427 research outputs found
Retarded Green's Functions In Perturbed Spacetimes For Cosmology and Gravitational Physics
Electromagnetic and gravitational radiation do not propagate solely on the
null cone in a generic curved spacetime. They develop "tails," traveling at all
speeds equal to and less than unity. If sizeable, this off-the-null-cone effect
could mean objects at cosmological distances, such as supernovae, appear dimmer
than they really are. Their light curves may be distorted relative to their
flat spacetime counterparts. These in turn could affect how we infer the
properties and evolution of the universe or the objects it contains. Within the
gravitational context, the tail effect induces a self-force that causes a
compact object orbiting a massive black hole to deviate from an otherwise
geodesic path. This needs to be taken into account when modeling the
gravitational waves expected from such sources. Motivated by these
considerations, we develop perturbation theory for solving the massless scalar,
photon and graviton retarded Green's functions in perturbed spacetimes,
assuming these Green's functions are known in the background spacetime. In
particular, we elaborate on the theory in perturbed Minkowski spacetime in
significant detail; and apply our techniques to compute the retarded Green's
functions in the weak field limit of the Kerr spacetime to first order in the
black hole's mass and angular momentum. Our methods build on and generalizes
work appearing in the literature on this topic to date, and lays the foundation
for a thorough, first principles based, investigation of how light propagates
over cosmological distances, within a spatially flat inhomogeneous
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) universe. This perturbative
scheme applied to the graviton Green's function, when pushed to higher orders,
may provide approximate analytic (or semi-analytic) results for the self-force
problem in the weak field limits of the Schwarzschild and Kerr black hole
geometries.Comment: 23 pages, 5 figures. Significant updates in v2: Scalar, photon and
graviton Green's functions calculated explicitly in Kerr black hole spacetime
up to first order in mass and angular momentum (Sec. V); Visser's van Vleck
determinant result shown to be equivalent to ours in Sec. II. v3: JWKB
discussion moved to introduction; to be published in PR
Brane Localization and Stabilization via Regional Physics
Extra-dimensional scenarios have become widespread among particle and
gravitational theories of physics to address several outstanding problems,
including cosmic acceleration, the weak hierarchy problem, and the quantization
of gravity. In general, the topology and geometry of the full spacetime
manifold will be non-trivial, even if our ordinary dimensions have the topology
of their covering space. Most compact manifolds are inhomogeneous, even if they
admit a homogeneous geometry, and it will be physically relevant where in the
extra-dimensions one is located. In this letter, we explore the use of both
local and global effects in a braneworld scenario to naturally provide
position-dependent forces that determine and stabilize the location of a single
brane. For illustrative purposes, we consider the 2-dimensional hyperbolic horn
and the Euclidean cone as toy models of the extra-dimensional manifold, and add
a brane wrapped around one of the two spatial dimensions. We calculate the
total energy due to brane tension and bending (extrinsic curvature) as well as
that due to the Casimir energy of a bulk scalar satisfying a Dirchlet boundary
condition on the brane. From the competition of at least two of these effects
there can exist a stable minimum of the effective potential for the brane
location. However, on more generic spaces (on which more symmetries are broken)
any one of these effects may be sufficient to stabilize the brane. We discuss
this as an example of physics that is neither local nor global, but regional.Comment: 4 pages, 2 figures. PRL submitte
It's Hard to Learn How Gravity and Electromagnetism Couple
We construct the most general effective Lagrangian coupling gravity and
electromagnetism up to mass dimension 6 by enumerating all possible non-minimal
coupling terms respecting both diffeomorphism and gauge invariance. In all,
there are only two unique terms after field re-definitions; one is known to
arise from loop effects in QED while the other is a parity violating term which
may be generated by weak interactions within the standard model of particle
physics. We show that neither the cosmological propagation of light nor,
contrary to earlier claims, solar system tests of General Relativity are useful
probes of these terms. These non-minimal couplings of gravity and
electromagnetism may remain a mystery for the foreseeable future.Comment: 9 pages. Minor corrections made. To appear in Phys. Rev.
Dynamical study of the hyperextended scalar-tensor theory in the empty Bianchi type I model
The dynamics of the hyperextended scalar-tensor theory in the empty Bianchi
type I model is investigated. We describe a method giving the sign of the first
and second derivatives of the metric functions whatever the coupling function.
Hence, we can predict if a theory gives birth to expanding, contracting,
bouncing or inflationary cosmology. The dynamics of a string inspired theory
without antisymetric field strength is analysed. Some exact solutions are
found.Comment: 18 pages, 3 figure
Information-preserving black holes still do not preserve baryon number and other effective global quantum numbers
It has been claimed recently that the black hole information-loss paradox has
been resolved: the evolution of quantum states in the presence of a black hole
is unitary and information preserving. We point out that, contrary to some
claims in literature, information-preserving black holes still violate baryon
number and any other quantum number which follows from an effective (and thus
approximate) or anomalous symmetry.Comment: Honorable Mention on Gravity Essay Competition 2005; Published in the
special Essay issue of Int.J.Mod.Phy
Results of the MRI substudy of the intravenous magnesium efficacy in stroke trial
<p><b>Background and Purpose:</b>Although magnesium is neuroprotective in animal stroke models, no clinical benefit was confirmed in the Intravenous Magnesium Efficacy in Stroke (IMAGES) trial of acute stroke patients. The Magnetic
Resonance in IMAGES (MR IMAGES) substudy investigated the effects of magnesium on the imaging surrogate
outcome of infarct growth.</p>
<p><b>Methods:</b> IMAGES trial patients in participating centers were randomized to receive either intravenous magnesium or placebo within 12 hours of stroke onset. Infarct growth was defined as volume difference between baseline diffusion-weighted imaging and day 90 fluid-attenuated inversion recovery image lesions. Patients who died were imputed the largest infarct growth observed.</p>
<p><b>Results:</b> Among the 90 patients included in the primary analysis, there was no difference in infarct growth (median absolute growth, P=0.639; median percentage growth, P=0.616; proportion with any growth, P=0.212) between the
46 treated with magnesium and 44 with placebo. Infarct growth correlated with NIHSS score change from baseline to
day 90. There was a trend showing baseline serum glucose correlated with infarct growth with magnesium treatment,
but not in the placebo group. The mismatch frequency was reduced from 73% to 47% by increasing the mismatch
threshold from >20% to >100% of core volume.</p>
<p><b>Conclusions:</b> Infarct growth, confirmed here as a surrogate for clinical progression, was similar between magnesium and placebo treatment, paralleling the main IMAGES trial clinical outcomes. Glucose was a covariate for infarct growth with magnesium treatment. A more stringent mismatch threshold to define penumbra more appropriately would have
excluded half of the patients in this 12-hour time window stroke study.</p>
SIMP (Strongly Interacting Massive Particle) Search
We consider laboratory experiments that can detect stable, neutral strongly
interacting massive particles (SIMPs). We explore the SIMP annihilation cross
section from its minimum value (restricted by cosmological bounds) to the barn
range, and vary the mass values from a GeV to a TeV. We also consider the
prospects and problems of detecting such particles at the Tevatron.Comment: Latex. 7 pages, 1 eps figure. Proceedings to the 4th UCLA Symposium
on Dark Matter DM2000, Marina del Rey, CA, USA, Feb. 23-25, 200
Should we doubt the cosmological constant?
While Bayesian model selection is a useful tool to discriminate between
competing cosmological models, it only gives a relative rather than an absolute
measure of how good a model is. Bayesian doubt introduces an unknown benchmark
model against which the known models are compared, thereby obtaining an
absolute measure of model performance in a Bayesian framework. We apply this
new methodology to the problem of the dark energy equation of state, comparing
an absolute upper bound on the Bayesian evidence for a presently unknown dark
energy model against a collection of known models including a flat LambdaCDM
scenario. We find a strong absolute upper bound to the Bayes factor B between
the unknown model and LambdaCDM, giving B < 3. The posterior probability for
doubt is found to be less than 6% (with a 1% prior doubt) while the probability
for LambdaCDM rises from an initial 25% to just over 50% in light of the data.
We conclude that LambdaCDM remains a sufficient phenomenological description of
currently available observations and that there is little statistical room for
model improvement.Comment: 10 pages, 2 figure
The Big Occulting Steerable Satellite (BOSS)
Natural (such as lunar) occultations have long been used to study sources on
small angular scales, while coronographs have been used to study high contrast
sources. We propose launching the Big Occulting Steerable Satellite (BOSS), a
large steerable occulting satellite to combine both of these techniques. BOSS
will have several advantages over standard occulting bodies. BOSS would block
all but about 4e-5 of the light at 1 micron in the region of interest around
the star for planet detections. Because the occultation occurs outside the
telescope, scattering inside the telescope does not degrade this performance.
BOSS could be combined with a space telescope at the Earth-Sun L2 point to
yield very long integration times, in excess of 3000 seconds. If placed in
Earth orbit, integration times of 160--1600 seconds can be achieved from most
major telescope sites for objects in over 90% of the sky. Applications for BOSS
include direct imaging of planets around nearby stars. Planets separated by as
little as 0.1--0.25 arcseconds from the star they orbit could be seen down to a
relative intensity as little as 1e-9 around a magnitude 8 (or brighter) star.
Other applications include ultra-high resolution imaging of compound sources,
such as microlensed stars and quasars, down to a resolution as little as 0.1
milliarcseconds.Comment: 25pages, 4 figures, uses aaspp4, rotate, and epsfig. Submitted to the
Astrophysical Journal. For more details see
http://erebus.phys.cwru.edu/~boss
Gravitational Lenses in Generalized Einstein-Aether theory: the Bullet Cluster
We study the lensing properties of an asymmetric mass distribution and vector
field in Generalized Einstein-Aether (GEA) theory. As vector field fluctuations
are responsible in GEA for seeding baryonic structure formation, vector field
concentrations can exist independently of baryonic matter. Such concentrations
would not be expected to be tied to baryonic matter except gravitationally, and
so, like dark matter halos, would become separated from baryonic matter in
interacting systems such as the Bullet Cluster. These vector field
concentrations cause metric deviations that affect weak lensing. Therefore, the
distribution of weak lensing deviates from that which would be inferred from
the luminous mass distribution, in a way that numerical calculations
demonstrate can be consistent with observations. This suggests that
MOND-inspired theories can reproduce weak lensing observations, but makes clear
the price: the existence of a coherent large-scale fluctuation of a field(s)
weakly tied to the baryonic matter, not completely dissimilar to a dark matter
halo.Comment: 7 pages, 10 figure
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