796 research outputs found
Regularity of the stress-energy tensor for extremal Reissner-Nordstrom black holes
We calculate the expectation values of the stress-energy tensor for both a
massless minimally-coupled and dilaton-coupled 2D field propagating on an
extremal Reissner-Nordstrom black hole, showing its regularity on the horizon
in contrast with previous claims in the literature.Comment: 10 pages, 1 figure; Talk given at QG05, Cala Gonone (Italy),
September 200
Covariant Galileon
We consider the recently introduced "galileon" field in a dynamical
spacetime. When the galileon is assumed to be minimally coupled to the metric,
we underline that both field equations of the galileon and the metric involve
up to third-order derivatives. We show that a unique nonminimal coupling of the
galileon to curvature eliminates all higher derivatives in all field equations,
hence yielding second-order equations, without any extra propagating degree of
freedom. The resulting theory breaks the generalized "Galilean" invariance of
the original model.Comment: 10 pages, no figure, RevTeX4 format; v2 adds footnote 1, Ref. [12],
reformats the link in Ref. [14], and corrects very minor typo
Counting the degrees of freedom of generalized Galileons
We consider Galileon models on curved spacetime, as well as the counterterms
introduced to maintain the second-order nature of the field equations of these
models when both the metric and the scalar are made dynamical. Working in a
gauge invariant framework, we first show how all the third-order time
derivatives appearing in the field equations -- both metric and scalar -- of a
Galileon model or one defined by a given counterterm can be eliminated to leave
field equations which contain at most second-order time derivatives of the
metric and of the scalar. The same is shown to hold for arbitrary linear
combinations of such models, as well as their k-essence-like/Horndeski
generalizations. This supports the claim that the number of degrees of freedom
in these models is only 3, counting 2 for the graviton and 1 for the scalar. We
comment on the arguments given previously in support of this claim. We then
prove that this number of degrees of freedom is strictly less that 4 in one
particular such model by carrying out a full-fledged Hamiltonian analysis. In
contrast to previous results, our analyses do not assume any particular gauge
choice of restricted applicability.Comment: 27 pages, no figure; v2: short explanation added below Eq. (42),
improved Sec. II.B.
Sentencing Upon Revocation of Probation in Florida
The Supreme Court of Florida held that a trial court is free to impose any sentence upon revocation of probation which it might have originally imposed despite the fact that the trial court had originally imposed a lesser sentence. In so doing, the court overruled the overwhelming weight of authority exhibited by the lower appellate courts. The author suggests that the defendant\u27s constitutional protection against being twice placed in jeopardy for the same offense and his right to counsel may have been infringed upon in the process
Sentencing Upon Revocation of Probation in Florida
The Supreme Court of Florida held that a trial court is free to impose any sentence upon revocation of probation which it might have originally imposed despite the fact that the trial court had originally imposed a lesser sentence. In so doing, the court overruled the overwhelming weight of authority exhibited by the lower appellate courts. The author suggests that the defendant\u27s constitutional protection against being twice placed in jeopardy for the same offense and his right to counsel may have been infringed upon in the process
Hawking radiation from extremal and non-extremal black holes
The relationship between Hawking radiation emitted by non extremal and
extremal Reissner Nordstrom black holes is critically analyzed. A careful study
of a series of regular collapsing geometries reveals that the stress energy
tensor stays regular in the extremal limit and is smoothly connected to that of
non extremal black holes. The unexpected feature is that the late time
transients which played little role in the non extremal case are necessary to
preserve the well defined character of the flux in the extremal case. The known
singular behavior of the static energy density of extremal black holes is
recovered from our series by neglecting these transients, when performing what
turns out to be an illegitimate late time limit. Although our results are
derived in two dimensional settings, we explain why they should also apply to
higher dimensional black holes.Comment: 18 pages, late
Quantum effects and superquintessence in the new age of precision cosmology
Recent observations of Type Ia supernova at high redshifts establish that the
dark energy component of the universe has (a probably constant) ratio between
pressure and energy density . The
conventional quintessence models for dark energy are restricted to the range
, with the cosmological constant corresponding to .
Conformally coupled quintessence models are the simplest ones compatible with
the marginally allowed superaccelerated regime (). However, they are
known to be plagued with anisotropic singularities.
We argue here that the extension of the classical approach to the
semiclassical one, with the inclusion of quantum counterterms necessary to
ensure the renormalization, can eliminate the anisotropic singularities
preserving the isotropic behavior of conformally coupled superquintessence
models. Hence, besides of having other interesting properties, they are
consistent candidates to describe the superaccelerated phases of the universe
compatible with the present experimental data.Comment: 7 pages. Essay selected for "Honorable Mention" in the 2004 Awards
for Essays on Gravitation, Gravity Research Foundatio
Semiclassical zero-temperature corrections to Schwarzschild spacetime and holography
Motivated by the quest for black holes in AdS braneworlds, and in particular
by the holographic conjecture relating 5D classical bulk solutions with 4D
quantum corrected ones, we numerically solve the semiclassical Einstein
equations (backreaction equations) with matter fields in the (zero temperature)
Boulware vacuum state. In the absence of an exact analytical expression for
in four dimensions we work within the s-wave approximation. Our
results show that the quantum corrected solution is very similar to
Schwarzschild till very close to the horizon, but then a bouncing surface for
the radial function appears which prevents the formation of an event horizon.
We also analyze the behavior of the geometry beyond the bounce, where a
curvature singularity arises. In the dual theory, this indicates that the
corresponding 5D static classical braneworld solution is not a black hole but
rather a naked singularity.Comment: 26 pages, 4 figures; revised version (title changed, conclusions
shortened), published as Phys. Rev. D73, 104023 (2006
Light scalar field constraints from gravitational-wave observations of compact binaries
Scalar-tensor theories are among the simplest extensions of general
relativity. In theories with light scalars, deviations from Einstein's theory
of gravity are determined by the scalar mass m_s and by a Brans-Dicke-like
coupling parameter \omega_{BD}. We show that gravitational-wave observations of
nonspinning neutron star-black hole binary inspirals can be used to set lower
bounds on \omega_{BD} and upper bounds on the combination
m_s/\sqrt{\omega_{BD}}$. We estimate via a Fisher matrix analysis that
individual observations with signal-to-noise ratio \rho would yield
(m_s/\sqrt{\omega_{BD}})(\rho/10)<10^{-15}, 10^{-16} and 10^{-19} eV for
Advanced LIGO, ET and eLISA, respectively. A statistical combination of
multiple observations may further improve these bounds.Comment: 9 pages, 4 figures. Matches version accepted in Physical Review
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