314 research outputs found
Scattering of massless bosonic fields by Kerr black holes: On-axis incidence
We study the scattering of monochromatic bosonic plane waves impinging upon a rotating black hole, in the special case that the direction of incidence is aligned with the spin axis. We present accurate numerical results for electromagnetic Kerr scattering cross sections for the first time, and give a unified picture of the Kerr scattering for all massless bosonic fields
Fermion scattering by a Schwarzschild black hole
We study the scattering of massive spin-half waves by a Schwarzschild black
hole using analytical and numerical methods. We begin by extending a recent
perturbation theory calculation to next order to obtain Born series for the
differential cross section and Mott polarization, valid at small couplings. We
continue by deriving an approximation for glory scattering of massive spinor
particles by considering classical timelike geodesics and spin precession.
Next, we formulate the Dirac equation on a black hole background, and outline a
simple numerical method for finding partial wave series solutions. Finally, we
present our numerical calculations of absorption and scattering cross sections
and polarization, and compare with theoretical expectations.Comment: Minor changes, 1 figure added. Version to appear in Phys. Rev. D. 36
pages, 13 figure
Rainbow scattering in the gravitational field of a compact object
We study the elastic scattering of a planar wave in the curved spacetime of a compact object such as a
neutron star, via a heuristic model: a scalar field impinging upon a spherically symmetric uniform density
star of radius R and mass M. For R<rc, there is a divergence in the deflection function at the light-ring
radius rc ¼ 3GM=c2, which leads to spiral scattering (orbiting) and a backward glory; whereas for R>rc,
there instead arises a stationary point in the deflection function which creates a caustic and rainbow
scattering. As in nuclear rainbow scattering, there is an Airy-type oscillation on a Rutherford-like cross
section, followed by a shadow zone. We show that, for R ∼ 3.5GM=c2, the rainbow angle lies close to 180°,
and thus there arises enhanced backscattering and glory. We explore possible implications for gravitational
wave astronomy and dark matter models
Regularization of the Teukolsky Equation for Rotating Black Holes
We show that the radial Teukolsky equation (in the frequency domain) with
sources that extend to infinity has well-behaved solutions. To prove that, we
follow Poisson approach to regularize the non-rotating hole, and extend it to
the rotating case. To do so we use the Chandrasekhar transformation among the
Teukolsky and Regge-Wheeler-like equations, and express the integrals over the
source in terms of solutions to the homogeneous Regge-Wheeler-like equation, to
finally regularize the resulting integral. We then discuss the applicability of
these results.Comment: 14 pages, 1 Table, REVTE
Inferring black hole charge from backscattered electromagnetic radiation
We compute the scattering cross section of Reissner-Nordström black holes for the case of an incident electromagnetic wave. We describe how scattering is affected by both the conversion of electromagnetic to gravitational radiation, and the parity dependence of phase shifts induced by the black hole charge. The latter effect creates a helicity-reversed scattering amplitude that is nonzero in the backward direction. We show that from the character of the electromagnetic wave scattered in the backward direction it is possible, in principle, to infer if a static black hole is charged
Electromagnetic wave scattering by Schwarzschild black holes
We analyze the scattering of a planar monochromatic electromagnetic wave
incident upon a Schwarzschild black hole. We obtain accurate numerical results
from the partial wave method for the electromagnetic scattering cross section,
and show that they are in excellent agreement with analytical approximations.
The scattering of electromagnetic waves is compared with the scattering of
scalar, spinor and gravitational waves. We present a unified picture of the
scattering of all massless fields for the first time.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Planckian Energy Scattering and Surface Terms in the Gravitational Action
This is a revised version of our previous paper by the same name and preprint
number. It contains various changes, two figures and new results in sect.5. We
propose a new approach to four-dimensional Planckian-energy scattering in which
the phase of the -matrix is written---to leading order in and
to all orders in ---in terms of the surface term of the gravity
action and of a boundary term for the colliding quanta. The proposal is checked
at the leading order in and also against some known examples of
scattering in strong gravitational fields.Comment: preprint CERN-TH.6904/93/rev (Latex file, 46 pages, 2 figures not
included
Spacetime Splitting, Admissible Coordinates and Causality
To confront relativity theory with observation, it is necessary to split
spacetime into its temporal and spatial components. The (1+3) timelike
threading approach involves restrictions on the gravitational potentials
, while the (3+1) spacelike slicing approach involves
restrictions on . These latter coordinate conditions protect
chronology within any such coordinate patch. While the threading coordinate
conditions can be naturally integrated into the structure of Lorentzian
geometry and constitute the standard coordinate conditions in general
relativity, this circumstance does not extend to the slicing coordinate
conditions. We explore the influence of chronology violation on wave motion. In
particular, we consider the propagation of radiation parallel to the rotation
axis of stationary G\"odel-type universes characterized by parameters and such that for ) chronology is
protected (violated). We show that in the WKB approximation such waves can
freely propagate only when chronology is protected.Comment: 25 pages, 3 figures; v2: minor typos corrected, accepted for
publication in Phys. Rev.
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Pre-vaccination prevalence of anogenital and oral human papillomavirus in young HIV-infected men who have sex with men.
The aims of this study were to: 1) determine prevalence of anogenital and oral HPV, 2) determine concordance between HPV at anal, perianal, scrotal/penile, and oral sites; and 3) describe factors associated with anogenital HPV types targeted by the 9-valent vaccine. Data were collected from 2012 to 2015 among men who have sex with men 18-26 years of age enrolled in a vaccine trial (N = 145). Penile/scrotal, perianal, anal, and oral samples were tested for 61 HPV types. Logistic regression was used to identify factors associated with types in the 9-valent vaccine. Participants' mean age was 23.0 years, 55.2% were African-American, and 26.2% were Hispanic; 93% had anal, 40% penile, and 6% oral HPV. Among those with anogenital infection, 18% had HPV16. Concordance was low between anogenital and oral sites. Factors independently associated with a 9-valent vaccine-type HPV were: race (African-American vs. White, OR=2.67, 95% CI=1.11-6.42), current smoking (yes vs. no, OR=2.37, 95% CI=1.03-5.48), and number of recent receptive anal sex partners (2+ vs. 0, OR=3.47, 95% CI=1.16-10.4). Most MSM were not infected with HPV16 or HPV18, suggesting that they may still benefit from HPV vaccination, but anogenital HPV was very common, highlighting the importance of vaccinating men before sexual initiation. CLINICAL TRIAL NUMBER: NCT01209325
Scattering of massless scalar waves by Reissner-Nordstr\"om black holes
We present a study of scattering of massless planar scalar waves by a charged
non-rotating black hole. Partial wave methods are applied to compute scattering
and absorption cross sections, for a range of incident wavelengths. We compare
our numerical results with semi-classical approximations from a geodesic
analysis, and find excellent agreement. The glory in the backward direction is
studied, and its properties are shown to be related to the properties of the
photon orbit. The effects of black hole charge upon scattering and absorption
are examined in detail. As the charge of the black hole is increased, we find
that the absorption cross section decreases, and the angular width of the
interference fringes of the scattering cross section at large angles increases.
In particular, the glory spot in the backward direction becomes wider. We
interpret these effects under the light of our geodesic analysis.Comment: 9 pages, 10 figure
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