Debashis Ghoshal

c = 1 string as the topological theory of the conifold

Abstract We show that the non-critical c = 1 string at the self-dual radius is equivalent to topological strings based on the deformation of the conifold singularity of Calabi-Yau threefolds. The Penner sum giving the genus expansion of the free energy of the c = 1 string theory at the self-dual radius therefore gives the universal behaviour of the topological partition function of a Calabi-Yau threefold near a conifold point.

On the cosmological relevance of the tachyon

Abstract We analyse of the effective action of the tachyon field on a D-brane, of both bosonic as well as superstring theory. We find that the non-standard kinetic term of the tachyon field requires a correction to the Born–Infeld type Lagrangian. The cosmological significance of the resulting dynamics is explored. We also examine if the rolling tachyon can provide an effective cosmological constant and contrast its behaviour with quintessence.

Tachyon Condensation and Brane Descent Relations in p-adic String Theory

It has been conjectured that an extremum of the tachyon potential of a bosonic D-brane represents the vacuum without any D-brane, and that various tachyonic lump solutions represent D-branes of lower dimension. We show that the tree level effective action of p-adic string theory, the expression for which is known exactly, provides an explicit realisation of these conjectures.

Stringy corrections to the Wilson loop in Script N = 4 super Yang-Mills theory

We study stringy fluctuations as a source for corrections to the Wilson loop as obtained from the superstring on AdS5 × S5/ = 4 SYM correspondence. We give a formal expression in terms of determinants of two-dimensional operators for the leading order correction.

Hybrid inflation and brane–antibrane system

We study a string theory inspired model for hybrid inflation in the context of a brane–antibrane system partially compactified on a compact submanifold of (a caricature of) a Calabi–Yau manifold. The interbrane distance acts as the inflaton, whereas the end of the inflationary epoch is brought about by the rapid rolling of the tachyon. The number of e-foldings is sufficiently large and is controlled by the initial conditions. The slow roll parameters, however, are essentially determined by the geometry and have little parametric dependence. Primordial density fluctuations can be made consistent with current data at the cost of reducing the string scale.

Normalization of the background independent open string field theory action

It has been shown recently that the background independent open string field theory provides an exact description of the tachyon condensation on unstable D-branes of bosonic string theory. In this analysis the overall normalisation of the action was chosen so that it reproduces the conjectured relations involving tachyon condensation. In this paper we fix this normalisation by comparing the on-shell three tachyon amplitude computed from the background independent open string field theory with the same amplitude computed from the cubic open string field theory, which in turn agrees with the result of the first quantised theory. We find that this normalisation factor is in precise agreement with the one required for verifying the conjectured properties of the tachyon potential.

Topological Landau-Ginzburg model of two-dimensional string theory

Abstract We study a topological Landau-Ginzburg model with superpotential W ( X ) = X −1 . This is argued to be equivalent to c = 1 string theory compactified at the self-dual radius. We compute the tree-level correlation function of N tachyons in this theory and show their agreement with matrix-model results. We also discuss the nature of contract terms, the perturbed superpotential and the flow of operators in the small phase space. The role of gravitational descendants in this theory is examined, and the tachyon two-point function in genus 1 is obtained using a conjectured modification of the gravitational recursion relations.

p-adic string theories provide lattice discretization to the ordinary string worldsheet

The amplitudes for the tree-level scattering of the open string tachyons, generalised to the field of p-adic numbers, define the p-adic string theory. There is empirical evidence of its relation to the ordinary string theory in the p_to_1 limit. We revisit this limit from a worldsheet perspective and argue that it is naturally thought of as a continuum limit in the sense of the renormalisation group.

Topological 2D string theory: Higher-genus amplitudes and W∞ identities

We investigate Landau-Ginzburg string theory with the singular superpotential X−1 on arbitrary Riemann surfaces. This theory, which is a topological version of the c = 1 string at the self-dual radius, is solved using results from intersection theory and from the analysis of matter Landau-Ginzburg systems, and consistency requirements. Higher-genus amplitudes decompose as a sum of contributions from the bulk and the boundary of moduli space. These amplitudes generate the W∞ algebra.

Generalised hyperKähler manifolds in string theory

We discuss the notion of generalised hyperKahler structure in the context of string theory and discuss examples of this geometry.