211 research outputs found
Quantum stress tensor for massive vector field in the space-time of a cylindrical black hole
The components of the renormalized quantum Energy-Momentum tensor for a
massive vector field coupled to the gravitational field configuration of a
static Black-String are analytically evaluated using the Schwinger-DeWitt
approximation. The general results are employed to investigate the pointwise
energy conditions for the quantized matter field, and it is shown that they are
violated at some regions of the spacetime, in particular the horizon of the
black hole.Comment: 4 pages, 3 figure
Is there a black hole minimum mass?
Applying the first and generalised second laws of thermodynamics for a
realistic process of near critical black hole formation, we derive an entropy
bound, which is identical to Bekenstein's one for radiation. Relying upon this
bound, we derive an absolute minimum mass ,
where and is the effective degrees of freedom for the
initial temparature and the Planck mass, respectively. Since this minimum mass
coincides with the lower bound on masses of which black holes can be regarded
as classical against the Hawking evaporation, the thermodynamical argument will
not prohibit the formation of the smallest classical black hole. For more
general situations, we derive a minimum mass, which may depend on the initial
value for entropy per particle. For primordial black holes, however, we show
that this minimum mass can not be much greater than the Planck mass at any
formation epoch of the Universe, as long as is within a reasonable
range. We also derive a size-independent upper bound on the entropy density of
a stiff fluid in terms of the energy density.Comment: 4 pages, accepted for publication in Physical Review D, minor
correctio
Dark matter from dark energy-baryonic matter couplings
We present a scenario in which a scalar field dark energy is coupled to the
trace of the energy momentum tensor of the baryonic matter fields. In the
slow-roll regime, this interaction could give rise to the cosmological features
of dark matter. We work out the cosmological background solutions and fit the
parameters of the model using the Union 2 supernovae data set. Then, we develop
the cosmological perturbations up to linear order, and we find that the
perturbed variables have an acceptable behavior, in particular the density
contrast of baryonic matter grows similar to that in the CDM model for
a suitable choice of the strength parameter of the coupling.Comment: 10 pages, 8 figures, in this version small typos are corrected and it
matches the published version in Phys. Rev. D15, January 201
Born-Infeld type Gravity
Generalizations of gravitational Born-Infeld type lagrangians are
investigated. Phenomenological constraints (reduction to Einstein-Hilbert
action for small curvature, spin two ghost freedom and absence of Coulomb like
Schwarschild singularity) select one effective lagrangian whose dynamics is
dictated by the tensors g_{\mu\nu} and R_{\mu\nu\rho\sigma}(not R_{\mu\nu} or
the scalar R).Comment: 7 pages, 3 figures, revte
Regularization of fluctuations near the sonic horizon due to the quantum potential and its influence on the Hawking radiation
We consider dynamics of fluctuations in transonically accelerating
Bose-Einstein condensates and luminous liquids (coherent light propagating in a
Kerr nonlinear medium) using the hydrodynamic approach. It is known that
neglecting the quantum potential (QP) leads to a singular behavior of quantum
and classical fluctuations in the vicinity of the Mach (sonic) horizon, which
in turn gives rise to the Hawking radiation. The neglect of QP is well founded
at not too small distances from the horizon, where is the
healing length. Taking the QP into account we show that a second characteristic
length exists, such that the linear fluctuation modes become
regularized for . At the modes keep their singular
behavior, which however is influenced by the QP. As a result we find a
deviation of the high frequency tail of the spectrum of Hawking radiation from
Planck's black body radiation distribution. Similar results hold for the wave
propagation in Kerr nonlinear media where the length and exist due
to the nonlinearity.Comment: 23 pages, 2 figure
The depletion in Bose Einstein condensates using Quantum Field Theory in curved space
Using methods developed in Quantum Field Theory in curved space we can
estimate the effects of the inhomogeneities and of a non vanishing velocity on
the depletion of a Bose Einstein condensate within the hydrodynamical
approximation.Comment: 4 pages, no figure. Discussion extended and references adde
Bound states due to an accelerated mirror
We discuss an effect of accelerated mirrors which remained hitherto
unnoticed, the formation of a field condensate near its surface for massive
fields. From the view point of an observer attached to the mirror, this is
effect is rather natural because a gravitational field is felt there. The
novelty here is that since the effect is not observer dependent even inertial
observers will detect the formation of this condensate. We further show that
this localization is in agreement with Bekenstein's entropy bound.Comment: Final version to appear in PR
Palatini Black Holes in Nonlinear Electrodynamics
The electrically charged Born-Infeld black holes in the Palatini formalism
for theories are analyzed. Specifically we study those supported by a
theory , where is Planck's curvature. These black
holes only differ from their General Relativity counterparts very close to the
center, but may give rise to different geometrical structures in terms of inner
horizons. The nature and strength of the central singularities are also
significantly affected. In particular, for the model the
singularity is shifted to a finite radius, , and the Kretschmann scalar
diverges only as .Comment: 14 pages, 9 figures, revtex4-1 clas
On the quantum stress tensor for extreme 2D Reissner-Nordstrom black holes
Contrary to previous claims, it is shown that the expectation values of the
quantum stress tensor for a massless scalar field propagating on a
two-dimensional extreme Reissner-Nordstrom black hole are indeed regular on the
horizon.Comment: 5 pages, revtex, 1 figur
Metric-scalar gravity with torsion and the measurability of the non-minimal coupling
The "measurability" of the non-minimal coupling is discussed by considering
the correction to the Newtonian static potential in the semi-classical
approach. The coefficient of the "gravitational Darwin term" (GDT) gets
redefined by the non-minimal torsion-scalar couplings. Based on a similar
analysis of the GDT in the effective field theory approach to non-minimal
scalar we conclude that for reasonable values of the couplings the correction
is very small.Comment: 10 pages, LaTex. Accepted for publication in Mod. Phys. Lett.
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