SA: A Background ‘Hum’ Pervades the Universe. Scientists Are Racing to Find Its Source

[Jim Leff]

Published by Scientific American on Friday

Astronomers are now seeking to pinpoint the origins of an exciting new form of gravitational waves that was announced earlier this year

In June, a new era in astronomy began with the apparent discovery of low-frequency gravitational waves, the ambient hum of spacetime ripples pervading the universe. That announcement came from a huge collaboration of researchers around the world. Groups in the U.S., Europe, India, Australia and China are each working on their own similar experiments and are pooling their data together to improve the result. With evidence for these never-before-seen gravitational waves now firmly in hand, all those disparate teams are now feverishly gathering more data for a grander goal: to understand exactly where this background hum is really coming from. Many experts suspect that the hum mostly emerges from pairs of supermassive black holes spiraling together in the gradual process of merging—but it could instead come from even stranger sources that might represent thrilling new branches of physics. “We’re right at the very beginning of the field,” says Chiara Mingarelli of Yale University, part of the U.S.-led collaboration, NANOGrav.


The announcement came on June 28 from NANOGrav and the other so-called pulsar timing arrays (PTAs), which use radio telescopes to track the precise arrival time of the regular flashes from pulsars, rapidly spinning neutron stars left behind after supernovae. Using dozens of pulsars and monitoring the arrival times of pulses to nanosecond-scale precision on decadal timescales, they can discern background gravitational waves passing through our solar system. Such waves slightly shrink or expand the intervening space between our planet and the targeted pulsars, creating telltale offsets in the arrival times of pulses. The astonishing result follows an earlier epoch of discovery that began in 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves produced by colliding stellar-mass black holes and neutron stars. LIGO, its European counterpart Virgo and similar facilities continue their hunt for these higher-frequency gravitational waves today.

[rstevenson]

I want to see video of the scientists racing.

Rob