11,892 research outputs found

### Finally, results from Gravity Probe-B

Nearly fifty years after its inception, the Gravity Probe B satellite mission
delivers the first measurements of how a spinning gyroscope precesses in the
gravitational warping of spacetime.Comment: A Viewpoint article, published in Physics 4, 43 (2011), available at
http://physics.aps.org/articles/v4/43 Submitted to the arXiv by permission of
the American Physical Societ

### Constraining Lorentz-violating, Modified Dispersion Relations with Gravitational Waves

Modified gravity theories generically predict a violation of Lorentz
invariance, which may lead to a modified dispersion relation for propagating
modes of gravitational waves. We construct a parametrized dispersion relation
that can reproduce a range of known Lorentz-violating predictions and
investigate their impact on the propagation of gravitational waves. A modified
dispersion relation forces different wavelengths of the gravitational wave
train to travel at slightly different velocities, leading to a modified phase
evolution observed at a gravitational-wave detector. We show how such
corrections map to the waveform observable and to the parametrized
post-Einsteinian framework, proposed to model a range of deviations from
General Relativity. Given a gravitational-wave detection, the lack of evidence
for such corrections could then be used to place a constraint on Lorentz
violation. The constraints we obtain are tightest for dispersion relations that
scale with small power of the graviton's momentum and deteriorate for a steeper
scaling.Comment: 11 pages, 3 figures, 2 tables: title changed slightly, published
versio

### Dynamic temperature selection for parallel-tempering in Markov chain Monte Carlo simulations

Modern problems in astronomical Bayesian inference require efficient methods
for sampling from complex, high-dimensional, often multi-modal probability
distributions. Most popular methods, such as Markov chain Monte Carlo sampling,
perform poorly on strongly multi-modal probability distributions, rarely
jumping between modes or settling on just one mode without finding others.
Parallel tempering addresses this problem by sampling simultaneously with
separate Markov chains from tempered versions of the target distribution with
reduced contrast levels. Gaps between modes can be traversed at higher
temperatures, while individual modes can be efficiently explored at lower
temperatures. In this paper, we investigate how one might choose the ladder of
temperatures to achieve more efficient sampling, as measured by the
autocorrelation time of the sampler. In particular, we present a simple,
easily-implemented algorithm for dynamically adapting the temperature
configuration of a sampler while sampling. This algorithm dynamically adjusts
the temperature spacing to achieve a uniform rate of exchanges between chains
at neighbouring temperatures. We compare the algorithm to conventional
geometric temperature configurations on a number of test distributions and on
an astrophysical inference problem, reporting efficiency gains by a factor of
1.2-2.5 over a well-chosen geometric temperature configuration and by a factor
of 1.5-5 over a poorly chosen configuration. On all of these problems a sampler
using the dynamical adaptations to achieve uniform acceptance ratios between
neighbouring chains outperforms one that does not.Comment: 21 pages, 21 figure

### Capture of non-relativistic particles in eccentric orbits by a Kerr black hole

We obtain approximate analytic expressions for the critical value of the
total angular momentum of a non-relativistic test particle moving in the Kerr
geometry, such that it will be captured by the black hole. The expressions
apply to arbitrary orbital inclinations, and are accurate over the entire range
of angular momentum for the Kerr black hole. The expressions can be easily
implemented in N-body simulations of the evolution of star clusters around
massive galactic black holes, where such captures play an important role.Comment: 8 pages, 1 figure, published versio

### Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. V. Evidence for the strong equivalence principle to second post-Newtonian order

Using post-Newtonian equations of motion for fluid bodies valid to the second
post-Newtonian order, we derive the equations of motion for binary systems with
finite-sized, non-spinning but arbitrarily shaped bodies. In particular we
study the contributions of the internal structure of the bodies (such as
self-gravity) that would diverge if the size of the bodies were to shrink to
zero. Using a set of virial relations accurate to the first post-Newtonian
order that reflect the stationarity of each body, and redefining the masses to
include 1PN and 2PN self-gravity terms, we demonstrate the complete
cancellation of a class of potentially divergent, structure-dependent terms
that scale as s^{-1} and s^{-5/2}, where s is the characteristic size of the
bodies. This is further evidence of the Strong Equivalence Principle, and
supports the use of post-Newtonian approximations to derive equations of motion
for strong-field bodies such as neutron stars and black holes. This extends
earlier work done by Kopeikin.Comment: 14 pages, submitted to Phys. Rev. D; small changes to coincide with
published versio

### Mutual benefit, added value? Doing research in the National Health Service

The National Health Service (NHS) has recently been the focus of government efforts to retain pharmaceutical research in the UK. Efforts to foster new partnerships between health care providers and industry have been framed with suggestions that clinical trials can offer patient benefit within the NHS, cutting across ethical and sociological concerns with the possible tension between doing research and offering care. This paper draws on ethnographic research to explore the sometimes awkward juxtapositions between trial protocols and everyday care, individual health and commercial profit, and thus the distribution of value produced through trials. While researchers appear to find the distinction between research and care useful, at least some of the time, both formal and informal strategies for living with this distinction may have the unintended consequence of making research appear supplementary to rather than simply different from clinical care

### Generation of Post-Newtonian Gravitational Radiation via Direct Integration of the Relaxed Einstein Equations

The completion of a network of advanced laser-interferometric
gravitational-wave observatories around 2001 will make possible the study of
the inspiral and coalescence of binary systems of compact objects (neutron
stars and black holes), using gravitational radiation. To extract useful
information from the waves, such as the masses and spins of the bodies,
theoretical general relativistic gravitational waveform templates of extremely
high accuracy will be needed for filtering the data, probably as accurate as
$O[(v/c)^6]$ beyond the predictions of the quadrupole formula. We summarize a
method, called DIRE, for Direct Integration of the Relaxed Einstein Equations,
which extends and improves an earlier framework due to Epstein and Wagoner, in
which Einstein's equations are recast as a flat spacetime wave equation with
source composed of matter confined to compact regions and gravitational
non-linearities extending to infinity. The new method is free of divergences or
undefined integrals, correctly predicts all gravitational wave ``tail'' effects
caused by backscatter of the outgoing radiation off the background curved
spacetime, and yields radiation that propagates asymptotically along true null
cones of the curved spacetime. The method also yields equations of motion
through $O[(v/c)^4]$, radiation-reaction terms at $O[(v/c)^5]$ and
$O[(v/c)^7]$, and gravitational waveforms and energy flux through $O[(v/c)^4]$,
in agreement with other approaches. We report on progress in evaluating the
$O[(v/c)^6]$ contributions.Comment: 11 pages, PTPTeX, submitted to Proceedings of the Yukawa-Kyoto
International Seminar 9

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