On the origin of thermal string gas

We investigate decaying D-branes as the origin of the thermal string gas of string gas cosmology. We consider initial configurations of low-dimensional branes and argue that they can time evolve to thermal string gas. We find that there is a range in the weak string coupling and fast brane decay time regimes, where the initial configuration could drive the evolution of the dilaton to values, where exactly three spacelike directions grow large.Comment: 16 pages, 4 figures, v2: references adde

c=1 Boundary Conformal Field Theory Revisited

Correlation functions of discrete primary fields in the c=1 boundary conformal field theory of a scalar field in a critical periodic boundary potential are computed using the underlying SU(2) symmetry of the model. Bulk amplitudes are unambigously determined and we give a prescription for amplitudes involving discrete boundary fields.Comment: LaTex, 8 pages, 1 figure, uses IOP style files. To appear in the proceedings of the RTN workshop "The quantum structure of spacetime and the geometric nature of fundamental interactions", Copenhagen, September 15-20, 200

Alpha-Vacua, Black Holes, and AdS/CFT

The Schwarzschild, Schwarzschild-AdS, and Schwarzschild-de Sitter solutions all admit freely acting discrete involutions which commute with the continuous symmetries of the spacetimes. Intuitively, these involutions correspond to the antipodal map of the corresponding spacetimes. In analogy with the ordinary de Sitter example, this allows us to construct new vacua by performing a Mottola-Allen transform on the modes associated with the Hartle-Hawking, or Euclidean, vacuum. These vacua are the `alpha'-vacua for these black holes. The causal structure of a typical black hole may ameliorate certain difficulties which are encountered in the case of de Sitter alpha-vacua. For Schwarzschild-AdS black holes, a Bogoliubov transformation which mixes operators of the two boundary CFT's provides a construction of the dual CFT alpha-states. Finally, we analyze the thermal properties of these vacua.Comment: 40 pages REVTeX and AMSLaTeX, 17 black&white eps figures. v3: references added. v4: details of the pinch singularity avoidance for the string quantization of the Rindler space toy model have been added in both the body of the paper and in a new 7 page appendix. Other clarifications and references added. This is the version accepted for publication in Class. Quant. Gra

Proper acceleration, geometric tachyon and dynamics of a fundamental string near Dpp branes

We present a detailed analysis of our recent observation that the origin of the geometric tachyon, which arises when a D$p$-brane propagates in the vicinity of a stack of coincident NS5-branes, is due to the proper acceleration generated by the background dilaton field. We show that when a fundamental string (F-string), described by the Nambu-Goto action, is moving in the background of a stack of coincident D$p$-branes, the geometric tachyon mode can also appear since the overall conformal mode of the induced metric for the string can act as a source for proper acceleration. We also studied the detailed dynamics of the F-string as well as the instability by mapping the Nambu-Goto action of the F-string to the tachyon effective action of the non-BPS D-string. We qualitatively argue that the condensation of the geometric tachyon is responsible for the (F,D$p$) bound state formation.Comment: 26 pages, v2: added references, v3: one ref. updated, to appear in Class. and Quant. Gravit

2+1 Black Hole with a Dynamical Conical Singularity

We find black hole solutions to Euclidean 2+1 gravity coupled to a relativistic particle which have a dynamical conical singularity at the horizon. These solutions mimic the tree level contribution to the partition function of gravity coupled to a quantum field theory. They are found to violate the standard area law for black hole entropy, their entropy being proportional to the total opening angle. Since each solution depends on the number of windings of the particle path around the horizon, the significance of their summation in the path integral is considered.Comment: 11 pages, LaTeX, no figures. Revised version to appear in Class. Quant. Grav. The main changes are an expanded introduction and a better presentation of the body of the paper. Three paragraphs have also been added to the conclusio

Exact Microscopic Entropy of Non-Supersymmetric Extremal Black Rings

In this brief note we show that the horizon entropy of the largest known class of non-supersymmetric extremal black rings, with up to six parameters, is exactly reproduced for all values of the ring radius using the same conformal field theory of the four-charge four-dimensional black hole. A particularly simple case is a dipole black ring without any conserved charges. The mass gets renormalized, but the first corrections it receives can be easily understood as an interaction potential energy. Finally, we stress that even if the entropy is correctly reproduced, this only implies that one sector of chiral excitations has been identified, but an understanding of excitations in the other sector is still required in order to capture the black ring dynamics.Comment: 7 pages. v2: minor improvements, ref adde

How Not to Construct an Asymptotically de Sitter Universe

Observational evidence suggests that our universe is currently evolving towards an asymptotically de Sitter future. Unfortunately and in spite of much recent attention, various quantum, holographic and cosmological aspects of de Sitter space remain quite enigmatic. With such intrigue in mind, this paper considers the ``construction'' of a toy model that describes an asymptotically de Sitter universe. More specifically, we add fluid-like matter to an otherwise purely de Sitter spacetime, formulate the relevant solutions and then discuss the cosmological and holographic implications. If the objective is to construct an asymptotically de Sitter universe that is free of singularities and has a straightforward holographic interpretation, then the results of this analysis are decidedly negative. Nonetheless, this toy model nicely illustrates the pitfalls that might be encountered in a more realistic type of construction.Comment: 25 pages, Latex; references and footnotes added (other minor changes

A Comment on the Geometric Entropy and Conical Space

It has been recently pointed out that a definition of the geometric entropy using the partition function in a conical space does not in general lead to a positive definite quantity. For a scalar field model with a non-minimal coupling we clarify the origin of the anomalous behavior from the viewpoint of the canonical formulation.Comment: No Figures. To appear in Classical and Quantum Gravit

The quantum structure of black holes

We give an elementary review of black holes in string theory. We discuss black hole entropy from string microstates and Hawking radiation from these states. We then review the structure of 2-charge microstates, and explore how `fractionation' can lead to quantum effects over macroscopic length scales of order the horizon radius.Comment: Review article, 58 pages, 2 figures; references added, note about topics covere

Massless black holes and black rings as effective geometries of the D1-D5 system

We compute correlation functions in the AdS/CFT correspondence to study the emergence of effective spacetime geometries describing complex underlying microstates. The basic argument is that almost all microstates of fixed charges lie close to certain "typical" configurations. These give a universal response to generic probes, which is captured by an emergent geometry. The details of the microstates can only be observed by atypical probes. We compute two point functions in typical ground states of the Ramond sector of the D1-D5 CFT, and compare with bulk two-point functions computed in asymptotically AdS_3 geometries. For large central charge (which leads to a good semiclassical limit), and sufficiently small time separation, a typical Ramond ground state of vanishing R-charge has the M=0 BTZ black hole as its effective description. At large time separation this effective description breaks down. The CFT correlators we compute take over, and give a response whose details depend on the microstate. We also discuss typical states with nonzero R-charge, and argue that the effective geometry should be a singular black ring. Our results support the argument that a black hole geometry should be understood as an effective coarse-grained description that accurately describes the results of certain typical measurements, but breaks down in general.Comment: 47 pages, 4 figures. v2: references added. v3: minor corrections to Appendix A, references adde