Massive scalar field instability in Kerr spacetime

We study the Klein-Gordon equation for a massive scalar field in Kerr spacetime in the time-domain. We demonstrate that under conditions of super-radiance, the scalar field becomes unstable and its amplitude grows without bound. We also estimate the growth rate of this instability.Comment: 10 pages, 5 figure

Quaternion Gravi-Electromagnetism

Defining the generalized charge, potential, current and generalized fields as complex quantities where real and imaginary parts represent gravitation and electromagnetism respectively, corresponding field equation, equation of motion and other quantum equations are derived in manifestly covariant manner. It has been shown that the field equations are invariant under Lorentz as well as duality transformations. It has been shown that the quaternionic formulation presented here remains invariant under quaternion transformations.Comment: Key Words: Quaternion, dyons, gravito-dyons, gravi-electromagnetism. PACS No.: 04.90. +e ; 14.80. H

Renormalization Group Flow in Scalar-Tensor Theories. II

We study the UV behaviour of actions including integer powers of scalar curvature and even powers of scalar fields with Functional Renormalization Group techniques. We find UV fixed points where the gravitational couplings have non-trivial values while the matter ones are Gaussian. We prove several properties of the linearized flow at such a fixed point in arbitrary dimensions in the one-loop approximation and find recursive relations among the critical exponents. We illustrate these results in explicit calculations in $d=4$ for actions including up to four powers of scalar curvature and two powers of the scalar field. In this setting we notice that the same recursive properties among the critical exponents, which were proven at one-loop order, still hold, in such a way that the UV critical surface is found to be five dimensional. We then search for the same type of fixed point in a scalar theory with minimal coupling to gravity in $d=4$ including up to eight powers of scalar curvature. Assuming that the recursive properties of the critical exponents still hold, one would conclude that the UV critical surface of these theories is five dimensional.Comment: 14 pages. v.2: Minor changes, some references adde

Perturbative evolution of particle orbits around Kerr black holes: time domain calculation

Treating the Teukolsky perturbation equation numerically as a 2+1 PDE and smearing the singularities in the particle source term by the use of narrow Gaussian distributions, we have been able to reproduce earlier results for equatorial circular orbits that were computed using the frequency domain formalism. A time domain prescription for a more general evolution of nearly geodesic orbits under the effects of radiation reaction is presented. This approach can be useful when tackling the more realistic problem of a stellar-mass black hole moving on a generic orbit around a supermassive black hole under the influence of radiation reaction forces.Comment: 8 pages, 5 figures, problems with references and double-printing fixe

Perturbative evolution of conformally flat initial data for a single boosted black hole

The conformally flat families of initial data typically used in numerical relativity to represent boosted black holes are not those of a boosted slice of the Schwarzschild spacetime. If such data are used for each black hole in a collision, the emitted radiation will be partially due to the ``relaxation'' of the individual holes to ``boosted Schwarzschild'' form. We attempt to compute this radiation by treating the geometry for a single boosted conformally flat hole as a perturbation of a Schwarzschild black hole, which requires the use of second order perturbation theory. In this we attempt to mimic a previous calculation we did for the conformally flat initial data for spinning holes. We find that the boosted black hole case presents additional subtleties, and although one can evolve perturbatively and compute radiated energies, it is much less clear than in the spinning case how useful for the study of collisions are the radiation estimates for the ``spurious energy'' in each hole. In addition to this we draw some lessons on which frame of reference appears as more favorable for computing black hole collisions in the close limit approximation.Comment: 11 pages, RevTex, 4 figures included with psfig, to appear in PR

Effect of α7 nicotinic acetylcholine receptor activation on cardiac fibroblasts: A mechanism underlying RV fibrosis associated with cigarette smoke exposure

INTRODUCTION: Right ventricular dysfunction is associated with numerous smoking-related illnesses including chronic obstructive pulmonary disease (COPD) where it is present even in absence of pulmonary hypertension. It is unknown if exposure to cigarette smoke has direct effects on RV function and cardiac fibroblast proliferation or collagen synthesis. In this study, we evaluated cardiac function and fibrosis in mice exposed to cigarette smoke (CS) and determined mechanisms of smoke-induced changes in cardiac fibroblast signaling and fibrosis. METHODS: AKR mice were exposed to cigarette smoke for six weeks followed by echocardiography and evaluation of cardiac hypertrophy, collagen content, and pulmonary muscularization. Proliferation and collagen content were evaluated in primary isolated rat cardiac fibroblasts (CF) exposed to cigarette smoke extract (CSE) or nicotine. Markers of cell proliferation, fibrosis, and proliferative signaling were determined by immunoblot or Sircol collagen assay. RESULTS: Mice exposed to CS had significantly decreased RV function as determined by TAPSE. There were no changes in LV parameters. RV collagen content was significantly elevated but there was no change in RV hypertrophy or pulmonary vascular muscularization. CSE directly increased cardiac fibroblast proliferation and collagen content in CF. Nicotine alone reproduced these effects. CSE and nicotine-induced fibroblast proliferation and collagen content were mediated through α7 nicotinic acetylcholine receptors and were dependent on PKC-α, PKC-δ, and reduced p38-MAPK phosphorylation. CONCLUSION: CS and nicotine have direct effects on cardiac fibroblasts to induce proliferation and fibrosis which may negatively affect right heart function