Building string field theory around non-conformal backgrounds

The main limitations of string field theory arise because its present formulation requires a background representing a classical solution, a background defined by a strictly conformally invariant theory. Here we sketch a construction for a gauge-invariant string field action around non-conformal backgrounds. The construction makes no reference to any conformal theory. Its two-dimensional field-theoretic aspect is based on a generalized BRST operator satisfying a set of Weyl descent equations. Its geometric aspect uses a complex of moduli spaces of two-dimensional Riemannian manifolds having ordinary punctures, and organized by the number of special punctures which goes from zero to infinity. In this complex there is a Batalin-Vilkovisky algebra that includes naturally the operator which adds one special puncture. We obtain a classical field equation that appears to relax the condition of conformal invariance usually taken to define classical string backgrounds.     

Holography and string dynamics in time-dependent backgrounds

We study the dynamics of D-branes in a smooth time-dependent background. The theory on the branes is a time-dependent noncommutative field theory. We find the metric and fluxes that determine the dual holographic closed string theory. This provides a concrete example of holography in a cosmological setting.     

String propagation in gravitational wave backgrounds

The Conformal Field Theory of the current algebra of the centrally extended 2-d Euclidean group is analyzed. Its representations can be written in terms of four free fields (without background charge) with signature (-+++). We construct all irreducible representations of the current algebra with unitary base out of the free fields and their orbifolds. This is used to investigate the spectrum and scattering of strings moving in the background of a gravitational wave. We find that all the dynamics happens in the transverse space or the longitudinal one but not both.     

Five-dimensional black hole string backgrounds and brane universe acceleration

We examine background solutions of black hole type arising from the string effective action in five dimensions. We derive the unique metric — dilaton vacuum which is a Schwarzschild type black hole. It is found that it can be extended to incorporate electric charge without changing the topology of the three space. Kalb–Ramond charge may also be introduced if the three space is closed. The basic features of the cosmology induced on a three brane evolving in this background are also discussed.     

Universal fermionic spectral functions from string theory

We carry out the first holographic calculation of a fermionic response function for a strongly coupled d=3 system with an explicit D=10 or D=11 supergravity dual. By considering the supersymmetry current, we obtain a universal result applicable to all d=3 N=2 SCFTs with such duals. Surprisingly, the spectral function does not exhibit a Fermi surface, despite the fact that the system is at finite charge density. We show that it has a phonino pole and at low frequencies there is a depletion of spectral weight with a power-law scaling which is governed by a locally quantum critical point.     

Quantum mechanical amplitude for string propagation

The quantum mechanical transition amplitude is calculated between an initial and a final string by using the canonical method based entirely on operators. The final expression for this amplitude is given in terms of the Jacobi ϑ₄-function and admits a suggestive geometrical interpretation.     

Sound propagation over curved boundary surfaces

A model for the determination of the variation of intensity of sound, due to a point source on a convex boundary surface, propagating over the surface is described. It is found that attenuation rates depend upon the surface compliant properties and are much higher for a pressure release surface than for a rigid one. The possibility of a surface giving still higher attenuation rates is explored. However, preliminary indications are that such a surface would have to be active.