Abstract
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 parametric models. Such models typically assume the distributions of binary parameters to be superpositions of “building block” features like power laws, peaks, dips, and breaks. Although this approach has yielded great progress in the initial characterization of the compact binary population, the strong assumptions entailed often leave it 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 an increase in the merger rate beyond redshift . Finally, we find that the distribution of black hole spin magnitudes is unimodal and concentrated at small but nonzero values, and that spin orientations span a wide range of spin-orbit misalignment angles but are also moderately unlikely to be truly isotropic.
- Received 21 February 2023
- Revised 8 November 2023
- Accepted 3 January 2024
DOI:https://doi.org/10.1103/PhysRevX.14.021005
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Viewpoint
ViewpointExploring the Black Hole Population with an Open Mind
Published 8 April 2024
A new model describes the population of black hole binaries without assumptions on the shape of their distribution—a capability that could boost the discovery potential of gravitational-wave observations.
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Popular Summary
Gravitational-wave astronomy is marching toward a big data regime, with hundreds or thousands of new black hole and neutron star collisions expected to be observed in the coming years. As gravitational-wave catalogs grow, a prime goal is to establish the ensemble properties of compact binary mergers: the distributions of their masses, spins, and locations throughout the Universe. In this paper, we present a new technique for flexibly measuring binary black hole demographics—one with no strong assumptions about what is expected.
Traditional methods for studying compact binary populations rely on a building-block approach. The distributions of binary masses, spins, etc. are assumed to follow simple functional forms like power laws or peaks, whose parameters are then fitted to observed data. These simple forms can be stacked to form more complex distributions. This approach, however, carries a risk: Because the ensemble properties of compact binaries are unknown, we might inadvertently pick a model that is a poor fit to reality.
We instead rely on a statistical model called an autoregressive process. Under this approach, the probability distributions of black hole properties are assumed to be continuous functions but otherwise do not possess predetermined forms. Using this technique, we systematically survey the properties of binary black holes and assess which features and trends arise. Along the way, we identify new features in the binary black hole population, such as a more complex structure in the mass distribution and unexpected evolution in the merger rate at earlier points in the Universe’s history.
Our approach allows one to not only agnostically study the “known unknowns” of the black hole population but also reveal the “unknown unknowns”—unexpected and impactful features that may otherwise be missed by the standard building-block method.
