203 research outputs found
Quantum effects can render w<-1 on cosmological scales
We report on a revision of our previous computation of the renormalized
expectation value of the stress-energy tensor of a massless, minimally coupled
scalar with a quartic self-interaction on a locally de Sitter background. This
model is important because it demonstrates that quantum effects can lead to
violations of the weak energy condition on cosmological scales - on average,
not just in fluctuations - although the effect in this particular model is far
too small to be observed. The revision consists of modifying the propagator so
that dimensional regularization can be used when the dimension of the
renormalized theory is not four. Although the finite part of the stress-energy
tensor does not change (in D=4) from our previous result, the counterterms do.
We also speculate that a certain, finite and separately conserved part of the
stress tensor can be subsumed into a natural correction of the initial state
from free Bunch-Davies vacuum.Comment: 9 pages, references adde
Reconstructing the potentials for the quintessence and tachyon dark energy, from the holographic principle
We propose an holographic quintessence and tachyon models of dark energy. The
correspondence between the quintessence and tachyon energy densities with the
holographic density, allows the reconstruction of the potentials and the
dynamics for the quintessence and tachyon fields, in flat FRW background. The
proposed infrared cut-off for the holographic energy density works for two
cases of the constant : for we reconstructed the holographic
quintessence model in the region before the crossing for the EoS
parameter. The cosmological dynamics for was also reconstructed for
the holographic quintessence and tachyon models.Comment: 21 pages, 18 figures, 2 table
Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology
We examine a class of braneworld models in which the expanding universe
encounters a "quiescent" future singularity. At a quiescent singularity, the
energy density and pressure of the cosmic fluid as well as the Hubble parameter
remain finite while all derivatives of the Hubble parameter diverge (i.e.,
, , etc. ). Since the Kretschmann invariant
diverges () at the singularity, one expects
quantum effects to play an important role as the quiescent singularity is
approached. We explore the effects of vacuum polarization due to massless
conformally coupled fields near the singularity and show that these can either
cause the universe to recollapse or, else, lead to a softer singularity at
which , , and remain finite while {\dddot H} and
higher derivatives of the Hubble parameter diverge. An important aspect of the
quiescent singularity is that it is encountered in regions of low density,
which has obvious implications for a universe consisting of a cosmic web of
high and low density regions -- superclusters and voids. In addition to vacuum
polarization, the effects of quantum particle production of non-conformal
fields are also likely to be important. A preliminary examination shows that
intense particle production can lead to an accelerating universe whose Hubble
parameter shows oscillations about a constant value.Comment: 19 pages, 3 figures, text slightly improved and references added.
Accepted for publication in Classical and Quantum Gravit
Cosmic Mimicry: Is LCDM a Braneworld in Disguise ?
For a broad range of parameter values, braneworld models display a remarkable
property which we call cosmic mimicry. Cosmic mimicry is characterized by the
fact that, at low redshifts, the Hubble parameter in the braneworld model is
virtually indistinguishable from that in the LCDM cosmology. An important point
to note is that the \Omega_m parameters in the braneworld model and in the LCDM
cosmology can nevertheless be quite different. Thus, at high redshifts (early
times), the braneworld asymptotically expands like a matter-dominated universe
with the value of \Omega_m inferred from the observations of the local matter
density. At low redshifts (late times), the braneworld model behaves almost
exactly like the LCDM model but with a renormalized value of the cosmological
density parameter \Omega_m^{LCDM}. The redshift which characterizes cosmic
mimicry is related to the parameters in the higher-dimensional braneworld
Lagrangian. Cosmic mimicry is a natural consequence of the scale-dependence of
gravity in braneworld models. The change in the value of the cosmological
density parameter is shown to be related to the spatial dependence of the
effective gravitational constant in braneworld theory. A subclass of mimicry
models lead to an older age of the universe and also predict a redshift of
reionization which is lower than z_{reion} \simeq 17 in the LCDM cosmology.
These models might therefore provide a background cosmology which is in better
agreement both with the observed quasar abundance at z \gsim 4 and with the
large optical depth to reionization measured by the Wilkinson Microwave
Anisotropy Probe.Comment: 22 pages, 4 figures. A subsection and references added; main results
remain unchanged. Accepted for publication in JCA
The case for dynamical dark energy revisited
We investigate the behaviour of dark energy using the recently released
supernova data of Riess et al ~(2004) and a model independent parameterization
for dark energy (DE). We find that, if no priors are imposed on
and , DE which evolves with time provides a better fit to the SNe data than
CDM. This is also true if we include results from the WMAP CMB data.
From a joint analysis of SNe+CMB, the best-fit DE model has at the
present epoch and the transition from deceleration to acceleration occurs at
. However, DE evolution becomes weaker if the CDM
based CMB results , are
incorporated in the analysis. In this case, . Our results
also show that the extent of DE evolution is sensitive to the manner in which
the supernova data is sampled.Comment: 16 pages, 8 figures. Discussions enhanced, new references added.
Matches version published in JCA
Variable Modified Chaplygin Gas and Accelerating Universe
In this letter, I have proposed a model of variable modified Chaplygin gas
and shown its role in accelerating phase of the universe. I have shown that the
equation of state of this model is valid from the radiation era to quiessence
model. The graphical representations of statefinder parameters characterize
different phase of evolution of the universe. All results presented in the
letter concerns the case .Comment: 7 Latex pages, 5 figures, revtex styl
Curvature driven acceleration : a utopia or a reality ?
The present work shows that a combination of nonlinear contribution from the
Ricci curvature in Einstein field equations can drive a late time acceleration
of expansion of the universe. The transit from the decelerated to the
accelerated phase of expansion takes place smoothly without having to resort to
a study of asymptotic behaviour. This result emphasizes the need for thorough
and critical examination of models with nonlinear contribution from the
curvature.Comment: 8 pages, 4 figure
Induced cosmological constant and other features of asymmetric brane embedding
We investigate the cosmological properties of an "induced gravity" brane
scenario in the absence of mirror symmetry with respect to the brane. We find
that brane evolution can proceed along one of four distinct branches. By
contrast, when mirror symmetry is imposed, only two branches exist, one of
which represents the self-accelerating brane, while the other is the so-called
normal branch. This model incorporates many of the well-known possibilities of
brane cosmology including phantom acceleration (w < -1), self-acceleration,
transient acceleration, quiescent singularities, and cosmic mimicry.
Significantly, the absence of mirror symmetry also provides an interesting way
of inducing a sufficiently small cosmological constant on the brane. A small
(positive) Lambda-term in this case is induced by a small asymmetry in the
values of bulk fundamental constants on the two sides of the brane.Comment: 17 pages, 4 figures. New results and two figures discussing transient
acceleration are included. Version accepted for publication in JCA
Networks of cosmological histories, crossing of the phantom divide line and potentials with cusps
We discuss the phenomenon of the smooth dynamical gravity induced crossing of
the phantom divide line in a framework of simple cosmological models where it
appears to occur rather naturally, provided the potential of the unique scalar
field has some kind of cusp. The behavior of cosmological trajectories in the
vicinity of the cusp is studied in some detail and a simple mechanical analogy
is presented. The phenomenon of certain complementarity between the smoothness
of the spacetime geometry and matter equations of motion is elucidated. We
introduce a network of cosmological histories and qualitatively describe some
of its properties
4D static solutions with interacting phantom fields
Three static models with two interacting phantom and ghost scalar fields were
considered: a model of a traversable wormhole, a brane-like model and a
spherically symmetric problem. It was shown numerically that regular solutions
exist for all three cases.Comment: final versio
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