The first-ever direct detection in 2015 of the ripples in spacetime known as gravitational waves revealed an extraordinary hidden realm to astrophysicists. Ever since, scientists have been able to study mergers of black holes—cataclysmic, yet clandestine events which produce no other detectable forms of radiation—and other extreme cosmic phenomena, such as black hole and neutron star smashups.
But these gravitational waves are just one of the several kinds that theorists think nature can cook up. Researchers have accordingly remained on the hunt for evidence of gravitational waves with frequency ranges that cannot be detected by LIGO and other existing gravitational wave observatories around the world.
A group of researchers at the Kavli Institute for Astronomy and Astrophysics at Peking University is particularly interested in ultra-low frequency gravitational waves. These undulations are theoretically produced by supermassive black holes—monsters which pack in millions of times more mass than the black holes spied through gravitational wave readings thus far. The tangoing of pairs of these colossal objects at the centers of galaxies is expected to flood the universe with low-frequency gravitational waves. The overlapping waves then hypothetically create a background, jumbled murmur—sort of like the hubbub of voices in a crowded room—that comes to Earth from all over the sky.
Other phenomena might be adding to this so-called gravitational wave background, including exotic objects called cosmic string, speculated to have formed very early in the universe's existence as the Big Bang unfolded. A cosmic string is equivalent to a defect formed when a material undergoes a phase transition, like when water freezes into crystalline ice. The early universe is thought to have undergone its own phase transitions as it rapidly cooled and expanded. Cosmic strings formed by this process would dissipate their energy as low-frequency gravitational waves, adding to the background clamor.
All in all, then, there are good scientific reasons to bring the frequency range of gravitational waves we can capture down, down, down.
"Ultra-low frequency gravitational waves open another window of the gravitational wave spectrum and will allow us to look at the universe in a completely different way," says Siyuan Chen, a fellow at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University.
"A detection will give us new insights in how supermassive black holes evolve and how they can influence their host galaxies," Chen explains. "Other possible sources are more exotic and could have formed and emitted gravitational waves just a bit after the Big Bang. The gravitational waves emitted could still be detected today, and if they were, we would have a way to look earlier into the universe's origin than with electromagnetic photons", or light.
Chen is the corresponding author on a new paper that is reporting significant progress on the quickening search for ultra-low frequency gravitational waves. Chen is a member of both the European Pulsar Timing Array (EPTA) and the North American Nanohertz Observatory for Gravitational Waves (NANOGrav)—two of the four research collaborations that comprise the International Pulsar Timing Array (IPTA).
The collaborations have each been gathering up their own data sets for analysis and then pooling them with other collaborations to independently review the findings. The data sets are composed of radio waves collected at radio telescopes. The waves themselves originally were beamed out by pulsars. These fascinating objects are the rapidly spinning remnants of massive stars that have exploded as supernovae. As pulsars whirl about, they blast a beam of radiation through the cosmos, rather like a hyperactive lighthouse.
The pulsars of interest to IPTA researchers are of the millisecond variety, meaning the stellar remnants whip around hundreds of times per second and thus produce a steady barrage of radio pulses. The pulses arrive so regularly, in fact, that any deviation in their clockwork precision can be chalked up to space itself changing. Gravitational waves, which distort regions of space as they propagate through them, fit the bill as such disrupters. The objective for IPTA and its constituent radio telescopes is therefore to detect and characterize these anticipated glitches in pulsar timing.
In the new paper, the IPTA published its second tranche of results, dubbed Data Release 2 or DR2, which shows signs of this sort of glitch in a set of 65 surveyed pulsars. While not set in stone and in need of corroboration, the finding is nonetheless cause for cautious optimism.
"For DR2, we found that there is an extra type of noise," says Kejia Lee, an associate professor and leader of the Pulsar Group at KIAA, which Chen recently joined. "To understand if the noise is really caused by gravitational waves, we need better, high-precision, and longer data."
Fortunately, this data is already being collected by the IPTA, which also includes the third founding member, Parkes Pulsar Timing Array in Australia (PPTA), and the newest, fourth member, the Indian Pulsar Timing Array (InPTA), which joined IPTA in May 2021. Together, these four efforts are compiling a DR3 expected to be ready in a few years' time.
Chen, Lee, and others are looking to bring in even more sources of data, for instance from the Five-hundred-meter Aperture Spherical Telescope (FAST) in China, a project KIAA members are involved in, as well as the MeerKAT telescope in South Africa and the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which involves researchers from the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology.
All the additional observations of millisecond pulsars would serve in strengthening the signal glimpsed so far—if indeed it originates from a cacophony of binary supermassive black holes and cosmic strings.
"Either we can gain significant knowledge of the astrophysics of massive structures or an insight into the beginning of the universe," says Chen. "Both are great possibilities, so a detection of ultra-low frequency gravitational waves will be a huge breakthrough."