776 research outputs found
Brane-world Cosmologies with non-local bulk effects
It is very common to ignore the non-local bulk effects in the study of
brane-world cosmologies using the brane-world approach. However, we shall
illustrate through the use of three different scenarios, that the non-local
bulk-effect does indeed have significant impact on both the
initial and future behaviour of brane-world cosmologies.Comment: 17 pages, no figures, iopart.cls, submitted to CQ
Teleparallel Theories of Gravity: Illuminating a Fully Invariant Approach
Teleparallel gravity and its popular generalization gravity can be
formulated as fully invariant (under both coordinate transformations and local
Lorentz transformations) theories of gravity. Several misconceptions about
teleparallel gravity and its generalizations can be found in the literature,
especially regarding their local Lorentz invariance. We describe how these
misunderstandings may have arisen and attempt to clarify the situation. In
particular, the central point of confusion in the literature appears to be
related to the inertial spin connection in teleparallel gravity models. While
inertial spin connections are commonplace in special relativity, and not
something inherent to teleparallel gravity, the role of the inertial spin
connection in removing the spurious inertial effects within a given frame of
reference is emphasized here. The careful consideration of the inertial spin
connection leads to the construction of a fully invariant theory of
teleparallel gravity and its generalizations. Indeed, it is the nature of the
spin connection that differentiates the relationship between what have been
called good tetrads and bad tetrads and clearly shows that, in principle, any
tetrad can be utilized. The field equations for the fully invariant formulation
of teleparallel gravity and its generalizations are presented and a number of
examples using different assumptions on the frame and spin connection are
displayed to illustrate the covariant procedure. Various modified teleparallel
gravity models are also briefly reviewed.Comment: v2: 72 pages, revised version, references added, matches published
versio
Scalar Field Cosmologies with Barotropic Matter: Models of Bianchi class B
We investigate in detail the qualitative behaviour of the class of Bianchi
type B spatially homogeneous cosmological models in which the matter content is
composed of two non-interacting components; the first component is described by
a barotropic fluid having a gamma-law equation of state, whilst the second is a
non-interacting scalar field (phi) with an exponential potential V=Lambda exp(k
phi). In particular, we study the asymptotic properties of the models both at
early and late times, paying particular attention on whether the models
isotropize (and inflate) to the future, and we discuss the genericity of the
cosmological scaling solutions.Comment: 18 pages, 1 figure, uses revtex and epsf to insert figur
Bianchi Type Cosmological Models in Tele-parallel Gravity
Symmetry assumptions on the geometrical framework have provided successful
mechanisms to develop physically meaningful solutions to many problems. In
tele-parallel gravity, invariance of the frame and spin-connection under a
group of motions defines an affine symmetry group. Here, we assume there exists
a three-dimensional group of affine symmetries acting simply transitively on a
spatial hypersurface and that this group of symmetry actions defines our affine
frame symmetry group. We determine the general form of the co-frame and spin
connection for each spatially homogeneous Bianchi type. We then construct the
corresponding field equations for tele-parallel gravity. We show that if
the symmetry group is of Bianchi type A (, , , , or
) then there exists a co-frame/spin connection pair that is consistent with
the antisymmetric part of the field equations of tele-parallel gravity.
For those geometries having a Bianchi type B symmetry group (, , ,
), we find that in general these geometries are inconsistent with the
antisymmetric part of the tele-parallel gravity field equations unless
the theory reduces to an analog of General Relativity with a cosmological
constant.Comment: 28 page
Qualitative Viscous Cosmology
The Full (non--truncated) Israel--Stewart theory of bulk viscosity is applied
to dissipative FRW spacetimes. Dimensionless variables and dimensionless
equations of state are used to write the Einstein--thermodynamic equations as a
plane autonomous system and the qualitative behaviour of this system is
determined. Entropy production in these models is also discussed.Comment: 13 pages, REVTeX, accepted for publication in Physical Review
The Dynamics of Multi-Scalar Field Cosmological Models and Assisted Inflation
We investigate the dynamical properties of a class of spatially homogeneous
and isotropic cosmological models containing a barotropic perfect fluid and
multiple scalar fields with independent exponential potentials. We show that
the assisted inflationary scaling solution is the global late-time attractor
for the parameter values for which the model is inflationary, even when
curvature and barotropic matter are included. For all other parameter values
the multi-field curvature scaling solution is the global late-time attractor
(in these solutions asymptotically the curvature is not dynamically
negligible). Consequently, we find that in general all of the scalar fields in
multi-field models with exponential potentials are non-negligible in late-time
behaviour, contrary to what is commonly believed. The early-time and
intermediate behaviour of the models is also studied. In particular, n-scalar
field models are investigated and the structure of the saddle equilibrium
points corresponding to inflationary m-field scaling solutions and
non-inflationary m-field matter scaling solutions are also studied (where m<n),
leading to interesting transient dynamical behaviour with new physical
scenarios of potential importance.Comment: 27 pages, uses REVTeX Added an appendix illustrating some of the
details needed to compute the stability of the assisted inflationary solutio
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