Standing gravitational waves from domain walls

We construct a plane symmetric, standing gravitational wave for a domain wall plus a massless scalar field. The scalar field can be associated with a fluid which has the properties of `stiff' matter, i.e. matter in which the speed of sound equals the speed of light. Although domain walls are observationally ruled out in the present era the solution has interesting features which might shed light on the character of exact non-linear wave solutions to Einstein's equations. Additionally this solution may act as a template for higher dimensional 'brane-world' model standing waves.Comment: 4 pages two-column format, no figures, added discussion of physical meaning of solution, added refernces, to be published PR

Superradiant instability of large radius doubly spinning black rings

We point out that 5D large radius doubly spinning black rings with rotation along S^1 and S^2 are afflicted by a robust instability. It is triggered by superradiant bound state modes. The Kaluza-Klein momentum of the mode along the ring is responsible for the bound state. This kind of instability in black strings and branes was first suggested by Marolf and Palmer and studied in detail by Cardoso, Lemos and Yoshida. We find the frequency spectrum and timescale of this instability in the black ring background, and show that it is active for large radius rings with large rotation along S^2. We identify the endpoint of the instability and argue that it provides a dynamical mechanism that introduces an upper bound in the rotation of the black ring. To estimate the upper bound, we use the recent black ring model of Hovdebo and Myers, with a minor extension to accommodate an extra small angular momentum. This dynamical bound can be smaller than the Kerr-like bound imposed by regularity at the horizon. Recently, the existence of higher dimensional black rings is being conjectured. They will be stable against this mechanism.Comment: 21 pages, 3 figures. Overall minor improvements in discussions added. Matches published version in PR

Perturbations and absorption cross-section of infinite-radius black rings

We study scalar field perturbations on the background of non-supersymmetric black rings and of supersymmetric black rings. In the infinite-radius limit of these geometries, we are able to separate the wave equation, and to study wave phenomena in its vicinities. In this limit, we show that (i) both geometries are stable against scalar field perturbations, (ii) the absorption cross-section for scalar fields is equal to the area of the event horizon in the supersymmetric case, and proportional to it in the non-supersymmetric situation.Comment: ReVTeX4. 15 pages, 3 figures. References added. Published versio

New counterterms induced by trans-Planckian physics in semiclassical gravity

We consider free and self-interacting quantum scalar fields satisfying modified dispersion relations in the framework of Einstein-Aether theory. Using adiabatic regularization, we study the renormalization of the equation for the mean value of the field in the self-interacting case, and the renormalization of the semiclassical Einstein-Aether equations for free fields. In both cases we consider Bianchi type I background spacetimes. Contrary to what happens for {\it free} fields in {\it flat} Robertson-Walker spacetimes, the self-interaction and/or the anisotropy produce non-purely geometric terms in the adiabatic expansion, i.e terms that involve both the metric $g_{\mu\nu}$ and the aether field $u_{\mu}$. We argue that, in a general spacetime, the renormalization of the theory would involve new counterterms constructed with $g_{\mu\nu}$ and $u_{\mu}$, generating a fine-tuning problem for the Einstein-Aether theory

Instability of non-supersymmetric smooth geometries

Recently certain non-supersymmetric solutions of type IIb supergravity were constructed [hep-th/0504181], which are everywhere smooth, have no horizons and are thought to describe certain non-BPS microstates of the D1-D5 system. We demonstrate that these solutions are all classically unstable. The instability is a generic feature of horizonless geometries with an ergoregion. We consider the endpoint of this instability and argue that the solutions decay to supersymmetric configurations. We also comment on the implications of the ergoregion instability for Mathur's `fuzzball' proposal.Comment: v2: typos corrected, reference adde

Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory

We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen ``paradox,'' we demonstrate strong polarization correlations of the entanged photons. Bell's idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds $S = 2.307 \pm 0.035$, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon.Comment: 10 pages, 6 figure

Primordial Black Hole: Mass and Angular Momentum Evolution

The evolution of the primordial low mass black hole (PBH) in hot universe is considered. Increase of mass and decrease of PBH spin due to the accretion of radiation dominated matter are estimated with using of results of numerical simulation of PBH formation and approximate relations for accretion to a rotating black hole.Comment: Gravitation and Cosmology, accepted, 3 pages, Talk presented at the russian summer school-seminar "Modern theoretical problems of gravitation and cosmology" (GRACOS-2007), September 9-16, 2007, Kazan-Yalchik, Russi

Self-similar cosmological solutions with dark energy. II: black holes, naked singularities and wormholes

We use a combination of numerical and analytical methods, exploiting the equations derived in a preceding paper, to classify all spherically symmetric self-similar solutions which are asymptotically Friedmann at large distances and contain a perfect fluid with equation of state $p=(\gamma -1)\mu$ with $0<\gamma<2/3$. The expansion of the Friedmann universe is accelerated in this case. We find a one-parameter family of self-similar solutions representing a black hole embedded in a Friedmann background. This suggests that, in contrast to the positive pressure case, black holes in a universe with dark energy can grow as fast as the Hubble horizon if they are not too large. There are also self-similar solutions which contain a central naked singularity with negative mass and solutions which represent a Friedmann universe connected to either another Friedmann universe or some other cosmological model. The latter are interpreted as self-similar cosmological white hole or wormhole solutions. The throats of these wormholes are defined as two-dimensional spheres with minimal area on a spacelike hypersurface and they are all non-traversable because of the absence of a past null infinity.Comment: 12 pages, 19 figures, 1 table, final version to appear in Physical Review

Self-Energy Correction to the Two-Photon Decay Width in Hydrogenlike Atoms

We investigate the gauge invariance of the leading logarithmic radiative correction to the two-photon decay width in hydrogenlike atoms. It is shown that an effective treatment of the correction using a Lamb-shift "potential" leads to equivalent results in both the length as well as the velocity gauges provided all relevant correction terms are taken into account. Specifically, the relevant radiative corrections are related to the energies that enter into the propagator denominators, to the Hamiltonian, to the wave functions, and to the energy conservation condition that holds between the two photons; the form of all of these effects is different in the two gauges, but the final result is shown to be gauge invariant, as it should be. Although the actual calculation only involves integrations over nonrelativistic hydrogenic Green functions, the derivation of the leading logarithmic correction can be regarded as slightly more complex than that of other typical logarithmic terms. The dominant radiative correction to the 2S two-photon decay width is found to be -2.020536 (alpha/pi) (Zalpha)^2 ln[(Zalpha)^-2] in units of the leading nonrelativistic expression. This result is in agreement with a length-gauge calculation [S. G. Karshenboim and V. G. Ivanov, e-print physics/9702027], where the coefficient was given as -2.025(1).Comment: 9 pages, RevTe

Instanton Theory of Burgers Shocks and Intermittency

A lagrangian approach to Burgers turbulence is carried out along the lines of the field theoretical Martin-Siggia-Rose formalism of stochastic hydrodynamics. We derive, from an analysis based on the hypothesis of unbroken galilean invariance, the asymptotic form of the probability distribution function of negative velocity-differences. The origin of Burgers intermittency is found to rely on the dynamical coupling between shocks, identified to instantons, and non-coherent background fluctuations, which, then, cannot be discarded in a consistent statistical description of the flow.Comment: 7 pages; LaTe