David L. Wiltshire

Spacetime as a membrane in higher dimensions

By means of a simple model we investigate the possibility that spacetime is a membrane embedded in higher dimensions. We present cosmological solutions of d-dimensional Einstein-Maxwell theory which compactify to two dimensions. These solutions are analytically continued to obtain dual solutions in which a (d − 2)-dimensional Einstein spacetime “membrane” is embedded in d dimensions. The membrane solutions generalise Melvins 4-dimensional flux tube solution. The flat membrane is shown to be classically stable. It is shown that there are zero mode solutions of the d-dimensional Dirac equation which are confined to a neighbourhood of the membrane and move within it like massless chiral (d − 2)-dimensional fermions. An investigation of the spectrum of scalar perturbations shows that a well-defined mass gap between the zero modes and massive modes can be obtained if there is a positive cosmological term in (d − 2) dimensions or a negative cosmological term in d dimensions.

Super p-Branes

Abstract It is shown that the extension of the spacetime supersymmetric Green-Schwarz covariant action p -dimensional extended objects ( p -branes) is possible if and only if the on-shell p -dimensional Bose and Fermi degrees of freedom are equal. This is further evidence for world-tube supersymmetry in these models. All the p -branes models are related to superstring actions in d =3, 4, 6 or 10 dimensions by double dimensional reduction (which we generalise to reduction on arbitrary compact spaces), and we also show how they may be considered as topological defects of supergravity theories.

Black Holes in Kaluza-Klein Theory

Stationary, spherically symmetric, asymptotically flat solutions of the Kaluza-Klein theory corresponding to regular black holes in four dimensions are discussed and classified. The thermodynamic properties of the holes are discussed and Smarr-type relations obtained. The thermal evaporation of the holes is considered. In the magnetically neutral case discharging of holes (by pyrgon emission) is only significant for solutions close to the extreme configuration Q 2 = 4κ 2 M 2 , 4πΣ = √3κ M . For holes with nonvanishing magnetic charge an evolution towards the extreme configuration corresponding to the Gross-Perry-Sorkin monopole is expected. The gyromagnetic ratio of rotating black holes is briefly discussed and compared to that of pyrgons and massive 5-dimensional fields.

Spherically Symmetric Solutions of Einstein-maxwell Theory With a {Gauss-Bonnet} Term

The low energy expansion of supersymmetric string theory suggests that the leading correction to the Einstein action is given by the Gauss-Bonnet invariant. A generalisation of Birkhoffs theorem in the case of Einstein-Maxwell theory modified by a Gauss-Bonnet term is proved. The only spherically symmetric solutions of the theory are shown to be generalisations of the Reissner-Nordstrom and Robinson-Bertotti solutions. The “Reissner-Nordstrom” solutions have asymptotically flat and asymptotically anti-de Sitter branches, however, the latter are unstable.

Exact solution to the averaging problem in cosmology

The exact solution of a two-scale Buchert average of the Einstein equations is derived for an inhomogeneous universe that represents a close approximation to the observed universe. The two scales represent voids, and the bubble walls surrounding them within which clusters of galaxies are located. As described elsewhere [New J. Phys. 9, 377 (2007)10.1088/1367-2630/9/10/377], apparent cosmic acceleration can be recognized as a consequence of quasilocal gravitational energy gradients between observers in bound systems and the volume-average position in freely expanding space. With this interpretation, the new solution presented here replaces the Friedmann solutions, in representing the average evolution of a matter-dominated universe without exotic dark energy, while being observationally viable.

What is dust?—Physical foundations of the averaging problem in cosmology

The problems of coarse-graining and averaging of inhomogeneous cosmologies, and their backreaction on average cosmic evolution, are reviewed from a physical viewpoint. A particular focus is placed on comparing different notions of average spatial homogeneity, and on the interpretation of observational results. Among the physical questions we consider are the nature of an average Copernican principle, the role of Machs principle, the issue of quasilocal gravitational energy and the different roles of spacetime, spatial and null cone averages. The observational interpretation of the timescape scenario is compared to other approaches to cosmological averaging, and outstanding questions are discussed.

Cosmic acceleration from M theory on twisted spaces

In a recent paper [I. P. Neupane and D. L. Wiltshire, Phys. Lett. B 619, 201 (2005).] we have found a new class of accelerating cosmologies arising from a time-dependent compactification of classical supergravity on product spaces that include one or more geometric twists along with nontrivial curved internal spaces. With such effects, a scalar potential can have a local minimum with positive vacuum energy. The existence of such a minimum generically predicts a period of accelerated expansion in the four-dimensional Einstein conformal frame. Here we extend our knowledge of these cosmological solutions by presenting new examples and discuss the properties of the solutions in a more general setting. We also relate the known (asymptotic) solutions for multiscalar fields with exponential potentials to the accelerating solutions arising from simple (or twisted) product spaces for internal manifolds.

Black holes in higher derivative gravity theories

We study static spherically symmetric solutions of Einstein gravity plus an action polynomial in the Ricci scalar

Gravitational Energy as Dark Energy: Concordance of Cosmological Tests

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Charged dilaton black holes with a cosmological constant

of arbitrary degree