55 research outputs found
Stringy Dark Energy Model with Cold Dark Matter
Cosmological consequences of adding the Cold Dark Matter (CDM) to the exactly
solvable stringy Dark Energy (DE) model are investigated. The model is
motivated by the consideration of our Universe as a slowly decaying D3-brane.
The decay of this D-brane is described in the String Field Theory framework.
Stability conditions of the exact solution with respect to small fluctuations
of the initial value of the CDM energy density are found. Solutions with large
initial value of the CDM energy density attracted by the exact solution without
CDM are constructed numerically. In contrast to the LambdaCDM model the Hubble
parameter in the model is not a monotonic function of time. For specific
initial data the DE state parameter w_{DE} is also not monotonic function of
time. For these cases there are two separate regions of time where w_{DE} being
less than -1 is close to -1.Comment: 15 pages, 6 figures; references adde
Role of Brans-Dicke Theory with or without self-interacting potential in cosmic acceleration
In this work we have studied the possibility of obtaining cosmic acceleration
in Brans-Dicke theory with varying or constant (Brans- Dicke
parameter) and with or without self-interacting potential, the background fluid
being barotropic fluid or Generalized Chaplygin Gas. Here we take the power law
form of the scale factor and the scalar field. We show that accelerated
expansion can also be achieved for high values of for closed Universe.Comment: 12 Latex pages, 20 figures, RevTex styl
Causality and statistics on the Groenewold-Moyal plane
Quantum theories constructed on the noncommutative spacetime called the
Groenewold-Moyal plane exhibit many interesting properties such as Lorentz and
CPT noninvariance, causality violation and twisted statistics. We show that
such violations lead to many striking features that may be tested
experimentally. These theories predict Pauli forbidden transitions due to
twisted statistics, anisotropies in the cosmic microwave background radiation
due to correlations of observables in spacelike regions and Lorentz and CPT
violations in scattering amplitudes.Comment: 12 pages, 1 figure. Based on the talk given by APB at the Workshop
"Theoretical and Experimental Aspects of the Spin Statisics Connection and
Related Symmetries", Stazione Marittima Conference Center, Trieste, Italy
from the 21st to the 25th of October 200
Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua
We investigate a string-inspired scenario associated with a rolling massive
scalar field on D-branes and discuss its cosmological implications. In
particular, we discuss cosmological evolution of the massive scalar field on
the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of
the warp factor of the anti-D3 brane, it is possible to obtain the required
level of amplitude of density perturbations. We study the spectra of scalar and
tensor perturbations generated during the rolling scalar inflation and show
that our scenario satisfies the observational constraint coming from the Cosmic
Microwave Background anisotropies and other observational data. We also
implement the negative cosmological constant arising from the stabilization of
the modulus fields in the KKLT vacua and find that this leads to a successful
reheating in which the energy density of the scalar field effectively scales as
a pressureless dust. The present dark energy can be also explained in our
scenario provided that the potential energy of the massive rolling scalar does
not exactly cancel with the amplitude of the negative cosmological constant at
the potential minimum.Comment: RevTex4, 15 pages, 5 eps figures, minor clarifications and few
references added, final version to appear in PR
Dynamical stability of infinite homogeneous self-gravitating systems: application of the Nyquist method
We complete classical investigations concerning the dynamical stability of an
infinite homogeneous gaseous medium described by the Euler-Poisson system or an
infinite homogeneous stellar system described by the Vlasov-Poisson system
(Jeans problem). To determine the stability of an infinite homogeneous stellar
system with respect to a perturbation of wavenumber k, we apply the Nyquist
method. We first consider the case of single-humped distributions and show
that, for infinite homogeneous systems, the onset of instability is the same in
a stellar system and in the corresponding barotropic gas, contrary to the case
of inhomogeneous systems. We show that this result is true for any symmetric
single-humped velocity distribution, not only for the Maxwellian. If we
specialize on isothermal and polytropic distributions, analytical expressions
for the growth rate, damping rate and pulsation period of the perturbation can
be given. Then, we consider the Vlasov stability of symmetric and asymmetric
double-humped distributions (two-stream stellar systems) and determine the
stability diagrams depending on the degree of asymmetry. We compare these
results with the Euler stability of two self-gravitating gaseous streams.
Finally, we determine the corresponding stability diagrams in the case of
plasmas and compare the results with self-gravitating systems
Dark energy from quantum wave function collapse of dark matter
Dynamical wave function collapse models entail the continuous liberation of a
specified rate of energy arising from the interaction of a fluctuating scalar
field with the matter wave function. We consider the wave function collapse
process for the constituents of dark matter in our universe. Beginning from a
particular early era of the universe chosen from physical considerations, the
rate of the associated energy liberation is integrated to yield the requisite
magnitude of dark energy around the era of galaxy formation. Further, the
equation of state for the liberated energy approaches
asymptotically, providing a mechanism to generate the present acceleration of
the universe.Comment: 5 pages in Elsevier style to match with version published in Phys.
Lett.
Kaluza-Klein Type Robertson Walker Cosmological Model With Dynamical Cosmological Term
In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW)
cosmological models by considering three different forms of variable :
, and
. It is found that, the connecting free parameters of the
models with cosmic matter and vacuum energy density parameters are equivalent,
in the context of higher dimensional space time. The expression for the look
back time, luminosity distance and angular diameter distance are also derived.
This work has thus generalized to higher dimensions the well-known results in
four dimensional space time. It is found that there may be significant
difference in principle at least, from the analogous situation in four
dimensional space time.Comment: 16 pages, no figur
Some remarks on the dynamical systems approach to fourth order gravity
Building on earlier work, we discuss a general framework for exploring the
cosmological dynamics of Higher Order Theories of Gravity. We show that once
the theory of gravity has been specified, the cosmological equations can be
written as a first-order autonomous system and we give several examples which
illustrate the utility of our method. We also discuss a number of results which
have appeared recently in the literature.Comment: 19 pages, LaTe
Scenario of Accelerating Universe from the Phenomenological \Lambda- Models
Dark matter, the major component of the matter content of the Universe,
played a significant role at early stages during structure formation. But at
present the Universe is dark energy dominated as well as accelerating. Here,
the presence of dark energy has been established by including a time-dependent
term in the Einstein's field equations. This model is compatible with
the idea of an accelerating Universe so far as the value of the deceleration
parameter is concerned. Possibility of a change in sign of the deceleration
parameter is also discussed. The impact of considering the speed of light as
variable in the field equations has also been investigated by using a well
known time-dependent model.Comment: Latex, 9 pages, Major change
Towards a quantum universe
In this short review we study the state of the art of the great problems in
cosmology and their interrelationships. The reconciliation of these problems
passes undoubtedly through the idea of a quantum universe.Comment: 7 pages, Accepted for publication in Astrophysics & Space Scienc
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