2,073 research outputs found
A new null diagnostic customized for reconstructing the properties of dark energy from BAO data
Baryon Acoustic Oscillations (BAO) provide an important standard ruler which
can be used to probe the recent expansion history of our universe. We show how
a simple extension of the Om diagnostic, which we call Om3, can combine
standard ruler information from BAO with standard candle information from type
Ia supernovae (SNIa) to yield a powerful novel null diagnostic of the
cosmological constant hypothesis. A unique feature of Om3 is that it requires
minimal cosmological assumptions since its determination does not rely upon
prior knowledge of either the current value of the matter density and the
Hubble constant, or the distance to the last scattering surface. Observational
uncertainties in these quantities therefore do not affect the reconstruction of
Om3. We reconstruct Om3 using the Union 2.1 SNIa data set and BAO data from
SDSS, WiggleZ and 6dFGS. Our results are consistent with dark energy being the
cosmological constant. We show how Om and Om3 can be used to obtain accurate
model independent constraints on the properties of dark energy from future data
sets such as BigBOSS.Comment: 9 pages, 4 figures, discussions extended, results unchanged, matches
the final version published in PR
Asymmetric embedding in brane cosmology
We derive a system of cosmological equations for a braneworld with induced
curvature which is a junction between several bulk spaces. The permutation
symmetry of the bulk spaces is not imposed, and the values of the fundamental
constants, and even the signatures of the extra dimension, may be different on
different sides of the brane. We then consider the usual partial case of two
asymmetric bulk spaces and derive an exact closed system of scalar equations on
the brane. We apply this result to the cosmological evolution on such a brane
and describe its various partial cases.Comment: 10 page
Evolution of perturbations in distinct classes of canonical scalar field models of dark energy
Dark energy must cluster in order to be consistent with the equivalence
principle. The background evolution can be effectively modelled by either a
scalar field or by a barotropic fluid.The fluid model can be used to emulate
perturbations in a scalar field model of dark energy, though this model breaks
down at large scales. In this paper we study evolution of dark energy
perturbations in canonical scalar field models: the classes of thawing and
freezing models.The dark energy equation of state evolves differently in these
classes.In freezing models, the equation of state deviates from that of a
cosmological constant at early times.For thawing models, the dark energy
equation of state remains near that of the cosmological constant at early times
and begins to deviate from it only at late times.Since the dark energy equation
of state evolves differently in these classes,the dark energy perturbations too
evolve differently. In freezing models, since the equation of state deviates
from that of a cosmological constant at early times, there is a significant
difference in evolution of matter perturbations from those in the cosmological
constant model.In comparison, matter perturbations in thawing models differ
from the cosmological constant only at late times. This difference provides an
additional handle to distinguish between these classes of models and this
difference should manifest itself in the ISW effect.Comment: 11 pages, 6 figures, accepted for publication in Phys. Rev.
Thermodynamical properties of dark energy
We have investigated the thermodynamical properties of dark energy. Assuming
that the dark energy temperature and considering that the volume
of the Universe enveloped by the apparent horizon relates to the temperature,
we have derived the dark energy entropy. For dark energy with constant equation
of state and the generalized Chaplygin gas, the derived entropy can be
positive and satisfy the entropy bound. The total entropy, including those of
dark energy, the thermal radiation and the apparent horizon, satisfies the
generalized second law of thermodynamics. However, for the phantom with
constant equation of state, the positivity of entropy, the entropy bound, and
the generalized second law cannot be satisfied simultaneously.Comment: 5 two column pages, 2 figures; v2: discussion on thermal equilibrium
with the horizon is added, v3: minor corrections, published in PR
Observational constraints on interacting quintessence models
We determine the range of parameter space of Interacting Quintessence Models
that best fits the recent WMAP measurements of Cosmic Microwave Background
temperature anisotropies. We only consider cosmological models with zero
spatial curvature. We show that if the quintessence scalar field decays into
cold dark matter at a rate that brings the ratio of matter to dark energy
constant at late times,the cosmological parameters required to fit the CMB data
are: \Omega_x = 0.43 \pm 0.12, baryon fraction \Omega_b = 0.08 \pm 0.01, slope
of the matter power spectrum at large scals n_s = 0.98 \pm 0.02 and Hubble
constant H_0 = 56 \pm 4 km/s/Mpc. The data prefers a dark energy component with
a dimensionless decay parameter c^2 =0.005 and non-interacting models are
consistent with the data only at the 99% confidence level. Using the Bayesian
Information Criteria we show that this exra parameter fits the data better than
models with no interaction. The quintessence equation of state parameter is
less constrained; i.e., the data set an upper limit w_x \leq -0.86 at the same
level of significance. When the WMAP anisotropy data are combined with
supernovae data, the density parameter of dark energy increases to \Omega_x
\simeq 0.68 while c^2 augments to 6.3 \times 10^{-3}. Models with quintessence
decaying into dark matter provide a clean explanation for the coincidence
problem and are a viable cosmological model, compatible with observations of
the CMB, with testable predictions. Accurate measurements of baryon fraction
and/or of matter density independent of the CMB data, would support/disprove
these models.Comment: 16 pages, Revtex4, 5 eps figures, to appear in Physical Review
Phantom Accretion by Five Dimensional Charged Black Hole
This paper deals with the dynamical behavior of phantom field near five
dimensional charged black hole. We formulate equations of motion for
steady-state spherically symmetric flow of phantom fluids. It is found that
phantom energy accretes onto black holes for . Further, the location of
critical point of accretion are evaluated that leads to mass to charge ratio
for 5D charged black hole. This ratio implies that accretion cannot transform a
black hole into a naked singularity. We would like to mention here that this
work is an irreducible extension of 4D charged black hole.Comment: 8 pages, accepted for publication in Mod. Phys. Lett.
String inspired explanation for the super-acceleration of our universe
We investigate the effect of the bulk content in the general Gauss-Bonnet
braneworld on the evolution of the universe. We find that the Gauss-Bonnet term
and the combination of the dark radiation and the matter content of the bulk
play a crucial role in the universe evolution. We show that our model can
describe the super-acceleration of our universe with the equation of state of
the effective dark energy in agreement with observations.Comment: 12 pages, 9 figures, references adde
APSIS - an Artificial Planetary System in Space to probe extra-dimensional gravity and MOND
A proposal is made to test Newton's inverse-square law using the perihelion
shift of test masses (planets) in free fall within a spacecraft located at the
Earth-Sun L2 point. Such an Artificial Planetary System In Space (APSIS) will
operate in a drag-free environment with controlled experimental conditions and
minimal interference from terrestrial sources of contamination. We demonstrate
that such a space experiment can probe the presence of a "hidden" fifth
dimension on the scale of a micron, if the perihelion shift of a "planet" can
be measured to sub-arc-second accuracy. Some suggestions for spacecraft design
are made.Comment: 17 pages, revtex, references added. To appear in Special issue of
IJMP
Quantum mechanical study of molecules - Eigenvalues and eigenvectors of real symmetric matrices
Computer methods for calculating eigenvalue and eigenvectors of real symmetric matrices arising in problems of molecular quantum mechanic
Cosmological scalar fields that mimic the cosmological model
We look for cosmologies with a scalar field (dark energy without cosmological
constant), which mimic the standard cosmological model yielding
exactly the same large-scale geometry described by the evolution of the Hubble
parameter (i.e. photometric distance and angular diameter distance as functions
on ). Asymptotic behavior of the field solutions is studied in the case of
spatially flat Universe with pressureless matter and separable scalar field
Lagrangians (power-law kinetic term + power-law potential). Exact analytic
solutions are found in some special cases. A number of models have the field
solutions with infinite behavior in the past or even singular behavior at
finite redshifts. We point out that introduction of the cosmological scalar
field involves some degeneracy leading to lower precision in determination of
. To remove this degeneracy additional information is needed beyond
the data on large-scale geometry.Comment: VIII International Conference "Relativistic Astrophysics, Gravitation
and Cosmology": May 21-23, 2008, Kyiv, Ukrain
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