29 research outputs found
Born-Infeld Cosmologies
We present a model for an inhomogeneous and anisotropic early universe filled
with a nonlinear electromagnetic field of Born-Infeld (BI) type. The effects of
the BI field are compared with the linear case (Maxwell). Since the curvature
invaria nts are well behaved then we conjecture that our model does not present
an initial big bang singularity. The existence of the BI field modifies the
curvature invariants at t=0 as well as sets bounds on the amplitude of the
conformal metric functionComment: 13 pages, latex, 6 eps figure
Exact inhomogeneous Einstein-Maxwell-Dilaton cosmologies
We present solution generating techniques which permit to construct exact
inhomogeneous and anisotropic cosmological solutions to a four-dimensional low
energy limit of string theory containing non-minimally interacting
electromagnetic and dilaton fields. Some explicit homogeneous and inhomogeneous
cosmological solutions are constructed. For example, inhomogeneous exact
solutions presenting Gowdy - type EMD universe are obtained. The asymptotic
behaviour of the solutions is investigated. The asymptotic form of the metric
near the initial singularity has a spatially varying Kasner form. The character
of the space-time singularities is discussed. The late evolution of the
solutions is described by a background homogeneous and anisotropic universe
filled with weakly interacting gravitational, dilatonic and electromagnetic
waves.Comment: 10 pages, latex; v2: English corrected, new comments and reference
added in section 1, acknowledgments added; v3: final version to be published
in Phys. Rev.
Solution generating in scalar-tensor theories with a massless scalar field and stiff perfect fluid as a source
We present a method for generating solutions in some scalar-tensor theories
with a minimally coupled massless scalar field or irrotational stiff perfect
fluid as a source. The method is based on the group of symmetries of the
dilaton-matter sector in the Einstein frame. In the case of Barker's theory the
dilaton-matter sector possesses SU(2) group of symmetries. In the case of
Brans-Dicke and the theory with "conformal coupling", the dilaton- matter
sector has as a group of symmetries. We describe an explicit
algorithm for generating exact scalar-tensor solutions from solutions of
Einstein-minimally-coupled-scalar-field equations by employing the nonlinear
action of the symmetry group of the dilaton-matter sector. In the general case,
when the Einstein frame dilaton-matter sector may not possess nontrivial
symmetries we also present a solution generating technique which allows us to
construct exact scalar-tensor solutions starting with the solutions of
Einstein-minimally-coupled-scalar-field equations. As an illustration of the
general techniques, examples of explicit exact solutions are constructed. In
particular, we construct inhomogeneous cosmological scalar-tensor solutions
whose curvature invariants are everywhere regular in space-time. A
generalization of the method for scalar-tensor-Maxwell gravity is outlined.Comment: 10 pages,Revtex; v2 extended version, new parts added and some parts
rewritten, results presented more concisely, some simple examples of
homogeneous solutions replaced with new regular inhomogeneous solutions,
typos corrected, references and acknowledgements added, accepted for
publication 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
Phenomenology of the Gowdy Universe on
Numerical studies of the plane symmetric, vacuum Gowdy universe on yield strong support for the conjectured asymptotically velocity term
dominated (AVTD) behavior of its evolution toward the singularity except,
perhaps, at isolated spatial points. A generic solution is characterized by
spiky features and apparent ``discontinuities'' in the wave amplitudes. It is
shown that the nonlinear terms in the wave equations drive the system
generically to the ``small velocity'' AVTD regime and that the spiky features
are caused by the absence of these terms at isolated spatial points.Comment: 19 pages, 21 figures, uses Revtex, psfi
Acceleration of the universe, vacuum metamorphosis, and the large-time asymptotic form of the heat kernel
We investigate the possibility that the late acceleration observed in the
rate of expansion of the universe is due to vacuum quantum effects arising in
curved spacetime. The theoretical basis of the vacuum cold dark matter (VCDM),
or vacuum metamorphosis, cosmological model of Parker and Raval is revisited
and improved. We show, by means of a manifestly nonperturbative approach, how
the infrared behavior of the propagator (related to the large-time asymptotic
form of the heat kernel) of a free scalar field in curved spacetime causes the
vacuum expectation value of its energy-momentum tensor to exhibit a resonance
effect when the scalar curvature R of the spacetime reaches a particular value
related to the mass of the field. we show that the back reaction caused by this
resonance drives the universe through a transition to an accelerating expansion
phase, very much in the same way as originally proposed by Parker and Raval.
Our analysis includes higher derivatives that were neglected in the earlier
analysis, and takes into account the possible runaway solutions that can follow
from these higher-derivative terms. We find that the runaway solutions do not
occur if the universe was described by the usual classical FRW solution prior
to the growth of vacuum energy-density and negative pressure (i.e., vacuum
metamorphosis) that causes the transition to an accelerating expansion of the
universe in this theory.Comment: 33 pages, 3 figures. Submitted to Physical Review D15 (Dec 23, 2003).
v2: 1 reference added. No other change
Initial Conditions in String Cosmology
We take a critical look at a recent conjecture concerning the past attractor
in the pre-big-bang scenario. We argue that the Milne universe is unlikely to
be a general past attractor for such models and support this with a number of
examples.Comment: 10 pages standard Latex format, no figures. Submitted to Phys. Rev.
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.