602 research outputs found
A Parameter-Free Tour of the Binary Black Hole Population
The continued operation of the Advanced LIGO and Advanced Virgo
gravitational-wave detectors is enabling the first detailed measurements of the
mass, spin, and redshift distributions of the merging binary black hole
population. Our present knowledge of these distributions, however, is based
largely on strongly parameteric models; such models typically assume the
distributions of binary parameters to be superpositions of power laws, peaks,
dips, and breaks, and then measure the parameters governing these "building
block" features. Although this approach has yielded great progress in initial
characterization of the compact binary population, the strong assumptions
entailed leave it often unclear which physical conclusions are driven by
observation and which by the specific choice of model. In this paper, we
instead model the merger rate of binary black holes as an unknown
autoregressive process over the space of binary parameters, allowing us to
measure the distributions of binary black hole masses, redshifts, component
spins, and effective spins with near-complete agnosticism. We find the primary
mass spectrum of binary black holes to be doubly-peaked, with a fairly flat
continuum that steepens at high masses. We identify signs of unexpected
structure in the redshift distribution of binary black holes: a
uniform-in-comoving volume merger rate at low redshift followed by a rise in
the merger rate beyond redshift . Finally, we find that the
distribution of black hole spin magnitudes is unimodal and concentrated at
small but non-zero values, and that spin orientations span a wide range of
spin-orbit misalignment angles but are also unlikely to be truly isotropic.Comment: 24 pages, 14 figures; code can be found at
http://github.com/tcallister/autoregressive-bbh-inference and data can be
download from https://zenodo.org/record/761609
The metallicity dependence and evolutionary times of merging binary black holes: Combined constraints from individual gravitational-wave detections and the stochastic background
The advent of gravitational-wave astronomy is now allowing for the study of
compact binary merger demographics throughout the Universe. This information
can be leveraged as tools for understanding massive stars, their environments,
and their evolution. One active question is the nature of compact binary
formation: the environmental and chemical conditions required for black hole
birth and the time delays experienced by binaries before they merge.
Gravitational-wave events detected today, however, primarily occur at low or
moderate redshifts due to current interferometer sensitivity, therefore
limiting our ability to probe the high redshift behavior of these quantities.
In this work, we circumvent this limitation by using an additional source of
information: observational limits on the gravitational-wave background from
unresolved binaries in the distant Universe. Using current gravitational-wave
data from the first three observing runs of LIGO-Virgo-KAGRA, we combine
catalogs of directly detected binaries and limits on the stochastic background
to constrain the time-delay distribution and metallicity dependence of binary
black hole evolution. Looking to the future, we also explore how these
constraints will be improved at the Advanced LIGO A+ sensitivity. We conclude
that, although binary black hole formation cannot be strongly constrained with
today's data, the future detection (or a non-detection) of the
gravitational-wave background with Advanced LIGO A+ will carry strong
implications for the evolution of binary black holes
Polarization-based Tests of Gravity with the Stochastic Gravitational-Wave Background
The direct observation of gravitational waves with Advanced LIGO and Advanced
Virgo offers novel opportunities to test general relativity in strong-field,
highly dynamical regimes. One such opportunity is the measurement of
gravitational-wave polarizations. While general relativity predicts only two
tensor gravitational-wave polarizations, general metric theories of gravity
allow for up to four additional vector and scalar modes. The detection of these
alternative polarizations would represent a clear violation of general
relativity. The LIGO-Virgo detection of the binary black hole merger GW170814
has recently offered the first direct constraints on the polarization of
gravitational waves. The current generation of ground-based detectors, however,
is limited in its ability to sensitively determine the polarization content of
transient gravitational-wave signals. Observation of the stochastic
gravitational-wave background, in contrast, offers a means of directly
measuring generic gravitational-wave polarizations. The stochastic background,
arising from the superposition of many individually unresolvable
gravitational-wave signals, may be detectable by Advanced LIGO at
design-sensitivity. In this paper, we present a Bayesian method with which to
detect and characterize the polarization of the stochastic background. We
explore prospects for estimating parameters of the background, and quantify the
limits that Advanced LIGO can place on vector and scalar polarizations in the
absence of a detection. Finally, we investigate how the introduction of new
terrestrial detectors like Advanced Virgo aid in our ability to detect or
constrain alternative polarizations in the stochastic background. We find that,
although the addition of Advanced Virgo does not notably improve detection
prospects, it may dramatically improve our ability to estimate the parameters
of backgrounds of mixed polarization.Comment: 24 pages, 20 figures; Accepted by PRX. This version includes major
changes in response to referee comments and corrects an error in Eq. E
Who Ordered That? Unequal-Mass Binary Black Hole Mergers Have Larger Effective Spins
Hierarchical analysis of the binary black hole (BBH) detections by the
Advanced LIGO and Virgo detectors has offered an increasingly clear picture of
their mass, spin, and redshift distributions. Fully understanding the formation
and evolution of BBH mergers will require not just the characterization of
these marginal distributions, though, but the discovery of any correlations
that exist between the properties of BBHs. Here, we hierarchically analyze the
ensemble of BBHs discovered by the LIGO and Virgo with a model that allows for
intrinsic correlations between their mass ratios and effective inspiral
spins . At credibility, we find that the mean of
the distribution varies as a function of , such that
more unequal-mass BBHs exhibit systematically larger . We
find Bayesian odds ratio of in favor of a model that allows for such a
correlation over one that does not. Finally, we use simulated signals to verify
that our results are robust against degeneracies in the measurements of and
for individual events. While many proposed astrophysical
formation channels predict some degree correlation between spins and mass
ratio, these predicted correlations typically act in an opposite sense to the
trend we observationally identify in the data.Comment: Accepted in ApJL. New version includes edits made during productio
Implications for first-order cosmological phase transitions from the third LIGO-Virgo observing run
We place constrains on the normalised energy density in gravitational waves
from first-order strong phase transitions using data from Advanced LIGO and
Virgo's first, second and third observing runs. First, adopting a broken power
law model, we place confidence level upper limits simultaneously on the
gravitational-wave energy density at 25 Hz from unresolved compact binary
mergers, , and strong first-order phase
transitions, . We then consider two more
complex phenomenological models, limiting at 25 Hz the gravitational-wave
background due to bubble collisions to
and the background due to sound waves to
at confidence level for temperatures above GeV.Comment: 7 pages, 3 figure
A First Search for Prompt Radio Emission from a Gravitational-Wave Event
Multimessenger observations of the binary neutron star merger GW170817 have
enabled the discovery of a diverse array of electromagnetic counterparts to
compact binary mergers, including an unambiguous kilonova, a short gamma-ray
burst, and a late-time radio jet. Beyond these counterparts, compact binary
mergers are additionally predicted to be accompanied by prompt low-frequency
radio emission. The successful observation of a prompt radio counterpart would
be immensely valuable, but is made difficult by the short delay between the
gravitational-wave and prompt electromagnetic signals as well as the poor
localization of gravitational-wave sources. Here, we present the first search
for prompt radio emission accompanying a gravitational-wave event, targeting
the binary black hole merger GW170104 detected by the Advanced LIGO and Virgo
gravitational-wave observatories during their second (O2) observing run. Using
the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), we search
a region for transient radio emission within
approximately one hour of GW170104, obtaining an upper limit of
on its equivalent isotropic
luminosity between 27-84 MHz. We additionally discuss plans to target binary
neutron star mergers in Advanced LIGO and Virgo's upcoming O3 observing run.Comment: 13 pages + appendices, 7 figures, submitted to Ap
Proteome Analyses of Strains Cyanothece ATCC 51142 and PCC 7822 of the Diazotrophic Cyanobacterium Cyanothece sp. Under Culture Conditions Resulting in Enhanced H2 Production.
Cultures of the cyanobacterial genus Cyanothece have been shown to produce high levels of biohydrogen. These strains are diazotrophic and undergo pronounced diurnal cycles when grown under N2-fixing conditions in light-dark cycles. We seek to better understand the way in which proteins respond to these diurnal changes, and we performed quantitative proteome analysis of Cyanothecesp. strains ATCC 51142 and PCC 7822 grown under 8 different nutritional conditions. Nitrogenase expression was limited to N2-fixing conditions, and in the absence of glycerol, nitrogenase gene expression was linked to the dark period. However, glycerol induced expression of nitrogenase during part of the light period, together with cytochrome c oxidase (Cox), glycogen phosphorylase (Glp), and glycolytic and pentose phosphate pathway (PPP) enzymes. This indicated that nitrogenase expression in the light was facilitated via higher levels of respiration and glycogen breakdown. Key enzymes of the Calvin cycle were inhibited in Cyanothece ATCC 51142 in the presence of glycerol under H2-producing conditions, suggesting a competition between these sources of carbon. However, in Cyanothece PCC 7822, the Calvin cycle still played a role in cofactor recycling during H2 production. Our data comprise the first comprehensive profiling of proteome changes in Cyanothece PCC 7822 and allow an in-depth comparative analysis of major physiological and biochemical processes that influence H2 production in both strains. Our results revealed many previously uncharacterized proteins that may play a role in nitrogenase activity and in other metabolic pathways and may provide suitable targets for genetic manipulation that would lead to improvement of large-scale H2 production
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