335 research outputs found
Constraining the Solution to the Last Parsec Problem with Pulsar Timing
The detection of a stochastic gravitational-wave signal from the
superposition of many inspiraling supermassive black holes with pulsar timing
arrays (PTAs) is likely to occur within the next decade. With this detection
will come the opportunity to learn about the processes that drive
black-hole-binary systems toward merger through their effects on the
gravitational-wave spectrum. We use Bayesian methods to investigate the extent
to which effects other than gravitational-wave emission can be distinguished
using PTA observations. We show that, even in the absence of a detection, it is
possible to place interesting constraints on these dynamical effects for
conservative predictions of the population of tightly bound supermassive
black-hole binaries. For instance, if we assume a relatively weak signal
consistent with a low number of bound binaries and a low black-hole-mass to
galaxy-mass correlation, we still find that a non-detection by a simulated
array, with a sensitivity that should be reached in practice within a few
years, disfavors gravitational-wave-dominated evolution with an odds ratio of
30:1. Such a finding would suggest either that all existing astrophysical
models for the population of tightly bound binaries are overly optimistic, or
else that some dynamical effect other than gravitational-wave emission is
actually dominating binary evolution even at the relatively high
frequencies/small orbital separations probed by PTAs.Comment: 14 pages, 8 figure
Gravitational Wave Tests of General Relativity with the Parameterized Post-Einsteinian Framework
Gravitational wave astronomy has tremendous potential for studying extreme
astrophysical phenomena and exploring fundamental physics. The waves produced
by binary black hole mergers will provide a pristine environment in which to
study strong field, dynamical gravity. Extracting detailed information about
these systems requires accurate theoretical models of the gravitational wave
signals. If gravity is not described by General Relativity, analyses that are
based on waveforms derived from Einstein's field equations could result in
parameter biases and a loss of detection efficiency. A new class of
"parameterized post-Einsteinian" (ppE) waveforms has been proposed to cover
this eventuality. Here we apply the ppE approach to simulated data from a
network of advanced ground based interferometers (aLIGO/aVirgo) and from a
future spaced based interferometer (LISA). Bayesian inference and model
selection are used to investigate parameter biases, and to determine the level
at which departures from general relativity can be detected. We find that in
some cases the parameter biases from assuming the wrong theory can be severe.
We also find that gravitational wave observations will beat the existing bounds
on deviations from general relativity derived from the orbital decay of binary
pulsars by a large margin across a wide swath of parameter space.Comment: 16 pages, 10 figures. Modified in response to referee comment
'Walking the talk': How companies succeed in managing risk at sea
This paper draws upon research about risk and risk management conducted with the support of The Lloydâs Register Educational Trust1. The wider research was large in scale comprising the analysis of over 2,300 questionnaires and a range of data relating to maritime incidents. In addition the study incorporated a detailed analysis of five case study companies. It is this latter element of the research which forms the basis for this paper which considers the differences in perceptions between shore-based, and ship-based, staff working for ship operators in relation to risk management. The paper explores the means of communication utilised for the transmission of data and ideas about safety and risk management both to, and from, management. It then goes on to consider why it is that despite considerable efforts to write and to talk about safety and risk management, many companies are unsuccessful in encouraging their sea-staff to believe that safety is a genuine company priority and therefore to adhere closely to company policy in relation to safety management
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