26,719 research outputs found
Remark about Non-BPS D-Brane in Type IIA Theory
In this paper we would like to show simple mechanisms how from the action for
non-BPS D-brane we can obtain action describing BPS D(p-1)-brane in Type IIA
theory.Comment: 13 pages, completely rewritten pape
WMAP Constraints on a Quintessence Model
We use the results from the Wilkinson Microwave Anisotropy Probe (WMAP) for
the locations of peaks and troughs of the Cosmic Microwave Background (CMB)
power spectrum, together with constraints from large-scale structure, to study
a quintessence model in which the pure exponential potential is modified by a
polynomial factor. Our analysis, in the cosmological
parameters space shows that this quintessence model is favoured compared to
CDM for and relatively high values of early
quintessence; for , quintessence and CDM give similar results,
except for high values of early quintessence, in which case CDM is
favoured.Comment: 3 pages. Talk presented by N. M. C. Santos at the Tenth Marcel
Grossmann Meeting on General Relativity, Rio de Janeiro, July 200
WMAP Constraints on the Generalized Chaplygin Gas Model
The generalized Chaplygin gas (GCG) model explains the recent accelerated
expansion of the Universe via an exotic background fluid whose equation of
state is given by p=-A/\rho^\alpha, where A is a positive constant and
0<\alpha\le 1. The model is an interesting alternative to scenarios involving
scalar field potentials, with the ensuing unnatural fine tuning conditions for
the underlying particle physics theories. We derive constraints on the
parameter space of the model from bounds on the location of the first few peaks
and troughs of the the Cosmic Microwave Background Radiation (CMBR) power
spectrum arising from recent WMAP and BOOMERanG data.Comment: 18 pages, 5 figures, version to appear in Phys. Lett.
Phase Fluctuations in Strongly Coupled -Wave Superconductors
We present a numerically exact solution for the BCS Hamiltonian at any
temperature, including the degrees of freedom associated with classical phase,
as well as amplitude, fluctuations via a Monte Carlo (MC) integration. This
allows for an investigation over the whole range of couplings: from weak
attraction, as in the well-known BCS limit, to the mainly unexplored
strong-coupling regime of pronounced phase fluctuations. In the latter, for the
first time two characteristic temperatures and , associated with
short- and long-range ordering, respectively, can easily be identified in a
mean-field-motivated Hamiltonian. at the same time corresponds to the
opening of a gap in the excitation spectrum. Besides introducing a novel
procedure to study strongly coupled d-wave superconductors, our results
indicate that classical phase fluctuations are not sufficient to explain the
pseudo-gap features of high-temperature superconductors (HTS).Comment: 5 pages, 3 figure
Supernovae constraints on dark energy and modified gravity models
We use the Type Ia Supernova gold sample to constrain the parameters of dark
energy models namely the Cardassian, Dvali-Turner (DT) and generalized
Chaplygin gas (GCG) models. In our best fit analysis for these dark energy
proposals we consider flat and the non-flat priors. For all models, we find
that relaxing the flatness condition implies that data favors a positive
curvature; moreover, the GCG model is nearly flat, as required by Cosmic
Microwave Background (CMB) observations.Comment: 6 pages, Latex file + 9 eps figures + (jpconf.cls,jpconf11.clo), to
appear in the Proceedings of the Fourth Meeting on Constrained Dynamics and
Quantum Gravity (QG05), Cala Gonone (Italy) September 12-16 200
A note on the decay of noncommutative solitons
We propose an ansatz for the equations of motion of the noncommutative model
of a tachyonic scalar field interacting with a gauge field, which allows one to
find time-dependent solutions describing decaying solitons. These correspond to
the collapse of lower dimensional branes obtained through tachyon condensation
of unstable brane systems in string theory.Comment: 8 pages, no figures. Extended version, references adde
Evolution of Tachyon Kink with Electric Field
We investigate the decay of an inhomogeneous D1-brane wrapped on a with
an electric field. The model that we consider consists of an array of tachyon
kink and anti-kink with a constant electric flux. Beginning with an initially
static configuration, we numerically evolve the tachyon field with some
perturbations under a fixed boundary condition at diametrically opposite points
on the circle . When the electric flux is smaller than the critical value,
the tachyon kink becomes unstable; the tachyon field rolls down the potential,
and the lower dimensional D0- and -brane become thin, which
resembles the caustic formation known for this type of the system in the
literature. For the supercritical values of the electric flux, the tachyon kink
remains stable.Comment: 27 pages, 8 figures, some changes, one reference added, version to
appear in JHE
Cosmological Solution from D-brane motion in NS5-Branes background
We study dynamics of a D3-brane propagating in the vicinity of k coincident
NS5 branes. We show that when is small, there exists a regime in which
dynamics of the D-brane is governed by Dirac-Born-Infeld action while higher
order derivative in the expansion can not be neglected. This leads to a
restriction on how fast scalar field may roll. We analyze the motion of a
rolling scalar field in this regime, and extend the analysis to cosmological
systems obtained by coupling this type of field theory to four dimensinal
gravity. It also leads to some FRW cosmologies, some of which are related to
those obtained with tachyon matter.Comment: 15 pages, To appear in International Journal of Modern Physics
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