Researchers at Hirosaki University have suggested a method to identify the source of ultra-low-frequency gravitational waves by looking for 'beat' patterns in pulsar signals. This approach could distinguish between waves from cosmic inflation and those from supermassive black hole binaries. The proposal builds on 2023 evidence from pulsar timing arrays that falls short of full confirmation.
Pulsars, rapidly rotating neutron stars that emit steady radio pulses, serve as precise cosmic clocks for detecting subtle distortions in spacetime. In 2023, international collaborations including NANOGrav in the US and European teams reported strong evidence for nanohertz gravitational waves—waves with periods of months to years—based on timing variations in pulsar signals. As Hideki Asada, a professor at Hirosaki University, explained, "The signal was statistically reliable but below the 5-sigma threshold that particle physicists usually require." This evidence, while promising, has not yet reached the level of confirmed detection in the cosmology and astrophysics community.
Two primary explanations exist for these waves: primordial fluctuations from cosmic inflation in the early universe, stretched to vast scales, or gravitational waves from orbiting supermassive black hole binaries formed during galaxy mergers. Distinguishing between them has proven challenging due to similar correlation patterns in pulsar data. To address this, Asada and researcher Shun Yamamoto propose analyzing 'beat' effects, akin to acoustic interference, where waves from two nearby black hole binaries with nearly identical frequencies overlap and modulate pulsar arrival times.
"If two such systems have very similar frequencies, their waves can interfere and create a beat pattern, like in acoustics," Asada noted. By searching for this modulation in pulsar timing correlations, scientists could confirm if the signal originates from discrete nearby sources rather than a diffuse inflationary background. Their findings were published on October 14, 2025, in the Journal of Cosmology and Astroparticle Physics.
Asada anticipates that a 5-sigma confirmation may arrive within a few years, at which point this method could clarify the waves' origins. "I think once a confirmed detection at 5-sigma is achieved... our method could be useful to distinguish whether they come from inflation or from nearby supermassive black hole binaries," he concluded.