Edward Teo

Rotating traversable wormholes

The general form of a stationary, axially symmetric traversable wormhole is discussed. This provides an explicit class of rotating wormholes that generalizes the static, spherically symmetric ones first considered by Morris and Thorne. In agreement with general analyses, it is verified that such a wormhole generically violates the null energy condition at the throat. However, for suitable model wormholes, there can be classes of geodesics falling through it which do not encounter any energy-condition-violating matter. The possible presence of an ergoregion surrounding the throat is also noted.

A new form of the C-metric

The usual form of the C-metric has the structure function G(ξ) = 1 − ξ2 − 2mAξ3, whose cubic nature can make calculations cumbersome, especially when explicit expressions for its roots are required. In this paper, we propose a new form of the C-metric, with the explicitly factorizable structure function G(ξ) = (1 − ξ2)(1 + 2mAξ). Although this form is related to the usual one by a coordinate transformation, it has the advantage that its roots are now trivial to write down. We show that this leads to potential simplifications, for example, when casting the C-metric in Weyl coordinates. These results also extend to the charged C-metric, whose structure function can be written in a new form G(ξ) = (1 − ξ2)(1 + r+Aξ)(1 + r−Aξ), where r± are the usual locations of the horizons in the Reissner–Nordstrom solution. As a by-product, we explicitly cast the extremally charged C-metric in Weyl coordinates.

Bounds on negative energy densities in static space-times

We generalise results of Ford and Roman which place lower bounds – known as quantum inequalities – on the renormalised energy density of a quantum field averaged against a choice of sampling function. Ford and Roman derived their results for a specific non-compactly supported sampling function; here we use a different argument to obtain quantum inequalities for a class of smooth, even and non-negative sampling functions which are either compactly supported or decay rapidly at infinity. Our results hold in d-dimensional Minkowski space (d ≥ 2) for the free real scalar field of mass m ≥ 0. We discuss various features of our bounds in 2 and 4 dimensions. In particular, for massless field theory in 2-dimensional Minkowski space, we show that our quantum inequality is weaker than Flanagan’s optimal bound by a factor of 3 .

Spherical Photon Orbits Around a Kerr Black Hole

Two circular photon orbits are known to exist in the equatorial plane of the Kerr black hole. In this paper, we investigate so-called spherical photon orbits—orbits with constant coordinate radii that are not confined to the equatorial plane. A one-parameter class of solutions is found, which includes the circular orbits as special cases. The properties of these spherical orbits are then analyzed, with the aim of classifying them by qualitative differences in their behavior. Finally, representative orbits from each class are plotted out, including a zero-angular momentum photon orbit and one with non-fixed azimuthal direction.

Macroscopic and microscopic description of black diholes

Abstract We study configurations consisting of a pair of non-extremal black holes in four dimensions, both with the same mass, and with charges of the same magnitude but opposite sign — diholes, for short. We present such exact solutions for Einstein–Maxwell theory with arbitrary dilaton coupling, and also solutions to the U(1)4 theories that arise from compactified string/M-theory. Despite the fact that the solutions are very complicated, physical properties of these black holes, such as their area, charge, and interaction energy, admit simple expressions. We also succeed in providing a microscopic description of the entropy of these black holes using the ‘effective string’ model, and taking into account the interaction between the effective string and antistring.

Rod-structure classification of gravitational instantons with U(1)×U(1) isometry

Abstract The rod-structure formalism has played an important role in the study of black holes in D = 4 and 5 dimensions with R × U ( 1 ) D − 3 isometry. In this paper, we apply this formalism to the study of four-dimensional gravitational instantons with U ( 1 ) × U ( 1 ) isometry, which could serve as spatial backgrounds for five-dimensional black holes. We first introduce a stronger version of the rod structure with the rod directions appropriately normalised, and show how the regularity conditions can be read off from it. Requiring the absence of conical and orbifold singularities will in general impose periodicity conditions on the coordinates, and we illustrate this by considering known gravitational instantons in this class. Some previous results regarding certain gravitational instantons are clarified in the process. Finally, we show how the rod-structure formalism is able to provide a classification of gravitational instantons, and speculate on the existence of possible new gravitational instantons.

Rotating black lens solution in five dimensions

It has recently been shown that a stationary, asymptotically flat vacuum black hole in five space-time dimensions with two commuting axial symmetries must have an event horizon with either a spherical, ring or lens-space topology. In this paper, we study the third possibility, a so-called black lens with

Nonsingularity of the exact two-dimensional string black hole

L(n,1)

Black diholes with unbalanced magnetic charges

horizon topology. Using the inverse scattering method, we construct a black-lens solution with the simplest possible rod structure, and possessing a single asymptotic angular momentum. Its properties are then analyzed; in particular, it is shown that there must either be a conical singularity or a naked curvature singularity present in the space-time.

Black holes on gravitational instantons

In the semiclassical limit, the two-dimensional space-time which emerges from string theory is a black hole similar to the Schwarzschild solution. However, we find that in the exact space-time, the singularity is replaced by a regular surface of time-reflection symmetry. The maximally extended space-time thus consists of an infinite sequence of asymptotically flat regions linked by wormholes, rather analogous to the Reissner-Nordstrom space-time except that there are no singularities in this case. We also calculate the mass and temperature associated with this space-time