Gravitational waves from binary supermassive black holes missing in pulsar observations

TL;DR

This paper uses pulsar timing observations to set upper limits on the gravitational wave background from supermassive black hole binaries, suggesting these binaries evolve differently than current models predict.

Contribution

It provides the most stringent observational constraints to date on the gravitational wave background from supermassive black hole binaries, challenging existing models of their evolution.

Findings

Upper limit on gravitational wave background amplitude: < 1.0×10⁻¹⁵

Current models are inconsistent with observations at 91-99.7% probability

Binary evolution may be stalled or accelerated by galactic environments

Abstract

Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain the characteristic amplitude of this background, $A_{\rm c,yr}$, to be < $1.0\times10^{-15}$ with 95% confidence. This limit excludes predicted ranges for $A_{\rm c,yr}$ from current models with 91-99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments, and that higher-cadence and shorter-wavelength observations would result in an increased sensitivity to gravitational waves.