7,219 research outputs found
Decoding mode-mixing in black-hole merger ringdown
Optimal extraction of information from gravitational-wave observations of
binary black-hole coalescences requires detailed knowledge of the waveforms.
Current approaches for representing waveform information are based on
spin-weighted spherical harmonic decomposition. Higher-order harmonic modes
carrying a few percent of the total power output near merger can supply
information critical to determining intrinsic and extrinsic parameters of the
binary. One obstacle to constructing a full multi-mode template of merger
waveforms is the apparently complicated behavior of some of these modes;
instead of settling down to a simple quasinormal frequency with decaying
amplitude, some modes show periodic bumps characteristic of
mode-mixing. We analyze the strongest of these modes -- the anomalous
harmonic mode -- measured in a set of binary black-hole merger waveform
simulations, and show that to leading order, they are due to a mismatch between
the spherical harmonic basis used for extraction in 3D numerical relativity
simulations, and the spheroidal harmonics adapted to the perturbation theory of
Kerr black holes. Other causes of mode-mixing arising from gauge ambiguities
and physical properties of the quasinormal ringdown modes are also considered
and found to be small for the waveforms studied here.Comment: 15 pages, 10 figures, 2 tables; new version has improved Figs. 1-3,
consistent labelling of simulations between Tables I & II,
additional/corrected references, and extra hyphen
The Final Merger of Black-Hole Binaries
Recent breakthroughs in the field of numerical relativity have led to
dramatic progress in understanding the predictions of General Relativity for
the dynamical interactions of two black holes in the regime of very strong
gravitational fields. Such black-hole binaries are important astrophysical
systems and are a key target of current and developing gravitational-wave
detectors. The waveform signature of strong gravitational radiation emitted as
the black holes fall together and merge provides a clear observable record of
the process. After decades of slow progress, these mergers and the
gravitational-wave signals they generate can now be routinely calculated using
the methods of numerical relativity. We review recent advances in understanding
the predicted physics of events and the consequent radiation, and discuss some
of the impacts this new knowledge is having in various areas of astrophysics.Comment: 57 pages; 9 figures. Updated references & fixed typos. Published
version is at
http://www.annualreviews.org/doi/abs/10.1146/annurev.nucl.010909.08324
Black-hole binaries, gravitational waves, and numerical relativity
Understanding the predictions of general relativity for the dynamical
interactions of two black holes has been a long-standing unsolved problem in
theoretical physics. Black-hole mergers are monumental astrophysical events,
releasing tremendous amounts of energy in the form of gravitational radiation,
and are key sources for both ground- and space-based gravitational-wave
detectors. The black-hole merger dynamics and the resulting gravitational
waveforms can only be calculated through numerical simulations of Einstein's
equations of general relativity. For many years, numerical relativists
attempting to model these mergers encountered a host of problems, causing their
codes to crash after just a fraction of a binary orbit could be simulated.
Recently, however, a series of dramatic advances in numerical relativity has
allowed stable, robust black-hole merger simulations. This remarkable progress
in the rapidly maturing field of numerical relativity, and the new
understanding of black-hole binary dynamics that is emerging is chronicled.
Important applications of these fundamental physics results to astrophysics, to
gravitational-wave astronomy, and in other areas are also discussed.Comment: 54 pages, 42 figures. Some typos corrected & references updated.
Essentially final published versio
The experiences of women with polycystic ovary syndrome on a very low-calorie diet
The research was funded by an educational grant from LighterLife. Broom was the Medical Director for LighterLife at the time of the research. Johnson is the Head of Nutrition and Research at LighterLife. The authors report no other conflicts of interest in this work.Peer reviewedPublisher PD
Observing mergers of non-spinning black-hole binaries
Advances in the field of numerical relativity now make it possible to
calculate the final, most powerful merger phase of binary black-hole
coalescence for generic binaries. The state of the art has advanced well beyond
the equal-mass case into the unequal-mass and spinning regions of parameter
space. We present a study of the nonspinning portion of parameter space,
primarily using an analytic waveform model tuned to available numerical data,
with an emphasis on observational implications. We investigate the impact of
varied mass ratio on merger signal-to-noise ratios (SNRs) for several
detectors, and compare our results with expectations from the test-mass limit.
We note a striking similarity of the waveform phasing of the merger waveform
across the available mass ratios. Motivated by this, we calculate the match
between our 1:1 (equal mass) and 4:1 mass-ratio waveforms during the merger as
a function of location on the source sky, using a new formalism for the match
that accounts for higher harmonics. This is an indicator of the amount of
degeneracy in mass ratio for mergers of moderate-mass-ratio systems.Comment: 13 pages, 11 figures, submitted to Phys. Rev.
Prompt Electromagnetic Transients from Binary Black Hole Mergers
Binary black hole (BBH) mergers provide a prime source for current and future
interferometric GW observatories. Massive BBH mergers may often take place in
plasma-rich environments, leading to the exciting possibility of a concurrent
electromagnetic (EM) signal observable by traditional astronomical facilities.
However, many critical questions about the generation of such counterparts
remain unanswered. We explore mechanisms that may drive EM counterparts with
magnetohydrodynamic simulations treating a range of scenarios involving
equal-mass black-hole binaries immersed in an initially homogeneous fluid with
uniform, orbitally aligned magnetic fields. We find that the time development
of Poynting luminosity, which may drive jet-like emissions, is relatively
insensitive to aspects of the initial configuration. In particular, over a
significant range of initial values, the central magnetic field strength is
effectively regulated by the gas flow to yield a Poynting luminosity of
, with BBH mass
scaled to and ambient density . We also calculate the
direct plasma synchrotron emissions processed through geodesic ray-tracing.
Despite lensing effects and dynamics, we find the observed synchrotron flux
varies little leading up to merger.Comment: 22 pages, 21 figures; additional reference + clarifying text added to
match published versio
Exploring Russian Cyberspace: Digitally-Mediated Collective Action and the Networked Public Sphere
This paper summarizes the major findings of a three-year research project to investigate the Internet's impact on Russian politics, media and society. We employed multiple methods to study online activity: the mapping and study of the structure, communities and content of the blogosphere; an analogous mapping and study of Twitter; content analysis of different media sources using automated and human-based evaluation approaches; and a survey of bloggers; augmented by infrastructure mapping, interviews and background research. We find the emergence of a vibrant and diverse networked public sphere that constitutes an independent alternative to the more tightly controlled offline media and political space, as well as the growing use of digital platforms in social mobilization and civic action. Despite various indirect efforts to shape cyberspace into an environment that is friendlier towards the government, we find that the Russian Internet remains generally open and free, although the current degree of Internet freedom is in no way a prediction of the future of this contested space
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