93 research outputs found
Galacitic Collapse of Scalar Field Dark Matter
We present a scenario for galaxy formation based on the hypothesis of scalar field dark matter. We interpret galaxy formation through the collapse of a scalar field fluctuation. We find that a cosh potential for the self-interaction of the scalar field provides a reasonable scenario for galactic formation, which is in agreement with cosmological observations and phenomenological studies in galaxies
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
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
Exact anisotropic brane cosmologies
We present exact solutions of the gravitational field equations in the
generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I
and V geometry, with perfect fluid and scalar fields as matter sources. Under
the assumption of a conformally flat bulk (with vanishing Weyl tensor) for a
cosmological fluid obeying a linear barotropic equation of state the general
solution of the field equations can be expressed in an exact parametric form
for both Bianchi type I and V space-times. In the limiting case of a stiff
cosmological fluid with pressure equal to the energy density, for a Bianchi
type I Universe the solution of the field equations are obtained in an exact
analytic form. Several classes of scalar field models evolution on the brane
are also considered, corresponding to different choices of the scalar field
potential. For all models the behavior of the observationally important
parameters like shear, anisotropy and deceleration parameter is considered in
detail.Comment: revised version to appear in PR
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.
Vector field and rotational curves in dark galactic halos
We study equations of a non-gauge vector field in a spherically symmetric
static metric. The constant vector field with a scale arrangement of
components: the time component about the Planck mass m_{Pl} and the radial
component about M suppressed with respect to the Planck mass, serves as a
source of metric reproducing flat rotation curves in dark halos of spiral
galaxies, so that the velocity of rotation v_0 is determined by the hierarchy
of scales: \sqrt{2} v_0^2= M/m_{Pl}, and M\sim 10^{12} GeV. A natural estimate
of Milgrom's acceleration about the Hubble rate is obtained.Comment: 17 pages, iopart style, misprint remove
The Schrdinger-Poisson equations as the large-N limit of the Newtonian N-body system: applications to the large scale dark matter dynamics
In this paper it is argued how the dynamics of the classical Newtonian N-body
system can be described in terms of the Schrdinger-Poisson equations
in the large limit. This result is based on the stochastic quantization
introduced by Nelson, and on the Calogero conjecture. According to the Calogero
conjecture, the emerging effective Planck constant is computed in terms of the
parameters of the N-body system as , where is the gravitational constant, and are the
number and the mass of the bodies, and is their average density. The
relevance of this result in the context of large scale structure formation is
discussed. In particular, this finding gives a further argument in support of
the validity of the Schrdinger method as numerical double of the
N-body simulations of dark matter dynamics at large cosmological scales.Comment: Accepted for publication in the Euro. Phys. J.
Cosmological Evolution of Dirac-Born-Infeld Field
We investigate the cosmological evolution of the system of a
Dirac-Born-Infeld field plus a perfect fluid. We analyze the existence and
stability of scaling solutions for the AdS throat and the quadratic potential.
We find that the scaling solutions exist when the equation of state of the
perfect fluid is negative and in the ultra-relativistic limit.Comment: 9 pages, 1 figure, LaTeX2e, references added, accepted for
publication in JCA
Slow-roll, acceleration, the Big Rip and WKB approximation in NLS-type formulation of scalar field cosmology
Aspects of non-linear Schr\"{o}dinger-type (NLS) formulation of scalar
(phantom) field cosmology on slow-roll, acceleration, WKB approximation and Big
Rip singularity are presented. Slow-roll parameters for the curvature and
barotropic density terms are introduced. We reexpress all slow-roll parameters,
slow-roll conditions and acceleration condition in NLS form. WKB approximation
in the NLS formulation is also discussed when simplifying to linear case. Most
of the Schr\"{o}dinger potentials in NLS formulation are very slowly-varying,
hence WKB approximation is valid in the ranges. In the NLS form of Big Rip
singularity, two quantities are infinity in stead of three. We also found that
approaching the Big Rip, , which is the
same as effective phantom equation of state in the flat case.Comment: [7 pages, no figure, more reference added, accepted by JCAP
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