Charged gravitational instantons in five-dimensional Einstein-Gauss-Bonnet-Maxwell theory

We study a solution of the Einstein-Gauus-Bonnet theory in 5 dimensions coupled to a Maxwell field, whose euclidean continuation gives rise to an instanton describing black hole pair production. We also discuss the dual theory with a 3-form field coupled to gravity.Comment: 8 pages, plain Te

Necessary Dependence of Currents on Fields They Generate

It is shown that in local (proper) Lorentz-invariant theories involving axial-vector, or tensor currents (conserved or not), the latter must vanish, if they commute at equal times with the fields they generate. The need for explicit field dependence of currents is demonstrated for gradient-coupled spinless and massive spin-one fields, as well as for electrodynamics with minimal or nonminimal coupling. The field-dependence requirement is distinct from that (already needed for free fields) of "spreading points" to make the current operators well-defined. The relation between the two, however, essentially fixes the form of this dependence. Applications are made to partially conserved currents, ∂_μ^μ = αϕ; if j^0 commutes with ϕ, the latter vanishes

On the variable-charged black holes embedded into de Sitter space: Hawking's radiation

In this paper we study the Hawking evaporation of masses of variable-charged Reissner-Nordstrom and Kerr-Newman, black holes embedded into the de Sitter universe by considering the charge to be function of radial coordinate of the spherically symmetric metric.Comment: LaTex, p. 2

Interaction of Hawking radiation with static sources outside a Schwarzschild black hole

We show that the response rate of (i) a static source interacting with Hawking radiation of massless scalar field in Schwarzschild spacetime (with the Unruh vacuum) and that of (ii) a uniformly accelerated source with the same proper acceleration in Minkowski spacetime (with the Minkowski vacuum) are equal. We show that this equality will not hold if the Unruh vacuum is replaced by the Hartle-Hawking vacuum. It is verified that the source responds to the Hawking radiation near the horizon as if it were at rest in a thermal bath in Minkowski spacetime with the same temperature. It is also verified that the response rate in the Hartle-Hawking vacuum approaches that in Minkowski spacetime with the same temperature far away from the black hole. Finally, we compare our results with others in the literature.Comment: 18 pages (REVTEX

Brane Cosmology from Heterotic String Theory

We consider brane cosmologies within the context of five-dimensional actions with O(a') higher curvature corrections. The actions are compatible with bulk string amplitude calculations from heterotic string theory. We find wrapped solutions that satisfy the field equations in an approximate but acceptable manner given their complexity, where the internal four-dimensional scale factor is naturally inflating, having an exponential De-Sitter form. The temporal dependence of the metric components is non-trivial so that this metric cannot be factored as in a conformally flat case. The effective Planck mass is finite and the brane solutions localize four-dimensional gravity, while the four-dimensional gravitational constant varies with time. The Hubble constant can be freely specified through the initial value of the scalar field, to conform with recent data.Comment: 15 pages, 3 figures, LaTeX, Accepted for Publication in IJT

Consistency of Semiclassical Gravity

We discuss some subtleties which arise in the semiclassical approximation to quantum gravity. We show that integrability conditions prevent the existence of Tomonaga-Schwinger time functions on the space of three-metrics but admit them on superspace. The concept of semiclassical time is carefully examined. We point out that central charges in the matter sector spoil the consistency of the semiclassical approximation unless the full quantum theory of gravity and matter is anomaly-free. We finally discuss consequences of these considerations for quantum field theory in flat spacetime, but with arbitrary foliations.Comment: 12 pages, LATEX, Report Freiburg THEP-94/2

Causal Structure of Vacuum Solutions to Conformal(Weyl) Gravity

Using Penrose diagrams the causal structure of the static spherically symmetric vacuum solution to conformal (Weyl) gravity is investigated. A striking aspect of the solution is an unexpected physical singularity at $r=0$ caused by a linear term in the metric. We explain how to calculate the deflection of light in coordinates where the metric is manifestly conformal to flat i.e. in coordinates where light moves in straight lines.Comment: 18 pages, 2 figures, title and abstract changed, contents essentially unaltered accepted for publication in General Relativity and Gravitatio

Brane versus shell cosmologies in Einstein and Einstein-Gauss-Bonnet theories

We first illustrate on a simple example how, in existing brane cosmological models, the connection of a 'bulk' region to its mirror image creates matter on the 'brane'. Next, we present a cosmological model with no $Z_2$ symmetry which is a spherical symmetric 'shell' separating two metrically different 5-dimensional anti-de Sitter regions. We find that our model becomes Friedmannian at late times, like present brane models, but that its early time behaviour is very different: the scale factor grows from a non-zero value at the big bang singularity. We then show how the Israel matching conditions across the membrane (that is either a brane or a shell) have to be modified if more general equations than Einstein's, including a Gauss-Bonnet correction, hold in the bulk, as is likely to be the case in a low energy limit of string theory. We find that the membrane can then no longer be treated in the thin wall approximation. However its microphysics may, in some instances, be simply hidden in a renormalization of Einstein's constant, in which cases Einstein and Gauss-Bonnet membranes are identical.Comment: 15 pages, submitted to Phys. Rev.

On the instability of classical dynamics in theories with higher derivatives

The development of instability in the dynamics of theories with higher derivatives is traced in detail in the framework of the Pais-Uhlenbeck fourth oder oscillator. For this aim the external friction force is introduced in the model and the relevant solutions to equations of motion are investigated. As a result, the physical implication of the energy unboundness from below in theories under consideration is revealed.Comment: 9 pages, no figures and no tables, revtex4; a few misprints are correcte