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 $z\approx 0.5$. 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 $q$ and effective inspiral spins $\chi_\mathrm{eff}$. At $98.7\%$ credibility, we find that the mean of the $\chi_\mathrm{eff}$ distribution varies as a function of $q$, such that more unequal-mass BBHs exhibit systematically larger $\chi_\mathrm{eff}$. We find Bayesian odds ratio of $10.5$ 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 $q$ and $\chi_\mathrm{eff}$ 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 $95 \%$ confidence level upper limits simultaneously on the gravitational-wave energy density at 25 Hz from unresolved compact binary mergers, $\Omega_{\rm cbc} < 5.9 \times 10^{-9}$, and strong first-order phase transitions, $\Omega_{\rm bpl} < 2.8 \times 10^{-9}$. We then consider two more complex phenomenological models, limiting at 25 Hz the gravitational-wave background due to bubble collisions to $\Omega_{\rm pt} < 5.0\times 10^{-9}$ and the background due to sound waves to $\Omega_{\rm pt} < 5.8\times10^{-9}$ at $95 \%$ confidence level for temperatures above $10^8$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 $\sim900\,\mathrm{deg}^2$ region for transient radio emission within approximately one hour of GW170104, obtaining an upper limit of $2.5\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$ 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