937 research outputs found
Electromagnetic waves around dilatonic stars and naked singularities
We study the propagation of classical electromagnetic waves on the simplest
four-dimensional spherically symmetric metric with a dilaton background field.
Solutions to the relevant equations are obtained perturbatively in a parameter
which measures the strength of the dilaton field (hence parameterizes the
departure from Schwarzschild geometry). The loss of energy from outgoing modes
is estimated as a back-scattering process against the dilaton background, which
would affect the luminosity of stars with a dilaton field. The radiation
emitted by a freely falling point-like source on such a background is also
studied by analytical and numerical methods.Comment: 9 pages, 1 figur
CMB acoustic scale in the entropic-like accelerating universe
We consider generalizations of the entropic accelerating universe recently
proposed in Ref. [4,5] and show that their background equations can be made
equivalent to a model with a dark energy component with constant parameter of
state , where is related to the
coefficients of the new terms in the Friedman equations. After discussing all
the Friedman equations for an arbitrary , we show how to recover the
standard scalings for dust and radiation. The acoustic scale , related
to the peak positions in the pattern of the angular power spectrum of the
Cosmic Microwave Background anisotropies, is also computed and yields the
stringent bound . We then argue that future data might be able
to distinguish this model from pure CDM (corresponding to ).Comment: 6 pages, 2 figures. Accepted for publication in Physical Review
Hawking radiation and the Bloom-Gilman duality
The decay widths of the quantum black hole precursors, determined from the
poles of the resummed graviton propagator, are matched to the expected lifetime
given by the Hawking decay. In this way, we impose a sort of duality between a
perturbative description and an essentially non-perturbative description,
bearing some similarity with the Bloom-Gilman duality for the strong
interactions. General relations are then obtained for the widths and masses of
the poles in terms of the number of particle species and the renormalisation
scale of gravity.Comment: 7 pages, 3 figure
Perturbations in the Kerr-Newman Dilatonic Black Hole Background: Maxwell Waves, the Dilaton Background and Gravitational Lensing
In this paper we continue the analysis of our previous papers and study the
affect of the existence of a non-trivial dilaton background on the propagation
of electromagnetic waves in the Kerr-Newman dilatonic black hole space-time.
For this purpose we again employ the double expansion in both the background
electric charge and the wave parameters of the relevant quantities in the
Newman-Penrose formalism and then identify the first order at which the dilaton
background enters the Maxwell equations. We then assume that gravitational and
dilatonic waves are negligible (at that order in the charge parameter) with
respect to electromagnetic waves and argue that this condition is consistent
with the solutions already found in the previous paper. Explicit expressions
are given for the asymptotic behavior of scattered waves, and a simple physical
model is proposed in order to test the effects. An expression for the relative
intensity is obtained for Reissner-Nordstrom dilaton black holes using
geometrical optics. A comparison with the approximation of geometrical optics
for Kerr-Newman dilaton black holes shows that at the order to which the
calculations are carried out gravitational lensing of optical images cannot
probe the dilaton background.Comment: 9 pages, 1 figur
On boundary terms and conformal transformations in curved space-times
We intend to clarify the interplay between boundary terms and conformal
transformations in scalar-tensor theories of gravity. We first consider the
action for pure gravity in five dimensions and show that, on compactifing a la
Kaluza-Klein to four dimensions, one obtains the correct boundary terms in the
Jordan (or String) Frame form of the Brans-Dicke action. Further, we analyze
how the boundary terms change under the conformal transformations which lead to
the Pauli (or Einstein) frame and to the non-minimally coupled massless scalar
field. In particular, we study the behaviour of the total energy in
asymptotically flat space-times as it results from surface terms in the
Hamiltonian formalism.Comment: LaTeX 2e, 12 pages, no figure
A recollimation shock 80 mas from the core in the jet of the radio galaxy 3C120: Observational evidence and modeling
We present Very Long Baseline Array observations of the radio galaxy 3C120 at
5, 8, 12, and 15 GHz designed to study a peculiar stationary jet feature
(hereafter C80) located ~80 mas from the core, which was previously shown to
display a brightness temperature ~600 times lager than expected at such
distances. The high sensitivity of the images -- obtained between December 2009
and June 2010 -- has revealed that C80 corresponds to the eastern flux density
peak of an arc of emission (hereafter A80), downstream of which extends a large
(~20 mas in size) bubble-like structure that resembles an inverted bow shock.
The linearly polarized emission closely follows that of the total intensity in
A80, with the electric vector position angle distributed nearly perpendicular
to the arc-shaped structure. Despite the stationary nature of C80/A80,
superluminal components with speeds up to ~3 c have been detected downstream
from its position, resembling the behavior observed in the HST-1 emission
complex in M87. The total and polarized emission of the C80/A80 structure, its
lack of motion, and brightness temperature excess are best reproduced by a
model based on synchrotron emission from a conical shock with cone opening
angle \eta=10 degrees, jet viewing angle \theta=16 degrees, a completely
tangled upstream magnetic field, and upstream Lorentz factor \gamma=8.4. The
good agreement between our observations and numerical modeling leads us to
conclude that the peculiar feature associated with C80/A80 corresponds to a
conical recollimation shock in the jet of 3C120 located at a de-projected
distance of ~190 pc downstream from the nucleus.Comment: Accepted for publication in Ap
Brane-world black holes and the scale of gravity
A particle in four dimensions should behave like a classical black hole if
the horizon radius is larger than the Compton wavelength or, equivalently, if
its degeneracy (measured by entropy in units of the Planck scale) is large. For
spherically symmetric black holes in 4 + d dimensions, both arguments again
lead to a mass threshold MC and degeneracy scale Mdeg of the order of the
fundamental scale of gravity MG. In the brane-world, deviations from the
Schwarzschild metric induced by bulk effects alter the horizon radius and
effective four-dimensional Euclidean action in such a way that MC \simeq Mdeg
might be either larger or smaller than MG. This opens up the possibility that
black holes exist with a mass smaller than MG and might be produced at the LHC
even if M>10 TeV, whereas effects due to bulk graviton exchanges remain
undetectable because suppressed by inverse powers of MG. Conversely, even if
black holes are not found at the LHC, it is still possible that MC>MG and MG
\simeq 1TeV.Comment: 4 pages, no figur
bilateral robot therapy based on haptics and reinforcement learning feasibility study of a new concept for treatment of patients after stroke
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Superradiance by mini black holes with mirror
The superradiant scattering of massive scalar particles by a rotating mini
black hole is investigated. Imposing the mirror boundary condition, the system
becomes the so called black-hole bomb where the rotation energy of the black
hole is transferred to the scattered particle exponentially with time. Bulk
emissions as well as brane emissions are considered altogether. It is found
that the largest effects are expected for the brane emission of lower angular
modes with lighter mass and larger angular momentum of the black hole.
Possibilities of the forming the black-hole bomb at the LHC are discussed.Comment: 20 pages, 2 figures, 7 tables. More discussions. To appear in JHE
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