731 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
Arbitrary p-form Galileons
We show that scalar, 0-form, Galileon actions --models whose field equations
contain only second derivatives-- can be generalized to arbitrary even p-forms.
More generally, they need not even depend on a single form, but may involve
mixed p combinations, including equal p multiplets, where odd p-fields are also
permitted: We construct, for given dimension D, general actions depending on
scalars, vectors and higher p-form field strengths, whose field equations are
of exactly second derivative order. We also discuss and illustrate their
curved-space generalizations, especially the delicate non-minimal couplings
required to maintain this order. Concrete examples of pure and mixed actions,
field equations and their curved space extensions are presented.Comment: 8 pages, no figure, RevTeX4 format, v2: minor editorial changes
reflecting the published version in PRD Rapid Communication
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
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
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
COMPASS: a 2.6m telescope for CMBR polarization studies
COMPASS (COsmic Microwave Polarization at Small Scale) is an experiment devoted to measuring the polarization of the CMBR. Its design and characteristics are presented
Static quantum corrections to the Schwarzschild spacetime
We study static quantum corrections of the Schwarzschild metric in the
Boulware vacuum state. Due to the absence of a complete analytic expression for
the full semiclassical Einstein equations we approach the problem by
considering the s-wave approximation and solve numerically the associated
backreaction equations. The solution, including quantum effects due to pure
vacuum polarization, is similar to the classical Schwarzschild solution up to
the vicinity of the classical horizon. However, the radial function has a
minimum at a time-like surface close to the location of the classical event
horizon. There the g_{00} component of the metric reaches a very small but
non-zero value. The analysis unravels how a curvature singularity emerges
beyond this bouncing point. We briefly discuss the physical consequences of
these results by extrapolating them to a dynamical collapsing scenario.Comment: 10 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Generalized Gravity and a Ghost
We show that generalized gravity theories involving the curvature invariants
of the Ricci tensor and the Riemann tensor as well as the Ricci scalar are
equivalent to multi- scalar-tensor gravities with four derivatives terms. By
expanding the action around a vacuum spacetime, the action is reduced to that
of the Einstein gravity with four derivative terms, and consequently there
appears a massive spin-2 ghost in such generalized gravity theories in addition
to a massive spin-0 field.Comment: 8 pages, a reference adde
- âŠ