Sudhakar Panda

T-duality and actions for non-BPS D-branes

We employ T-duality to restrict the tachyon dependence of effective actions for non-BPS D-branes. For the Born-Infeld part the criteria of T-duality and supersymmetry are satisfied by a simple extension of the D-brane Born-Infeld action.

Assisted inflation via tachyon condensation.

In this paper we propose a new mechanism of inflating the Universe with non-BPS D4-branes which decay into stable D3-branes via tachyon condensation. In a single brane scenario the tachyon potential is very steep and unable to support inflation. However, if the universe lives in a stack of branes produced by a set of noninteracting unstable -branes, then the associated set of tachyons may drive inflation along our 3 spatial dimensions. After tachyon condensation the Universe is imagined to be filled with a set of parallel stable D3-branes. We study the scalar density perturbations and reheating within this setup.

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.

Newtonian gravity and the Bargmann algebra

We show how the Newton-Cartan formulation of Newtonian gravity can be obtained from gauging the Bargmann algebra, i.e., the centrally extended Galilean algebra. In this gauging procedure several curvature constraints are imposed. These convert the spatial (time) translational symmetries of the algebra into spatial (time) general coordinate transformations, and make the spin connection gauge fields dependent. In addition we require two independent Vielbein postulates for the temporal and spatial directions. In the final step we impose an additional curvature constraint to establish the connection with (on-shell) Newton-Cartan theory. We discuss a few extensions of our work that are relevant in the context of the AdS-CFT correspondence.

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.

Prospects of inflation in delicate D-brane cosmology

We study D-brane inflation in a warped conifold background that includes brane-position dependent corrections for the nonperturbative superpotential. Instead of stabilizing the volume modulus

De Sitter solutions in N = 4 matter coupled supergravity

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Potential and mass-matrix in gauged N = 4 supergravity

at instantaneous minima of the potential and studying the inflation dynamics with an effective single field (radial distance between a brane and an antibrane)

Axions as Quintessence in String Theory

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Fractional-level current algebras and the classification of characters

, we investigate the multifield inflation scenario involving these two fields. The two-field dynamics with the potential