31 research outputs found
Scalar Field Dark Energy Perturbations and their Scale Dependence
We estimate the amplitude of perturbation in dark energy at different length
scales for a quintessence model with an exponential potential. It is shown that
on length scales much smaller than hubble radius, perturbation in dark energy
is negligible in comparison to that in in dark matter. However, on scales
comparable to the hubble radius () the
perturbation in dark energy in general cannot be neglected. As compared to the
CDM model, large scale matter power spectrum is suppressed in a
generic quintessence dark energy model. We show that on scales , this suppression is primarily due to different background
evolution compared to CDM model. However, on much larger scales
perturbation in dark energy can effect matter power spectrum significantly.
Hence this analysis can act as a discriminator between CDM model and
other generic dark energy models with .Comment: 12 pages, 13 figures, added new section, accepted for publication in
Phys. Rev.
Observational constraints on low redshift evolution of dark energy: How consistent are different observations?
The dark energy component of the universe is often interpreted either in
terms of a cosmological constant or as a scalar field. A generic feature of the
scalar field models is that the equation of state parameter w= P/rho for the
dark energy need not satisfy w=-1 and, in general, it can be a function of
time. Using the Markov chain Monte Carlo method we perform a critical analysis
of the cosmological parameter space, allowing for a varying w. We use
constraints on w(z) from the observations of high redshift supernovae (SN), the
WMAP observations of CMB anisotropies and abundance of rich clusters of
galaxies. For models with a constant w, the LCDM model is allowed with a
probability of about 6% by the SN observations while it is allowed with a
probability of 98.9% by WMAP observations. The LCDM model is allowed even
within the context of models with variable w: WMAP observations allow it with a
probability of 99.1% whereas SN data allows it with 23% probability. The SN
data, on its own, favors phantom like equation of state (w<-1) and high values
for Omega_NR. It does not distinguish between constant w (with w<-1) models and
those with varying w(z) in a statistically significant manner. The SN data
allows a very wide range for variation of dark energy density, e.g., a
variation by factor ten in the dark energy density between z=0 and z=1 is
allowed at 95% confidence level. WMAP observations provide a better constraint
and the corresponding allowed variation is less than a factor of three.
Allowing for variation in w has an impact on the values for other cosmological
parameters in that the allowed range often becomes larger. (Abridged)Comment: 21 pages, PRD format (Revtex 4), postscript figures. minor
corrections to improve clarity; references, acknowledgement adde
Vacuum Fluctuations of Energy Density can lead to the observed Cosmological Constant
The energy density associated with Planck length is while the energy density associated with the Hubble length is
where . The observed value of the dark
energy density is quite different from {\it either} of these and is close to
the geometric mean of the two: .
It is argued that classical gravity is actually a probe of the vacuum {\it
fluctuations} of energy density, rather than the energy density itself. While
the globally defined ground state, being an eigenstate of Hamiltonian, will not
have any fluctuations, the ground state energy in the finite region of space
bounded by the cosmic horizon will exhibit fluctuations . When used as a source of gravity, this should
lead to a spacetime with a horizon size . This bootstrapping condition
leads naturally to an effective dark energy density which is precisely the observed value. The model
requires, either (i) a stochastic fluctuations of vacuum energy which is
correlated over about a Hubble time or (ii) a semi- anthropic interpretation.
The implications are discussed.Comment: r pages; revtex; comments welcom
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.
Different faces of the phantom
The SNe type Ia data admit that the Universe today may be dominated by some
exotic matter with negative pressure violating all energy conditions. Such
exotic matter is called {\it phantom matter} due to the anomalies connected
with violation of the energy conditions. If a phantom matter dominates the
matter content of the universe, it can develop a singularity in a finite future
proper time. Here we show that, under certain conditions, the evolution of
perturbations of this matter may lead to avoidance of this future singularity
(the Big Rip). At the same time, we show that local concentrations of a phantom
field may form, among other regular configurations, black holes with
asymptotically flat static regions, separated by an event horizon from an
expanding, singularity-free, asymptotically de Sitter universe.Comment: 6 pages, presented at IRGAC 2006, Barcelona, 11-15 July 200
Initial state of matter fields and trans-Planckian physics: Can CMB observations disentangle the two?
The standard, scale-invariant, inflationary perturbation spectrum will be
modified if the effects of trans-Planckian physics are incorporated into the
dynamics of the matter field in a phenomenological manner, say, by the
modification of the dispersion relation. The spectrum also changes if we retain
the standard dynamics but modify the initial quantum state of the matter field.
We show that, given {\it any} spectrum of perturbations, it is possible to
choose a class of initial quantum states which can exactly reproduce this
spectrum with the standard dynamics. We provide an explicit construction of the
quantum state which will produce the given spectrum. We find that the various
modified spectra that have been recently obtained from `trans-Planckian
considerations' can be constructed from suitable squeezed states above the
Bunch-Davies vacuum in the standard theory. Hence, the CMB observations can, at
most, be useful in determining the initial state of the matter field in the
standard theory, but it can {\it not} help us to discriminate between the
various Planck scale models of matter fields. We study the problem in the
Schrodinger picture, clarify various conceptual issues and determine the
criterion for negligible back reaction due to modified initial conditions.Comment: revtex4; 17 page
Observational constraints on the dark energy density evolution
We constrain the evolution of the dark energy density from Cosmic Microwave
Background, Large Scale Structure and Supernovae Ia measurements. While
Supernovae Ia are most sensitive to the equation of state of dark energy
today, the Cosmic Microwave Background and Large Scale Structure data best
constrains the dark energy evolution at earlier times. For the parametrization
used in our models, we find and the dark energy fraction at very
high redshift at 95 per cent confidence level.Comment: 5 pages, 10 figure
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
Legacy data and cosmological constraints from the angular-size/redshift relation for ultra-compact radio sources
We have re-examined an ancient VLBI survey of ultra-comact radio sources at
2.29 GHz, which gave fringe amplitudes for 917 such objects with total flux
density >0.5 Jy approximately. A number of cosmological investigations based
upon this survey have been published in recent years. We have updated the
sample with respect to both redshift and radio information, and now have full
data for 613 objects, significantly larger than the number (337) used in
earlier investigations. The corresponding angular-size/redshift diagram gives
Omega_m=0.25+0.04/-0.03, Omega_\Lambda=0.97+0.09/-0.13 and K=0.22+0.07/-0.10.
In combination with supernova data, and a simple-minded approach to CMB data
based upon the angular size of the acoustic horizon, our best figures are
Omega_m=0.298+0.025/-0.024, Omega_\Lambda=0.702+0.035/-0.036 and K=
0.000+0.021/-0.019. We have examined simple models of dynamical vacuum energy;
the first, based upon a scalar potential V(phi)=omega_C^2 phi^2/2, gives
w(0)=-1.00+0.06/-0.00, (dw/dz)_0=+0.00/-0.08; in this case conditions at z=0
require particular attention, to preclude behaviour in which phi becomes
singular as z -->infinity. For fixed w limits are w=-1.20+0.15/-0.14. The above
error bars are 68% confidence limits.Comment: 24 pages, 9 figure
Bayesian analysis of Friedmannless cosmologies
Assuming only a homogeneous and isotropic universe and using both the 'Gold'
Supernova Type Ia sample of Riess et al. and the results from the Supernova
Legacy Survey, we calculate the Bayesian evidence of a range of different
parameterizations of the deceleration parameter. We consider both spatially
flat and curved models. Our results show that although there is strong evidence
in the data for an accelerating universe, there is little evidence that the
deceleration parameter varies with redshift.Comment: 7 pages, 3 figure