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.

Exactly stable non-BPS spinors in heterotic string theory on tori

Considering SO(32) heterotic string theory compactified on a torus of dimension 4 and less, stability of non-supersymmetric states is studied. A non-supersymmetric state with robust stability is constructed, and its exact stability is proven in a large region of moduli space against all the possible decay mechanisms allowed by charge conservation. Using various T-duality transform matrices, we translate various selection rules about conserved charges into simpler problems resembling partition and parity of integers. For heterotic string on T^4, we give a complete list of BPS atoms with elementary excitations, and we study BPS and non-BPS molecules with various binding energies. Using string-string duality, the results are interpreted in terms of Dirichlet-branes in type IIA string theory compactified on an orbifold limit of a K3 surface.Comment: 47 pages, 14 figures, LaTe

Stable non-BPS states in string theory: a pedagogical review

We present a pedagogical review of the stable non-BPS states in string theory which have recently attracted some attention in the literature. In particular, following the analysis of A. Sen, we discuss in detail the case of the stable non-BPS D-particle of Type I theory whose existence is predicted (and required) by the heterotic/Type I duality. We show that this D-particle originates from an unstable bound state formed by a D1/anti-D1 pair of Type IIB in which the tachyon field acquires a solitonic kink configuration. The mechanism of tachyon condensation is discussed first at a qualitative level and then with an exact conformal field theory analysis.Comment: 58 pages, 1 figure; minor correction

Flows and Duality Symmetries in N=1 Supersymmetric Gauge Theories

We present more examples of dual N=1 SUSY gauge theories. This set of theories is connected by flows to both Seiberg's and Kutasov's dual theories. This provides a unifying picture of the various dual theories. We investigate the dual theories, their flat directions and mass perturbations.Comment: 31 pages, uses harvmac.tex and tables.tex Minor changes - version accepted to Nucl. Phys.

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

D-Brane Probe and Closed String Tachyons

We consider a D-brane probe in unstable string background associated with flux branes. The twist in spacetime metric reponsible for the supersymmetry breaking is shown to manifest itself in mixing of open Wilson lines with the phases of some adjoint matter fields, resulting in a nonlocal and nonsupersymmetric form of Yang-Mills theory as the probe dynamics. This provides a setup where one can study fate of a large class of unstable closed string theories that include as a limit type 0 theories and various orbifolds of type II and type 0 theories. We discuss the limit of ${\bf C}/Z_n$ orbifold in some detail and speculate on couplings with closed string tachyons.Comment: LaTeX, 17 pages, typos fixed, references update

Perturbation Theory for Singular Potentials in Quantum Mechanics

We study perturbation theory in certain quantum mechanics problems in which the perturbing potential diverges at some points, even though the energy eigenvalues are smooth functions of the coefficient of the potential. We discuss some of the unusual techniques which are required to obtain perturbative expansions of the energies in such cases. These include a point-splitting prescription for expansions around the Dirichlet (fermionic) limit of the $\delta$-function potential, and performing a similarity transformation to a non-Hermitian potential in the Calogero-Sutherland model. As an application of the first technique, we study the ground state of the $\delta$-function Bose gas near the fermionic limit.Comment: LaTeX, 19 pages, no figure

Lectures on Non-BPS Dirichlet branes

A comprehensive introduction to the boundary state approach to Dirichlet branes is given. Various examples of BPS and non-BPS Dirichlet branes are discussed. In particular, the non-BPS states in the duality of Type IIA on K3 and the heterotic string on T4 are analysed in detail.Comment: 46 pages, 5 figures, LaTeX; lectures given at the TMR network school on `Quantum aspects of gauge theories, supersymmetry and quantum gravity', Torino, 26 January - 2 February 2000, and at the `Spring workshop on Superstrings and related matters', Trieste, 27 March - 4 April 2000; references adde