29 research outputs found
Non-minimally coupled scalar field cosmology on the phase plane
In this publication we investigate dynamics of a flat FRW cosmological model
with a non-minimally coupled scalar field with the coupling term in the scalar field action. The quadratic potential function
is assumed. All the evolutional paths are visualized
and classified in the phase plane, at which the parameter of non-minimal
coupling plays the role of a control parameter. The fragility of global
dynamics with respect to changes of the coupling constant is studied in
details. We find that the future big rip singularity appearing in the phantom
scalar field cosmological models can be avoided due to non-minimal coupling
constant effects. We have shown the existence of a finite scale factor singular
point (future or past) where the Hubble function as well as its first
cosmological time derivative diverges.Comment: revtex4, 20 pages, 12 figs; (v2) title changed, analysis of critical
points at infinity added, accepted to JCA
Scalar field cosmology in the energy phase-space -- unified description of dynamics
In this letter we apply dynamical system methods to study all evolutional
paths admissible for all initial conditions of the FRW cosmological model with
a non-minimally coupled to gravity scalar field and a barotropic fluid. We
choose "energy variables" as phase variables. We reduce dynamics to a
3-dimensional dynamical system for an arbitrary potential of the scalar field
in the phase space variables. After postulating the potential parameter
as a function of (defined as ) we reduce whole
dynamics to a 3-dimensional dynamical system and study evolutional paths
leading to current accelerating expansion. If we restrict the form of the
potential then we will obtain a 2-dimensional dynamical system. We use the
dynamical system approach to find a new generic quintessence scenario of
approaching to the de Sitter attractor which appears only for the case of
non-vanishing coupling constant.Comment: revtex4, 16 pages, 3 figs; (v2) refs. added, published versio
Exploring the Expanding Universe and Dark Energy using the Statefinder Diagnostic
The coming few years are likely to witness a dramatic increase in high
quality Sn data as current surveys add more high redshift supernovae to their
inventory and as newer and deeper supernova experiments become operational.
Given the current variety in dark energy models and the expected improvement in
observational data, an accurate and versatile diagnostic of dark energy is the
need of the hour. This paper examines the Statefinder diagnostic in the light
of the proposed SNAP satellite which is expected to observe about 2000
supernovae per year. We show that the Statefinder is versatile enough to
differentiate between dark energy models as varied as the cosmological constant
on the one hand, and quintessence, the Chaplygin gas and braneworld models, on
the other. Using SNAP data, the Statefinder can distinguish a cosmological
constant () from quintessence models with and Chaplygin gas
models with at the level if the value of \om is
known exactly. The Statefinder gives reasonable results even when the value of
\om is known to only accuracy. In this case, marginalizing over
\om and assuming a fiducial LCDM model allows us to rule out quintessence
with and the Chaplygin gas with (both at
). These constraints can be made even tighter if we use the
Statefinders in conjunction with the deceleration parameter. The Statefinder is
very sensitive to the total pressure exerted by all forms of matter and
radiation in the universe. It can therefore differentiate between dark energy
models at moderately high redshifts of z \lleq 10.Comment: 21 pages, 17 figures. Minor typos corrected to agree with version
published in MNRAS. Results unchange
Confusing the extragalactic neutrino flux limit with a neutrino propagation limit
We study the possible suppression of the extragalactic neutrino flux due to a
nonstandard interaction during its propagation. In particular, we study
neutrino interaction with an ultra-light scalar field dark matter. It is shown
that the extragalactic neutrino flux may be suppressed by such an interaction,
leading to a new mechanism to reduce the ultra-high energy neutrino flux. We
study both the cases of non-self-conjugate as well as self-conjugate dark
matter. In the first case, the suppression is independent of the neutrino and
dark matter masses. We conclude that care must be taken when explaining limits
on the neutrino flux through source acceleration mechanisms only, since there
could be other mechanisms for the reduction of the neutrino flux.Comment: 15 pages, 4 figures. Important changes implemented. Abstract
modified. Conclusions remain. To be published in JCA
Modelling non-dust fluids in cosmology
Currently, most of the numerical simulations of structure formation use
Newtonian gravity. When modelling pressureless dark matter, or `dust', this
approach gives the correct results for scales much smaller than the
cosmological horizon, but for scenarios in which the fluid has pressure this is
no longer the case. In this article, we present the correspondence of
perturbations in Newtonian and cosmological perturbation theory, showing exact
mathematical equivalence for pressureless matter, and giving the relativistic
corrections for matter with pressure. As an example, we study the case of
scalar field dark matter which features non-zero pressure perturbations. We
discuss some problems which may arise when evolving the perturbations in this
model with Newtonian numerical simulations and with CMB Boltzmann codes.Comment: 5 pages; v2: typos corrected and refs added, submitted version; v3:
version to appear in JCA
Dynamical evolution of phantom scalar perturbation in the background of Schwarzschild black String spacetime
Using Leaver's continue fraction and time domain method, we study the wave
dynamics of phantom scalar perturbation in a Schwarzschild black string
spacetime. We find that the quasinormal modes contain the imprint from the
wavenumber of the fifth dimension. The late-time behaviors are dominated by
the difference between the wavenumber and the mass of the phantom
scalar perturbation. For , the phantom scalar perturbation in the
late-time evolution grows with an exponential rate as in the four-dimensional
Schwarzschild black hole spacetime. While, for , the late-time behavior
has the same form as that of the massless scalar field perturbation in the
background of a black hole. Furthermore, for , the late-time evolution
of phantom scalar perturbation is dominated by a decaying tail with an
oscillation which is consistent with that of the usual massive scalar field.
Thus, the Schwarzschild black string is unstable only against the phantom
scalar perturbations which satisfy the wavelength . These
information can help us know more about the wave dynamics of phantom scalar
perturbation and the properties of black string.Comment: 11 pages, 5 figures. Accepted by JHEP for publicatio
Scalar field exact solutions for non-flat FLRW cosmology: A technique from non-linear Schr\"odinger-type formulation
We report a method of solving for canonical scalar field exact solution in a
non-flat FLRW universe with barotropic fluid using non-linear Schr\"{o}dinger
(NLS)-type formulation in comparison to the method in the standard Friedmann
framework. We consider phantom and non-phantom scalar field cases with
exponential and power-law accelerating expansion. Analysis on effective
equation of state to both cases of expansion is also performed. We speculate
and comment on some advantage and disadvantage of using the NLS formulation in
solving for the exact solution.Comment: 12 pages, GERG format, Reference added. accepted by Gen. Relativ. and
Gra
Solution generating in scalar-tensor theories with a massless scalar field and stiff perfect fluid as a source
We present a method for generating solutions in some scalar-tensor theories
with a minimally coupled massless scalar field or irrotational stiff perfect
fluid as a source. The method is based on the group of symmetries of the
dilaton-matter sector in the Einstein frame. In the case of Barker's theory the
dilaton-matter sector possesses SU(2) group of symmetries. In the case of
Brans-Dicke and the theory with "conformal coupling", the dilaton- matter
sector has as a group of symmetries. We describe an explicit
algorithm for generating exact scalar-tensor solutions from solutions of
Einstein-minimally-coupled-scalar-field equations by employing the nonlinear
action of the symmetry group of the dilaton-matter sector. In the general case,
when the Einstein frame dilaton-matter sector may not possess nontrivial
symmetries we also present a solution generating technique which allows us to
construct exact scalar-tensor solutions starting with the solutions of
Einstein-minimally-coupled-scalar-field equations. As an illustration of the
general techniques, examples of explicit exact solutions are constructed. In
particular, we construct inhomogeneous cosmological scalar-tensor solutions
whose curvature invariants are everywhere regular in space-time. A
generalization of the method for scalar-tensor-Maxwell gravity is outlined.Comment: 10 pages,Revtex; v2 extended version, new parts added and some parts
rewritten, results presented more concisely, some simple examples of
homogeneous solutions replaced with new regular inhomogeneous solutions,
typos corrected, references and acknowledgements added, accepted for
publication in Phys.Rev.
Expanding Universe: Thermodynamical Aspects From Different Models
The pivotal point of the paper is to discuss the behavior of temperature,
pressure, energy density as a function of volume along with determination of
caloric EoS from following two model: & .
The time scale of instability for this two models is discussed. In the paper we
then generalize our result and arrive at general expression for energy density
irrespective of the model. The thermodynamical stability for both of the model
and the general case is discussed from this viewpoint. We also arrive at a
condition on the limiting behavior of thermodynamic parameter to validate the
third law of thermodynamics and interpret the general mathematical expression
of integration constant (what we get while integrating energy
conservation equation) physically relating it to number of micro states. The
constraint on the allowed values of the parameters of the models is discussed
which ascertains stability of universe. The validity of thermodynamical laws
within apparent and event horizon is discussed.Comment: 16 pages, 3 figures(Accepted for publication in "Astrophysics and
Space Science"
Galactic Halos of Fluid Dark Matter
Dwarf spiral galaxies - and in particular the prototypical DDO 154 - are
known to be completely dominated by an unseen component. The putative
neutralinos - so far the favored explanation for the astronomical dark matter -
fail to reproduce the well measured rotation curves of those systems because
these species tend to form a central cusp whose presence is not supported by
observation. We have considered here a self-coupled charged scalar field as an
alternative to neutralinos and investigated whether a Bose condensate of that
field could account for the dark matter inside DDO 154 and more generally
inside dwarf spirals. The size of the condensate turns out to be precisely
determined by the scalar mass m and self-coupling lambda of the field. We find
actually that for m^4 / lambda = 50 - 75 eV^4, the agreement with the
measurements of the circular speed of DDO 154 is impressive whereas it lessens
for larger systems. The cosmological behavior of the field is also found to be
consistent - yet marginally - with the limits set by BBN on the effective
number of neutrino families. We conclude that classical configurations of a
scalar and self-coupled field provide a possible solution to the astronomical
dark matter problem and we suggest further directions of research.Comment: 20 pages, 7 figures; one reference added, version to be published in
PR