39 research outputs found
Cosmological constraints on parameters of one-brane models with extra dimension
We study some aspects of cosmologies in 5D models with one infinite extra
dimension. Matter is confined to the brane, gravity extends to the bulk. Models
with positive and negative tension of the brane are considered. Cosmological
evolution of the 4D world is described by warped solutions of the generalized
Friedmann equation. Cosmological solutions on the brane are obtained with the
input of the present-time observational cosmological parameters. We estimate
the age of the Universe and abundance of produced in primordial
nucleosynthesis in different models. Using these estimates we find constraints
on dimensionless combinations of the 5D gravitational scale, scale of the warp
factor and coupling at the 4D curvature term in the action.Comment: 21 pages, 4 figure
CBR Anisotropy from Primordial Gravitational Waves in Two-Component Inflationary Cosmology
We examine stochastic temperature fluctuations of the cosmic background
radiation (CBR) arising via the Sachs-Wolfe effect from gravitational wave
perturbations produced in the early universe. We consider spatially flat,
perturbed FRW models that begin with an inflationary phase, followed by a mixed
phase containing both radiation and dust. The scale factor during the mixed
phase takes the form , where are
constants. During the mixed phase the universe smoothly transforms from being
radiation to dust dominated. We find analytic expressions for the graviton mode
function during the mixed phase in terms of spheroidal wave functions. This
mode function is used to find an analytic expression for the multipole moments
of the two-point angular correlation function
for the CBR anisotropy. The analytic expression for the multipole
moments is written in terms of two integrals, which are evaluated numerically.
The results are compared to multipoles calculated for models that are {\it
completely} dust dominated at last-scattering. We find that the multipoles
of the CBR temperature perturbations for are
significantly larger for a universe that contains both radiation and dust at
last-scattering. We compare our results with recent, similar numerical work and
find good agreement. The spheroidal wave functions may have applications to
other problems of cosmological interest.Comment: 28 pgs + 6 postscript figures, RevTe
Null energy condition and superluminal propagation
We study whether a violation of the null energy condition necessarily implies
the presence of instabilities. We prove that this is the case in a large class
of situations, including isotropic solids and fluids relevant for cosmology. On
the other hand we present several counter-examples of consistent effective
field theories possessing a stable background where the null energy condition
is violated. Two necessary features of these counter-examples are the lack of
isotropy of the background and the presence of superluminal modes. We argue
that many of the properties of massive gravity can be understood by associating
it to a solid at the edge of violating the null energy condition. We briefly
analyze the difficulties of mimicking in scalar tensor theories of
gravity.Comment: 46 pages, 6 figure
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
On a general class of brane-world black holes
We use the general solution to the trace of the 4-dimensional Einstein
equations for static, spherically symmetric configurations as a basis for
finding a general class of black hole (BH) metrics, containing one arbitrary
function which vanishes at some , the horizon
radius. Under certain reasonable restrictions, BH metrics are found with or
without matter and, depending on the boundary conditions, can be asymptotically
flat or have any other prescribed large behaviour. It is shown that this
procedure generically leads to families of solutions unifying non-extremal
globally regular BHs with a Kerr-like global structure, extremal BHs and
symmetric wormholes. Horizons in space-times with zero scalar curvature are
shown to be either simple or double. The same is generically true for horizons
inside a matter distribution, but in special cases there can be horizons of any
order. A few simple examples are discussed. A natural application of the above
results is the brane world concept, in which the trace of the 4D gravity
equations is the only unambiguous equation for the 4D metric, and its solutions
can be continued into the 5D bulk according to the embedding theorems.Comment: 9 pages, revtex
Aspects of Tachyonic Inflation with Exponential Potential
We consider issues related to tachyonic inflation with exponential potential.
We find exact solution of evolution equations in the slow roll limit in FRW
cosmology. We also carry out similar analysis in case of Brane assisted
tachyonic inflation. We investigate the phase space behavior of the system and
show that the dust like solution is a late time attractor. The difficulties
associated with reheating in the tachyonic model are also indicated.Comment: New References added. To appear in Phys. Rev.
Nonsingular FRW cosmology and nonlinear electrodynamics
The possibility to avoid the cosmic initial singularity as a consequence of
nonlinear effects on the Maxwell eletromagnetic theory is discussed. For a flat
FRW geometry we derive the general nonsingular solution supported by a magnetic
field plus a cosmic fluid and a nonvanishing vacuum energy density. The
nonsingular behavior of solutions with a time-dependent -term are
also examined. As a general result, it is found that the functional dependence
of can uniquely be determined only if the magnetic field remains
constant. All these models are examples of bouncing universes which may exhibit
an inflationary dynamics driven by the nonlinear corrections of the magnetic
field.Comment: 20 pages, 7 figure
Brane cosmology with curvature corrections
We study the cosmology of the Randall-Sundrum brane-world where the
Einstein-Hilbert action is modified by curvature correction terms: a
four-dimensional scalar curvature from induced gravity on the brane, and a
five-dimensional Gauss-Bonnet curvature term. The combined effect of these
curvature corrections to the action removes the infinite-density big bang
singularity, although the curvature can still diverge for some parameter
values. A radiation brane undergoes accelerated expansion near the minimal
scale factor, for a range of parameters. This acceleration is driven by the
geometric effects, without an inflaton field or negative pressures. At late
times, conventional cosmology is recovered.Comment: RevTex4, 8 pages, no figures, minor change
Cosmological evolution of interacting dark energy in Lorentz violation
The cosmological evolution of an interacting scalar field model in which the
scalar field interacts with dark matter, radiation, and baryon via Lorentz
violation is investigated. We propose a model of interaction through the
effective coupling . Using dynamical system analysis, we study the
linear dynamics of an interacting model and show that the dynamics of critical
points are completely controlled by two parameters. Some results can be
mentioned as follows. Firstly, the sequence of radiation, the dark matter, and
the scalar field dark energy exist and baryons are sub dominant. Secondly, the
model also allows the possibility of having a universe in the phantom phase
with constant potential. Thirdly, the effective gravitational constant varies
with respect to time through . In particular, we consider a simple
case where has a quadratic form and has a good agreement with the
modified CDM and quintessence models. Finally, we also calculate the
first post--Newtonian parameters for our model.Comment: 14 pages, published versio
Oxidised cosmic acceleration
We give detailed proofs of several new no-go theorems for constructing flat
four-dimensional accelerating universes from warped dimensional reduction.
These new theorems improve upon previous ones by weakening the energy
conditions, by including time-dependent compactifications, and by treating
accelerated expansion that is not precisely de Sitter. We show that de Sitter
expansion violates the higher-dimensional null energy condition (NEC) if the
compactification manifold M is one-dimensional, if its intrinsic Ricci scalar R
vanishes everywhere, or if R and the warp function satisfy a simple limit
condition. If expansion is not de Sitter, we establish threshold
equation-of-state parameters w below which accelerated expansion must be
transient. Below the threshold w there are bounds on the number of e-foldings
of expansion. If M is one-dimensional or R everywhere vanishing, exceeding the
bound implies the NEC is violated. If R does not vanish everywhere on M,
exceeding the bound implies the strong energy condition (SEC) is violated.
Observationally, the w thresholds indicate that experiments with finite
resolution in w can cleanly discriminate between different models which satisfy
or violate the relevant energy conditions.Comment: v2: corrections, references adde