Particle Production in Matrix Cosmology

We consider cosmological particle production in 1+1 dimensional string theory. The process is described most efficiently in terms of anomalies, but we also discuss the explicit mode expansions. In matrix cosmology the usual vacuum ambiguity of quantum fields in time-dependent backgrounds is resolved by the underlying matrix model. This leads to a finite energy density for the "in" state which cancels the effect of anomalous particle production.Comment: 24 pages, 1 figure; v2: references added, minor change

Observer dependent D-brane for strings propagating in pp-wave time dependent background

We study type IIB superstring in the pp-wave time-dependent background, which has a singularity at $t=0$. We show that this background can provide a toy model to study some ideas related to the stretched horizon paradigm and the complementary principle of black holes. To this end, we construct a unitary Bogoliubov generator which relates the asymptotically flat string Hilbert space, defined at $t =\pm \infty$, to the finite time Hilbert space. For asymptotically flat observers, the closed string vacuum close to the singularity appears as a boundary state which is in fact a D-brane described in the closed string channel. However, observers who go with the string towards to the singularity see the original vacuum.Comment: 12 pages, revtex 4, added references, corrected mistake

Holographic Description of AdS Cosmologies

To gain insight in the quantum nature of the big bang, we study the dual field theory description of asymptotically anti-de Sitter solutions of supergravity that have cosmological singularities. The dual theories do not appear to have a stable ground state. One regularization of the theory causes the cosmological singularities in the bulk to turn into giant black holes with scalar hair. We interpret these hairy black holes in the dual field theory and use them to compute a finite temperature effective potential. In our study of the field theory evolution, we find no evidence for a "bounce" from a big crunch to a big bang. Instead, it appears that the big bang is a rare fluctuation from a generic equilibrium quantum gravity state.Comment: 34 pages, 8 figures, v2: minor changes, references adde

Towards a Big Crunch Dual

We show there exist smooth asymptotically anti-de Sitter initial data which evolve to a big crunch singularity in a low energy supergravity limit of string theory. This opens up the possibility of using the dual conformal field theory to obtain a fully quantum description of the cosmological singularity. A preliminary study of this dual theory suggests that the big crunch is an endpoint of evolution even in the full string theory. We also show that any theory with scalar solitons must have negative energy solutions. The results presented here clarify our earlier work on cosmic censorship violation in N=8 supergravity.Comment: 27 pages, 3 figures;v2:minor correction

Closed String Tachyon Condensation on Twisted Circles

We study IIA/B string theory compactified on twisted circles. These models possess closed string tachyons and reduce to type 0B/A theory in a special limit. Using methods of gauged linear sigma models and mirror symmetry we construct a conformal field theory which interpolates between these models and flat space via an auxiliary Liouville direction. Interpreting motion in the Liouville direction as renormalization group flow, we argue that the end point of tachyon condensation in all these models (including 0B/A theory) is supersymmetric type II theory. We also find a zero-slope limit of these models which is best described in a T-dual picture as a type II NS-NS fluxbrane. In this limit tachyon condensation is an interesting and well posed problem in supergravity. We explicitly determine the tachyon as a fluctuation of supergravity fields, and perform a rudimentary numerical analysis of the relevant flows.Comment: 21 pages plus appendices (12 pages), harvmac, 1 fig, v2: minor changes and references added, v3: minor changes version published in JHE

Greybody Factors of Charged Dilaton Black Holes in 2 + 1 Dimensions

We have studied scalar perturbations of charged dilaton black holes in 2+1 dimensions. The black hole considered here is a solution to the low-energy string theory in 2+1 dimensions. The exact decay rates and the grey body factors for the massless minimally coupled scalar is computed for both the charged and the uncharged dilaton black holes. The charged and the uncharged black hole show similar behavior for grey body factors, reflection coefficients and decay rates.Comment: The equation for the potential and figure:1 are changed. The changes does not effect the result

AdS/CFT, Multitrace Deformations and New Instabilities of Nonlocal String Theories

We study "multitrace" deformations of large N master fields in models with a mass gap. In particular, we determine the conditions for the multitrace couplings to drive tachyonic instabilities. These tachyons represent new local instabilities of the associated nonlocal string theories. In the particular case of Dp-branes at finite temperature, we consider topology-changing phase transitions and the effect of multitrace perturbations on the corresponding phase diagrams.Comment: harvmac. 28 pages. 9 eps figure

Negative Energy in String Theory and Cosmic Censorship Violation

We find asymptotically anti de Sitter solutions in N=8 supergravity which have negative total energy. This is possible since the boundary conditions required for the positive energy theorem are stronger than those required for finite mass (and allowed by string theory). But stability of the anti de Sitter vacuum is still ensured by the positivity of a modified energy, which includes an extra surface term. Some of the negative energy solutions describe classical evolution of nonsingular initial data to naked singularities. Since there is an open set of such solutions, cosmic censorship is violated generically in supergravity. Using the dual field theory description, we argue that these naked singularities will be resolved in the full string theory.Comment: 23 pages, 2 figures, v2: argument for forming naked singularities clarified, references adde

Black Hole Graybody Factor and Black Hole Entropy

We have proposed the entropy formula of the black hole which is constructed by the intersecting D1-brane and D5-brane with no momentum, whose compactification radii are constrained by the surface gravities in ten-dimensions. We interpret the entropy of the black hole as the statistical entropy of the effective string living on the D5-brane. We further study the behavior of the absorption cross-section of the black hole using our entropy formula.Comment: 15 pages, RevTex, version to appear in PR

Closed-String Tachyons and the Hagedorn Transition in AdS Space

We discuss some aspects of the behaviour of a string gas at the Hagedorn temperature from a Euclidean point of view. Using AdS space as an infrared regulator, the Hagedorn tachyon can be effectively quasi-localized and its dynamics controled by a finite energetic balance. We propose that the off-shell RG flow matches to an Euclidean AdS black hole geometry in a generalization of the string/black-hole correspondence principle. The final stage of the RG flow can be interpreted semiclassically as the growth of a cool black hole in a hotter radiation bath. The end-point of the condensation is the large Euclidean AdS black hole, and the part of spacetime behind the horizon has been removed. In the flat-space limit, holography is manifest by the system creating its own transverse screen at infinity. This leads to an argument, based on the energetics of the system, explaining why the non-supersymmetric type 0A string theory decays into the supersymmetric type IIB vacuum. We also suggest a notion of `boundary entropy', the value of which decreases along the line of flow.Comment: 24 pages, Harvmac. 2 Figures. Typos corrected and reference adde