1,496 research outputs found
Perturbations on steady spherical accretion in Schwarzschild geometry
The stationary background flow in the spherically symmetric infall of a
compressible fluid, coupled to the space-time defined by the static
Schwarzschild metric, has been subjected to linearized perturbations. The
perturbative procedure is based on the continuity condition and it shows that
the coupling of the flow with the geometry of space-time brings about greater
stability for the flow, to the extent that the amplitude of the perturbation,
treated as a standing wave, decays in time, as opposed to the amplitude
remaining constant in the Newtonian limit. In qualitative terms this situation
simulates the effect of a dissipative mechanism in the classical Bondi
accretion flow, defined in the Newtonian construct of space and time. As a
result of this approach it becomes impossible to define an acoustic metric for
a conserved spherically symmetric flow, described within the framework of
Schwarzschild geometry. In keeping with this view, the perturbation, considered
separately as a high-frequency travelling wave, also has its amplitude reduced.Comment: 8 pages, no figur
Understanding the role of surfactants on the preparation of ZnS nanocrystals
We have synthesized surface modified ZnS nanoparticles of size 2–3 nm using non-ionic surfactant-stabilized reverse emulsions. The non-ionic surfactants in the Span series, i.e. sorbitan monolaurate (Span 20) and sorbitan monooleate (Span 80) of hydrophilic–lipophilic balance (HLB) values of 8.6 and 4.3, respectively, have been used for the stabilization of emulsions. The role of these surfactants in controlling the size and properties of the ZnS nanoparticles has been discussed. The triethylamine (TEA) has been proved to be the effective surface modifying (capping) agent for the preparation of free-standing ZnS nanoparticles. The Span 20 with the higher HLB value of 8.6 has been found to be highly suitable in synthesizing TEA-capped ZnS nanoparticles of smaller size and higher photophysical characteristics compared to that of the Span 80 of lower HLB value of 4.3. A mechanism for the formation of TEA-capped ZnS nanoparticles from the surfactant-stabilized reverse emulsions has been proposed
Synthesis and characterization of PVP-encapsulated ZnS nanoparticles
We report a simple soft chemical method for the synthesis of PVP-encapsulated ZnS nanoparticles and examine the optical properties of these ZnS nanoparticles with varying ageing time at the reaction temperature, concentrations of PVP and S2- ions. The observed photoluminescence peak of PVP capped ZnS nanoparticles at 407 nm, markedly blue shifted relative to that of the bulk ZnS, clearly indicates the strong quantum size effect. A mechanism for the formation of PVP encapsulated ZnS nanoclusters under varying PVP/Zn2+ mole ratio has also been suggeste
Cosmological evolution of interacting phantom (quintessence) model in Loop Quantum Gravity
The dynamics of interacting dark energy model in loop quantum cosmology (LQC)
is studied in this paper. The dark energy has a constant equation of state
and interacts with dark matter through a form . We
find for quintessence model () the cosmological evolution in LQC is the
same as that in classical Einstein cosmology; whereas for phantom dark energy
(), although there are the same critical points in LQC and classical
Einstein cosmology, loop quantum effect reduces significantly the parameter
spacetime () required by stability. If parameters and satisfy
the conditions that the critical points are existent and stable, the universe
will enter an era dominated by dark energy and dark matter with a constant
energy ratio between them, and accelerate forever; otherwise it will enter an
oscillatory regime. Comparing our results with the observations we find at
confidence level the universe will accelerate forever.Comment: 15 pages, 8 figures, to appear in JCA
Curvaton Dynamics in Brane-worlds
We study the curvaton dynamics in brane-world cosmologies. Assuming that the
inflaton field survives without decay after the end of inflation, we apply the
curvaton reheating mechanism to Randall-Sundrum and to its curvature
corrections: Gauss-Bonnet, induced gravity and combined Gauss-Bonnet and
induced gravity cosmological models. In the case of chaotic inflation and
requiring suppression of possible short-wavelength generated gravitational
waves, we constraint the parameters of a successful curvaton brane-world
cosmological model. If density perturbations are also generated by the curvaton
field then, the fundamental five-dimensional mass could be much lower than the
Planck massComment: 47 pages, 1 figure, references added, to be published in JCA
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
Loop Quantum Cosmology: A Status Report
The goal of this article is to provide an overview of the current state of
the art in loop quantum cosmology for three sets of audiences: young
researchers interested in entering this area; the quantum gravity community in
general; and, cosmologists who wish to apply loop quantum cosmology to probe
modifications in the standard paradigm of the early universe. An effort has
been made to streamline the material so that, as described at the end of
section I, each of these communities can read only the sections they are most
interested in, without a loss of continuity.Comment: 138 pages, 15 figures. Invited Topical Review, To appear in Classical
and Quantum Gravity. Typos corrected, clarifications and references adde
A Quintessentially Geometric Model
We consider string inspired cosmology on a solitary -brane moving in the
background of a ring of branes located on a circle of radius . The motion of
the -brane transverse to the plane of the ring gives rise to a radion field
which can be mapped to a massive non-BPS Born-Infeld type field with a cosh
potential. For certain bounds of the brane tension we find an inflationary
phase is possible, with the string scale relatively close to the Planck scale.
The relevant perturbations and spectral indices are all well within the
expected observational bounds. The evolution of the universe eventually comes
to be dominated by dark energy, which we show is a late time attractor of the
model. However we also find that the equation of state is time dependent, and
will lead to late time Quintessence.Comment: 11 pages, 3 figures. References and comments adde
Coupled dark energy: Towards a general description of the dynamics
In dark energy models of scalar-field coupled to a barotropic perfect fluid,
the existence of cosmological scaling solutions restricts the Lagrangian of the
field \vp to p=X g(Xe^{\lambda \vp}), where X=-g^{\mu\nu} \partial_\mu \vp
\partial_\nu \vp /2, is a constant and is an arbitrary function.
We derive general evolution equations in an autonomous form for this Lagrangian
and investigate the stability of fixed points for several different dark energy
models--(i) ordinary (phantom) field, (ii) dilatonic ghost condensate, and
(iii) (phantom) tachyon. We find the existence of scalar-field dominant fixed
points (\Omega_\vp=1) with an accelerated expansion in all models
irrespective of the presence of the coupling between dark energy and dark
matter. These fixed points are always classically stable for a phantom field,
implying that the universe is eventually dominated by the energy density of a
scalar field if phantom is responsible for dark energy. When the equation of
state w_\vp for the field \vp is larger than -1, we find that scaling
solutions are stable if the scalar-field dominant solution is unstable, and
vice versa. Therefore in this case the final attractor is either a scaling
solution with constant \Omega_\vp satisfying 0<\Omega_\vp<1 or a
scalar-field dominant solution with \Omega_\vp=1.Comment: 21 pages, 5 figures; minor clarifications added, typos corrected and
references updated; final version to appear in JCA
Evolution of ZnS Nanoparticles via Facile CTAB Aqueous Micellar Solution Route: A Study on Controlling Parameters
Synthesis of semiconductor nanoparticles with new photophysical properties is an area of special interest. Here, we report synthesis of ZnS nanoparticles in aqueous micellar solution of Cetyltrimethylammonium bromide (CTAB). The size of ZnS nanodispersions in aqueous micellar solution has been calculated using UV-vis spectroscopy, XRD, SAXS, and TEM measurements. The nanoparticles are found to be polydispersed in the size range 6–15 nm. Surface passivation by surfactant molecules has been studied using FTIR and fluorescence spectroscopy. The nanoparticles have been better stabilized using CTAB concentration above 1 mM. Furthermore, room temperature absorption and fluorescence emission of powdered ZnS nanoparticles after redispersion in water have also been investigated and compared with that in aqueous micellar solution. Time-dependent absorption behavior reveals that the formation of ZnS nanoparticles depends on CTAB concentration and was complete within 25 min
- …