffering from the graceful exit problem. Some of the solutions however show a
rather unexpected property: some of their spatial sections shrink to a point in
spite of winding modes wrapped around them. We also comment how more general,
anisotropic, solutions, with fewer Killing symmetries can be obtained with the
help of STU dualities.Comment: 22 pages, latex, no figures. Several minor typos correcte
Symmetries for generating string cosmologies
We discuss the symmetry properties of the low-energy effective action of the
type IIB superstring that may be employed to derive four-dimensional solutions.
A truncated effective action, compactified on a six-torus, but including both
Neveu/Schwarz-Neveu/Schwarz and Ramond-Ramond field strengths, can be expressed
as a non-linear sigma model which is invariant under global SL(3,R)
transformations. This group contains as a sub-group the SL(2,R) symmetry of the
ten-dimensional theory and a discrete Z2 reflection symmetry which leads to a
further SL(2,R) sub-group. The symmetries are employed to analyse a general
class of spatially homogeneous cosmological solutions with non-trivial
Ramond-Ramond fields.Comment: Substantially extended version with new sections on further
symmetries and anisotropic cosmological solutions. New title. To appear in
Physical Review D. 13 pages, LaTeX, no figure
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.
Effects of anisotropy and spatial curvature on the pre-big bang scenario
A class of exact, anisotropic cosmological solutions to the vacuum
Brans-Dicke theory of gravity is considered within the context of the pre-big
bang scenario. Included in this class are the Bianchi type III, V and VI_h
models and the spatially isotropic, negatively curved
Friedmann-Robertson-Walker universe. The effects of large anisotropy and
spatial curvature are determined. In contrast to negatively curved
Friedmann-Robertson-Walker model, there exist regions of the parameter space in
which the combined effects of curvature and anisotropy prevent the occurrence
of inflation. When inflation is possible, the necessary and sufficient
conditions for successful pre-big bang inflation are more stringent than in the
isotropic models. The initial state for these models is established and
corresponds in general to a gravitational plane wave.Comment: 15 pages, including 2 eps figure
Vacuum fluctuations in axion-dilaton cosmologies
We study axion-dilaton cosmologies derived from the low-energy string
effective action. We present the classical homogeneous
Friedmann-Robertson-Walker solutions and derive the semi-classical perturbation
spectra in the dilaton, axion and moduli fields in the pre-Big Bang scenario.
By constructing the unique S-duality invariant field perturbations for the
axion and dilaton fields we derive S-duality invariant solutions, valid when
the axion field is time-dependent as well as in a dilaton-vacuum cosmology.
Whereas the dilaton and moduli fields have steep blue perturbation spectra
(with spectral index n=4) we find that the axion spectrum depends upon the
expansion rate of the internal dimensions (0.54<n<4) which allows
scale-invariant (n=1) spectra. We note that for n<1 the metric is non-singular
in the conformal frame in which the axion is minimally coupled.Comment: LaTeX, 23 pages plus 6 figures, minor typos corrected and references
updated. To appear in Phys Rev