Conformal Invariance of Black Hole Temperature

It is shown that the surface gravity and temperature of a stationary black hole are invariant under conformal transformations of the metric that are the identity at infinity. More precisely, we find a conformal invariant definition of the surface gravity of a conformal Killing horizon that agrees with the usual definition(s) for a true Killing horizon and is proportional to the temperature as defined by Hawking radiation. This result is reconciled with the intimate relation between the trace anomaly and the Hawking effect, despite the {\it non}invariance of the trace anomaly under conformal transformations.Comment: 8 pages, plain LaTeX, NSF-ITP-93-9

Generalized entropy and Noether charge

We find an expression for the generalized gravitational entropy of Hawking in terms of Noether charge. As an example, the entropy of the Taub-Bolt spacetime is calculated.Comment: 6 pages, revtex, reference correcte

A non-singular black hole model as a possible end-product of gravitational collapse

In this paper we present a non-singular black hole model as a possible end-product of gravitational collapse. The depicted spacetime which is type [II,(II)], by Petrov classification, is an exact solution of the Einstein equations and contains two horizons. The equation of state in the radial direction, is a well-behaved function of the density and smoothly reproduces vacuum-like behavior near r=0 while tending to a polytrope at larger r, low density, values. The final equilibrium configuration comprises of a de Sitter-like inner core surrounded by a family of 2-surfaces of matter fields with variable equation of state. The fields are all concentrated in the vicinity of the radial center r=0. The solution depicts a spacetime that is asymptotically Schwarzschild at large r, while it becomes de Sitter-like for vanishing r. Possible physical interpretations of the macro-state of the black hole interior in the model are offered. We find that the possible state admits two equally viable interpretations, namely either a quintessential intermediary region or a phase transition in which a two-fluid system is in both dynamic and thermodynamic equilibrium. We estimate the ratio of pure matter present to the total energy and in both (interpretations) cases find it to be virtually the same, being 0.83. Finally, the well-behaved dependence of the density and pressure on the radial coordinate provides some insight on dealing with the information loss paradox.Comment: 12 Pages, 1 figure. Accepted for publication in Phys. Rev.

Black-hole information puzzle: A generic string-inspired approach

Given the insight steming from string theory, the origin of the black-hole (BH) information puzzle is traced back to the assumption that it is physically meaningful to trace out the density matrix over negative-frequency Hawking particles. Instead, treating them as virtual particles necessarily absorbed by the BH in a manner consistent with the laws of BH thermodynamics, and tracing out the density matrix only over physical BH states, the complete evaporation becomes compatible with unitarity.Comment: 8 pages, revised, title changed, to appear in Eur. Phys. J.

Information Loss in Black Holes

The question of whether information is lost in black holes is investigated using Euclidean path integrals. The formation and evaporation of black holes is regarded as a scattering problem with all measurements being made at infinity. This seems to be well formulated only in asymptotically AdS spacetimes. The path integral over metrics with trivial topology is unitary and information preserving. On the other hand, the path integral over metrics with non-trivial topologies leads to correlation functions that decay to zero. Thus at late times only the unitary information preserving path integrals over trivial topologies will contribute. Elementary quantum gravity interactions do not lose information or quantum coherence

Quantum Coherence and Closed Timelike Curves

Various calculations of the $S$ matrix have shown that it seems to be non unitary for interacting fields when there are closed timelike curves. It is argued that this is because there is loss of quantum coherence caused by the fact that part of the quantum state circulates on the closed timelike curves and is not measured at infinity. A prescription is given for calculating the superscattering matrix $\$$ on space times whose parameters can be analytically continued to obtain a Euclidean metric. It is illustrated by a discussion of a spacetime in with two disks in flat space are identified. If the disks have an imaginary time separation, this corresponds to a heat bath. An external field interacting with the heat bath will lose quantum coherence. One can then analytically continue to an almost real separation of the disks. This will give closed timelike curves but one will still get loss of quantum coherence.Comment: 13 page

Gravitational Entropy and Global Structure

The underlying reason for the existence of gravitational entropy is traced to the impossibility of foliating topologically non-trivial Euclidean spacetimes with a time function to give a unitary Hamiltonian evolution. In $d$ dimensions the entropy can be expressed in terms of the $d-2$ obstructions to foliation, bolts and Misner strings, by a universal formula. We illustrate with a number of examples including spaces with nut charge. In these cases, the entropy is not just a quarter the area of the bolt, as it is for black holes.Comment: 18 pages. References adde

New Scale Factor Measure

The computation of probabilities in an eternally inflating universe requires a regulator or "measure". The scale factor time measure truncates the universe when a congruence of timelike geodesics has expanded by a fixed volume factor. This definition breaks down if the generating congruence is contracting---a serious limitation that excludes from consideration gravitationally bound regions such as our own. Here we propose a closely related regulator which is well-defined in the entire spacetime. The New Scale Factor Cutoff restricts to events with scale factor below a given value. Since the scale factor vanishes at caustics and crunches, this cutoff always includes an infinite number of disconnected future regions. We show that this does not lead to divergences. The resulting measure combines desirable features of the old scale factor cutoff and of the light-cone time cutoff, while eliminating some of the disadvantages of each.Comment: 20 pages, 1 figure; v2: references adde

On the fate of black string instabilities: An Observation

Gregory and Laflamme (hep-th/9301052) have argued that an instability causes the Schwarzschild black string to break up into disjoint black holes. On the other hand, Horowitz and Maeda (arXiv:hep-th/0105111) derived bounds on the rate at which the smallest sphere can pinch off, showing that, if it happens at all, such a pinch-off can occur only at infinite affine parameter along the horizon. An interesting point is that, if a singularity forms, such an infinite affine parameter may correspond to a finite advanced time -- which is in fact a more appropriate notion of time at infinity. We argue below that pinch-off at a finite advanced time is in fact a natural expectation under the bounds derived by Horowitz and Maeda.Comment: 4 pages, RevTex, 1 figure, references adde