17 research outputs found
Can cosmological observations uniquely determine the nature of dark energy ?
The observational effect of all minimally coupled scalar field models of dark
energy can be determined by the behavior of the following two parameters : (1)
equation of state parameter , which relates dark energy pressure to its
energy density, and (2) effective speed of sound , which relates
dark energy pressure fluctuation to its density fluctuation. In this paper we
show that these two parameters do not uniquely determine the form of a scalar
field dark energy Lagrangian even after taking into account the perturbation in
the scalar field. We present this result by showing that two different forms of
scalar field Lagrangian can lead to the same values for these paired
parameters. It is well known that from the background evolution the Lagrangian
of the scalar field dark energy cannot be uniquely determined. The two models
of dark energy presented in this paper are indistinguishable from the evolution
of background as well as from the evolution of perturbations from a FRW metric.Comment: 9 pages, 2 figures, few references adde
Some Consequences of Dark Energy Density varying Exponentially with Scale Factor
In this paper we have explored the consequences of a model of dark energy
with its energy density varying exponentially with the scale factor. We first
consider the model with , where
is a constant. This is a kind of generalisation of the cosmological constant
model with . We show that such an exponentially varying dark energy
density with the scale factor naturally leads to an equivalent phantom field.
We also consider a model with and we show
that this also naturally leads to an equivalent phantom field.Comment: 22 pages, 6 figure
Cosmic acceleration in a model of scalar-tensor gravitation
In this paper we consider a model of scalar-tensor theory of gravitation in
which the scalar field, determines the gravitational coupling G and has
a Lagrangian of the form, . We study the cosmological consequence
of this theory in the matter dominated era and show that this leads to a
transition from an initial decelerated expansion to an accelerated expansion
phase at the present epoch. Using observational constraints, we see that the
effective equation of state today for the scalar field turns out to be
, with and that the transition
to an accelerated phase happened at a redshift of about 0.3.Comment: 12 pages, 2 figures, matches published versio
A note on perfect scalar fields
We derive a condition on the Lagrangian density describing a generic, single,
non-canonical scalar field, by demanding that the intrinsic, non-adiabatic
pressure perturbation associated with the scalar field vanishes identically.
Based on the analogy with perfect fluids, we refer to such fields as perfect
scalar fields. It is common knowledge that models that depend only on the
kinetic energy of the scalar field (often referred to as pure kinetic models)
possess no non-adiabatic pressure perturbation. While we are able to construct
models that seemingly depend on the scalar field and also do not contain any
non-adiabatic pressure perturbation, we find that all such models that we
construct allow a redefinition of the field under which they reduce to pure
kinetic models. We show that, if a perfect scalar field drives inflation, then,
in such situations, the first slow roll parameter will always be a
monotonically decreasing function of time. We point out that this behavior
implies that these scalar fields can not lead to features in the inflationary,
scalar perturbation spectrum.Comment: v1: 11 pages; v2: 11 pages, minor changes, journal versio
Mouthpart morphology of phyllosoma of the tropical spiny lobster Panulirus homarus (Linnaeus, 1758)
Mouthpart morphology of Panulirus homarus phyllosoma larva was studied under scanning electron microscope in order to
analyse the developmental changes during growth. Phyllosoma larvae have six pairs of mouthparts (mandibles, maxillule,
maxilla, maxillipeds I, II, and III), labrum, and paired paragnaths. Increased length of second and third maxillipeds in late stage phyllosoma resulted in the increase of oral field, thus increasing its ability to catch prey. Labrum and paired paragnaths form a semienclosed oral chamber where mastication by the mandibles occurs. The improved threshing and tearing efficiency in late instars (stage VI-VIII) is facilitated by morphological changes in the mouthparts viz., increase in oral field, increased robustness and number of spinose setations of maxillule, and lengthy maxillipeds, indicating that the late instar larvae can process fleshier prey as compared to the early instar counterparts
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.