40 research outputs found
Thawing quintessence with a nearly flat potential
The thawing quintessence model with a nearly flat potential provides a
natural mechanism to produce an equation of state parameter, w, close to -1
today. We examine the behavior of such models for the case in which the
potential satisfies the slow roll conditions: [(1/V)(dV/dphi)]^2 << 1 and
(1/V)(d^2 V/dphi^2) << 1, and we derive the analog of the slow-roll
approximation for the case in which both matter and a scalar field contribute
to the density. We show that in this limit, all such models converge to a
unique relation between 1+w, Omega_phi, and the initial value of
(1/V)(dV/dphi). We derive this relation, and use it to determine the
corresponding expression for w(a), which depends only on the present-day values
for w and Omega_phi. For a variety of potentials, our limiting expression for
w(a) is typically accurate to within delta w < 0.005 for w<-0.9. For redshift z
< 1, w(a) is well-fit by the Chevallier-Polarski-Linder parametrization, in
which w(a) is a linear function of a.Comment: 8 pages, 5 figures, discussion added, references updated, typos
corrected, to appear in Phys. Rev.
Generalized Global Defect Solutions
We investigate the presence of defect structures in generalized models
described by real scalar field in space-time dimensions. We work with
two distinct generalizations, one in the form of a product of functions of the
field and its derivative, and the other as a sum. We search for static
solutions and study the corresponding linear stability on general grounds. We
illustrate the results with several examples, where we find stable defect
structures of modified profile. In particular, we show how the new defect
solutions may give rise to evolutions not present in the standard scenario in
higher spatial dimensions.Comment: RevTex, 10 pages, 2 figures; version to appear in EPJ
Estimating Temperature Fluctuations in the Early Universe
A lagrangian for the essence field is constructed for a constant scalar
potential and its form determined when the scale factor was very small compared
to the present epoch but very large compared to the inflationary epoch. This
means that one is already in an expanding and flat universe. The form is
similar to that of an oscillator with time-dependent frequency. Expansion is
naturally built into the theory with the existence of growing classical
solutions of the scale factor. The formalism allows one to estimate
fluctuations of the temperature of the background radiation in these early
stages (compared to the present epoch) of the universe. If the temperature at
time is and at time the temperature is
(), then for small times, the probability for the logarithm of
inverse temperature evolution can be estimated to be given by
where
, is the Planck mass and Planck's constant and the
speed of light has been put equal to unity. There is the further possibility
that a single scalar field may suffice for an inflationary scenario as well as
the dark matter and dark energy realms.Comment: 8 pages, Revtex, title,abstract and format changed for journal
publication,no change in basic results, clarifications and a figure added.
Keywords: physics of the early universe,inflation, dark matter theory, dark
energy theory. PACS: 95.35.+d ; 95.36.+x ; 98.80.Cq ; 98.80.-
Reconstructing the properties of dark energy from recent observations
We explore the properties of dark energy from recent observational data,
including the Gold Sne Ia, the baryonic acoustic oscillation peak from SDSS,
the CMB shift parameter from WMAP3, the X-ray gas mass fraction in cluster and
the Hubble parameter versus redshift. The model with curvature
and two parameterized dark energy models are studied. For the
model, we find that the flat universe is consistent with observations at the
confidence level and a closed universe is slightly favored by these
data. For two parameterized dark energy models, with the prior given on the
present matter density, , with ,
and , our result seems to suggest that the
trend of dependence for an evolving dark energy from a
combination of the observational data sets is model-dependent.Comment: 16 pages, 15 figures, To appear in JCA
Deviation From \Lambda CDM With Cosmic Strings Networks
In this work, we consider a network of cosmic strings to explain possible
deviation from \Lambda CDM behaviour. We use different observational data to
constrain the model and show that a small but non zero contribution from the
string network is allowed by the observational data which can result in a
reasonable departure from \Lambda CDM evolution. But by calculating the
Bayesian Evidence, we show that the present data still strongly favour the
concordance \Lambda CDM model irrespective of the choice of the prior.Comment: 15 Pages, Latex Style, 4 eps figures, Revised Version, Accepted for
publication in European Physical Journal
Pure kinetic k-essence as the cosmic speed-up
In this paper, we consider three types of k-essence. These k-essence models
were presented in the parametric forms. The exact analytical solutions of the
corresponding equations of motion are found. It is shown that these k-essence
models for the presented solutions can give rise to cosmic acceleration.Comment: 10 pages, typos corrected, main results remain the same, minor
changes to match IJTP accepted versio
The -essence scalar field in the context of Supernova Ia Observations
A -essence scalar field model having (non canonical) Lagrangian of the
form where
with constant is shown to be consistent with luminosity
distance-redshift data observed for type Ia Supernova. For constant ,
satisfies a scaling relation which is used to set up a differential
equation involving the Hubble parameter , the scale factor and the
-essence field . and are extracted from SNe Ia data and using
the differential equation the time dependence of the field is found to
be: . The constants
have been determined. The time dependence is similar to that of the
quintessence scalar field (having canonical kinetic energy) responsible for
homogeneous inflation. Furthermore, the scaling relation and the obtained time
dependence of the field is used to determine the -dependence of the
function .Comment: 8 pages, 5 figures, Late
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
Statefinder diagnostic and analysis for interacting polytropic gas dark energy model
The interacting polytropic gas dark energy model is investigated from the
viewpoint of statefinder diagnostic tool and analysis. The
dependency of the statefinder parameters on the parameter of the model as well
as the interaction parameter between dark matter and dark energy is calculated.
We show that different values of the parameters of model and different values
of interaction parameter result different evolutionary trajectories in
and planes. The polytropic gas model of dark energy mimics the
standard CDM model at the early time.Comment: 17 pages, 4 figures, ijtp accepte