Is string theory a theory of quantum gravity?

Some problems in finding a complete quantum theory incorporating gravity are discussed. One is that of giving a consistent unitary description of high-energy scattering. Another is that of giving a consistent quantum description of cosmology, with appropriate observables. While string theory addresses some problems of quantum gravity, its ability to resolve these remains unclear. Answers may require new mechanisms and constructs, whether within string theory, or in another framework.Comment: Invited contribution for "Forty Years of String Theory: Reflecting on the Foundations," a special issue of Found. Phys., ed. by G 't Hooft, E. Verlinde, D. Dieks, S. de Haro. 32 pages, 5 figs., harvmac. v2: final version to appear in journal (small revisions

Cosmological diagrammatic rules

A simple set of diagrammatic rules is formulated for perturbative evaluation of ``in-in" correlators, as is needed in cosmology and other nonequilibrium problems. These rules are both intuitive, and efficient for calculational purposes.Comment: 7 pages, 3 figure

Linking the trans-Planckian and the information loss problems in black hole physics

The trans-Planckian and information loss problems are usually discussed in the literature as separate issues concerning the nature of Hawking radiation. Here we instead argue that they are intimately linked, and can be understood as "two sides of the same coin" once it is accepted that general relativity is an effective field theory.Comment: 10 pages, 2 figures. Replaced with the version to be published in General Relativity and Gravitatio

Scalar potential from de Sitter brane in 5D and effective cosmological constant

We derive the scalar potential in zero mode effective action arising from a de Sitter brane embedded in five dimensions with bulk cosmological constant $\Lambda$. The scalar potential for a scalar field canonically normalized is given by the sum of exponential potentials. In the case of $\Lambda=0$ and $\Lambda>0$, we point out that the scalar potential has an unstable local maximum at the origin and exponentially vanishes for large positive scalar field. In the case of $\Lambda<0$, the scalar potential has an unstable local maximum at the origin and a stable local minimum, it is shown that the positive cosmological constant in brane is reduced by negative potential energy of scalar at minimum.Comment: 14 pages, 5 figures, add the section of cosmological implication

Closed String Amplitudes from Gauge Fixed String Field Theory

Closed string diagrams are derived from cubic open string field theory using a gauge fixed kinetic operator. The basic idea is to use a string propagator that does not generate a boundary to the world sheet. Using this propagator and the closed string vertex, the moduli space of closed string surfaces is covered, so closed string scattering amplitudes should be reproduced. This kinetic operator could be a gauge fixed form of the string field theory action around the closed string vacuum.Comment: 10 pages, revtex, 3 figures. Discussion on the covering of moduli expanded, version to appear in PR

High-energy head-on collisions of particles and hoop conjecture

We investigate the apparent horizon formation for high-energy head-on collisions of particles in multi-dimensional spacetime. The apparent horizons formed before the instance of particle collision are obtained analytically. Using these solutions, we discuss the feature of the apparent horizon formation in the multi-dimensional spacetime from the viewpoint of the hoop conjecture.Comment: 4pages, 4figure

Charged Randall-Sundrum black holes and N=4 super Yang-Mills in AdS(2)xS(2)

We obtain some exact results for black holes in the Randall-Sundrum model with a single brane. We consider an extreme black hole charged with respect to a Maxwell field on the brane. The near-horizon geometry is determined. The induced metric on the brane and the black hole entropy are compared with the predictions of 4d General Relativity. There is good agreement for large black holes, with calculable subleading corrections. As a separate application, the bulk solution provides a gravitational dual for (strongly coupled, large N) N=4 SYM in AdS(2)xS(2) for arbitrary relative size of AdS(2) and S(2).Comment: 13 page

Effective action and brane running

We address the renormalized effective action for a Randall-Sundrum brane running in 5d bulk space. The running behavior of the brane action is obtained by shifting the brane-position without changing the background and the fluctuations. After an appropriate renormalization, we obtain an effective, low energy braneworld action, in which the effective 4d Planck mass is independent of the running-position. We address some implications of this effective action.Comment: 15 pages, no figur

Charge Asymmetry in the Brane World and Formation of Charged Black Holes

In theories with an infinite extra dimension, free particles localized on the brane can leak out to the extra space. We argue that if there were color confinement in the bulk, electrons would be more able to escape than quarks and than protons (which are composed states). Thus, this process generates an electric charge asymmetry on brane matter densities. A primordial charge asymmetry during Big Bang Nucleosynthesis era is predicted. We use current bounds on this and on electron disappearance to constrain the parameter space of these models. Although the generated asymmetry is generically small, it could be particularly enhanced on large densities as in astrophysical objects, like massive stars. We suggest the possibility that such accumulation of charge may be linked, upon supernova collapse, to the formation of a charged Black Hole and the generation of Gamma-Ray Bursts.Comment: Four pages, one figure. Minor changes, conclusions remai

D-branes in 2d Lorentzian black hole

We study D-branes in the Lorentzian signature 2D black hole string theory. We use the technique of gauged WZW models to construct the associated boundary conformal field theories. The main focus of this work is to discuss the (semi-classical) world-volume geometries of the D-branes. We also discuss comparison of our work with results in related gauged WZW models.Comment: 24 pages, 5 figures, uses JHEP3.cl