ASTRONOMERS LISTEN IN ON THE COSMOS’ BACKGROUND HUM
GRAVITATIONAL WAVES are ripples in space-time that arise from extremely energetic events, such as the collisions of neutron stars or black holes. Since their first detection in 2016 by the Laser Interferometer Gravitational-wave Observatory (LIGO), gravitational waves have given us a new way to study the universe — and 2023 brought a fresh twist.
Scientists are limited to studying a narrow range of gravitational waves. That’s because their wavelength, or the distance between successive crests of each wave, is proportional to the masses of and the distance between the objects creating them. This means a pair of stars orbiting in a tight binary create shorter-wavelength gravitational waves than do merging supermassive black holes with millions or billions of times the mass of the Sun. In fact, supermassive black hole mergers can create gravitational waves with crests tens of light-years apart.
Detecting such long-wavelength gravitational waves is beyond current observatories like LIGO and Virgo, which only catch the high-pitched “chirps” of binary objects a few to about 100 times the Sun’s mass. These signals represent the last minutes or seconds of a merger, as the objects circle ever closer before slamming together, all the while releasing angular momentum as gravitational waves.
For supermassive black holes, this process plays out over a much greater span of time. When galaxies merge, their individual supermassive black holes sink to the center and eventually merge over some 100 million to 200 million years. During that time, other galaxies elsewhere in the universe will merge as well, and their black holes will begin their own hundredmillion-year inward spiral.
“If there’s a lot of these [longwavelength] gravitational-wave signals, they can add together and give you a gravitational-wave background,” said Yale University Assistant Professor of Physics Chiara Mingarelli in a video release. Mingarelli is part of the North American
Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration, which announced the first-ever detection of this background June 29 in several papers in The Astrophysical Journal Letters.
Without instruments tuned to long wavelengths, the NANOGrav collaboration looked to fast-rotating neutron stars called pulsars. As a pulsar spins, it shoots beams of radiation from its poles; every revolution, these beams sweep over Earth like light from a lighthouse. The beams’ arrival is incredibly regular, down to a fraction of a second, turning each pulsar into its own highly accurate cosmic clock.
NANOGrav monitored a network of 67 pulsars throughout the Milky Way for 15 years, looking for tiny shifts in the timing of the arrival of their beams. These occur when a gravitational wave passes by, subtly squeezing or stretching the spacetime between the pulsar and Earth, causing the signals to arrive slightly sooner or later than expected, respectively. “Like a huge ocean swell, the stars in our galaxy are all moving in concert to waves in
space-time that take more than a decade just to complete one cycle of the wave,” says Kelly Holley-Bockelmann of Vanderbilt University, a gravitational-wave researcher who is not part of NANOGrav.
That’s why NANOGrav had to monitor the pulsars for so long. And it was worth it. The resulting pattern of timing disruptions matches exactly what is expected if there is a background of gravitational waves humming throughout the cosmos. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe,” said NANOGrav collaborator Stephen Taylor, also of Vanderbilt, in a statement.
The detection has now clinched the case that supermassive black holes do merge — previously a long-standing question in astrophysics. It has also revealed surprises: The gravitational-wave background is twice as loud as expected. Perhaps supermassive black holes are larger or more numerous than current estimates. But perhaps something previously unimagined is contributing to the volume as well. “We’ll need to keep observing to reveal the true nature of these gravitational waves,” says Holley-Bockelmann.