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Placing bounds on gravitational wave detection
Gravitational waves are expected to be generated by the interaction of the massive bodies in black-hole binary systems. As gravitational waves distort spacetime, it should be possible to verify their existence as they interfere with the pulses emitted by millisecond pulsars. However, after monitoring 24 pulsars with the Parkes radio telescope for 12 years, Shannon et al. found no detectable variation in pulsar records. This nondetection result indicates that a new detection strategy for gravitational waves is needed.
Science, this issue p. 1522
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 would modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, Ac,yr, to be <1.0 × 10−15 with 95% confidence. This limit excludes predicted ranges for Ac,yr from current models with 91 to 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 be more sensitive to gravitational waves.