650 research outputs found
Dynamics of tachyon field in spatially curved FRW universe
The dynamics of a tachyon field plus a barotropic fluid is investigated in
spatially curved FRW universe. We perform a phase-plane analysis and obtain
scaling solutions accompanying with a discussion on their stability.
Furthermore, we construct the form of scalar potential which may give rise to
stable solutions for spatially open and closed universe separately.Comment: 16 pages, 2 figures, version to be published in PL
Mice null for brush border myosin I assemble brush borders and lack an overt phenotype
published_or_final_versio
A Note on Agegraphic Dark Energy
Recently a new model of dynamical dark energy, or time-varying , was
proposed by Cai [arXiv:0707.4049] by relating the energy density of quantum
fluctuations in a Minkowski space-time, namely ,
where and t is the cosmic time, to the present day dark
energy density. In this note, we show that the model can be adjusted to the
present values of dark energy density parameter () and
the equation of state () only if the numerical
coefficient takes a reasonably large value () or the present
value of the gravitational coupling of q-field to (dark) matter is also
nonzero, namely, where
is the present value of dark energy density fraction. We also
discuss some of the difficulties of this proposal as a viable dark energy model
with a constant ; especially, the bound imposed on the dark energy density
parameter during big bang nucleosynthesis (BBN) requires . To overcome this drawback, we outline a few modifications where such
constraints can be weakened or relaxed. Finally, by establishing a
correspondence between the agegraphic dark energy scenario and the standard
scalar-field model, we also point out some interesting features of an
agegraphic quintessence model.Comment: 18 pages, several eps figures; significant changes in Secs. 2 & 3,
Journal versio
A New Approach to Quintessence and a Solution of Multiple Attractors
We take a new approach to construct Quintessential models. With this
approach, we first easily obtain a tracker solution that is different from
those discovered before and straightforwardly find a solution of multiple
attractors, i.e., a solution with more than one attractor for a given set of
parameters. Then we propose a scenario of Quintessence where the field jumps
out of the scaling attractor to the de-Sitter-like attractor, by introducing a
field whose value changes a certain amount in a short time, leading to the
current acceleration. We also calculate the change the field needs for a
successful jump and suggest a possible mechanism that involves spontaneous
symmetry breaking to realize the sudden change of the field value.Comment: 6 pages, 2 figures, Revtex4; To appear in PL
Cell-specific expression of Cre recombinase in Purkinje and retinal bipolar neurons
published_or_final_versio
Constraints on Dark Energy and Modified Gravity models by the Cosmological Redshift Drift test
We study cosmological constraints on the various accelerating models of the
universe using the time evolution of the cosmological redshift of distant
sources. The important characteristic of this test is that it directly probes
the expansion history of the universe. In this work we analyze the various
models of the universe which can explain the late time acceleration, within the
framework of General Theory of Relativity (GR) (XCDM, scalar field potentials)
and beyond GR (f(R) gravity model).Comment: 7 pages, 10 figures, revised version, accepted for publication in
Physics Lett.
Observational constraints on thawing quintessence models
We use a dynamical systems approach to study thawing quintessence models,
using a multi-parameter extension of the exponential potential which can
approximate the form of typical thawing potentials. We impose observational
constraints using a compilation of current data, and forecast the tightening of
constraints expected from future dark energy surveys, as well as discussing the
relation of our results to analytical constraints already in the literature.Comment: 6 pages MNRAS style with 8 figures included. Minor updates to match
MNRAS accepted versio
Renormalization Group Approach to Generalized Cosmological models
We revisit here the problem of generalized cosmology using renormalization
group approach. A complete analysis of these cosmologies, where specific models
appear as asymptotic fixed-points, is given here along with their linearized
stability analysis.Comment: 10 pages, to appear in the International Journal of Theoretical
Physic
Applications of scalar attractor solutions to Cosmology
We develop a framework to study the phase space of a system consisting of a
scalar field rolling down an arbitrary potential with varying slope and a
background fluid, in a cosmological setting. We give analytical approximate
solutions of the field evolution and discuss applications of its features to
the issues of quintessence, moduli stabilisation and quintessential inflation.Comment: 9 pages, 7 figures. Accepted for publication in PR
Instability of agegraphic dark energy models
We investigate the agegraphic dark energy models which were recently proposed
to explain the dark energy-dominated universe. For this purpose, we calculate
their equation of states and squared speeds of sound. We find that the squared
speed for agegraphic dark energy is always negative. This means that the
perfect fluid for agegraphic dark energy is classically unstable. Furthermore,
it is shown that the new agegraphic dark energy model could describe the matter
(radiation)-dominated universe in the far past only when the parameter is
chosen to be , where the critical values are determined to be
numerically. It seems that the new agegraphic dark
energy model is no better than the holographic dark energy model for the
description of the dark energy-dominated universe, even though it resolves the
causality problem.Comment: 15 pages 4 figure
- …