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

Evolution of Tachyon Kink with Electric Field

We investigate the decay of an inhomogeneous D1-brane wrapped on a $S^1$ 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 $S^1$. 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 $\bar {\rm D}0$-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

Single-degree-of-freedom roll response due to two-dimensional vertical gusts

Single degree of freedom roll response due to vertical random two dimensional vertical gust

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.

Cross-spectral functions based on von Karman's spectral equation

Cross-spectral functions for the vertical and longitudinal components of turbulence of a two-dimensional gust field are derived from the point correlation function for turbulence due to von Karman. Closed form solutions in terms of Bessel functions of order 5/6 and 11/6 are found. An asymptotic expression for large values of the frequency argument, and series results for small values of frequency, are also given. These results now form the base for studying the effect of spanwise variations in turbulence for a turbulence environment which is characterized by the von Karman isotropic spectral relations. Previous studies were based mainly on the Dryden-type spectral representation

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

On Orientifold Constructions of Type IIA Dual Pairs

In this paper we analyze the earlier constructions of the type IIA dual pairs through orientifolding. By an appropriate choice of $\Gamma$-matrix basis for the spinor representations of the $U$-duality group, we give an explicit relationship between the orientifold models and their dual pairs.Comment: 11pages, Late

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 $g_s$ 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

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 $(\Omega_m, h, n_s)$ cosmological parameters space shows that this quintessence model is favoured compared to $\Lambda$CDM for $n_s\approx 1$ and relatively high values of early quintessence; for $n_s<1$, quintessence and $\Lambda$CDM give similar results, except for high values of early quintessence, in which case $\Lambda$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