Astronomers detect a cosmic “heartbeat” in pulsar signals
Scientists may soon “hear” the universe’s hidden rhythm through pulsars beating with gravitational waves.
- Date:
- October 15, 2025
- Source:
- Sissa Medialab
- Summary:
- Researchers analyzing pulsar data have found tantalizing hints of ultra-slow gravitational waves. A team from Hirosaki University suggests these signals might carry “beats” — patterns formed by overlapping waves from supermassive black holes. This subtle modulation could help scientists tell whether the waves stem from ancient cosmic inflation or nearby black hole binaries, potentially identifying the true source of spacetime’s gentle vibrations.
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Pulsars may be revealing that extremely low-frequency gravitational waves are moving through the universe. Observations reported in 2023 by international pulsar timing array collaborations could be caused by either a background of overlapping gravitational waves from countless distant sources or a single pair of nearby supermassive black holes orbiting each other. To determine which explanation fits, Hideki Asada, a theoretical physicist and Professor at Hirosaki University, and Shun Yamamoto, a researcher at the Graduate School of Science and Technology, Hirosaki University, have proposed a new approach. Their method takes advantage of "beat" effects that occur when two gravitational waves have nearly the same frequency, searching for their subtle influence on the arrival times of pulsars' radio signals.
Their findings were recently published in the Journal of Cosmology and Astroparticle Physics (JCAP).
The night sky contains remarkably precise "cosmic clocks": pulsars, which are dense neutron stars that emit radio pulses at steady intervals, ticking like perfectly timed metronomes. On Earth, radio telescopes track these pulses not only to learn about the pulsars themselves, but also to use them as tools for studying the structure and behavior of the wider universe.
If something unseen -- almost a "cosmic ghost" -- distorts spacetime between a pulsar and Earth, the timing of its pulses shifts slightly. These changes are not random; several pulsars in certain parts of the sky can show matching variations, as if a slow, invisible wave were passing through space.
"In 2023 several pulsar timing array collaborations -- NANOGrav in the US, and European teams -- announced strong evidence for nanohertz gravitational waves," Asada notes. Nanohertz means wave periods of months to years, with wavelengths of several light-years. To probe such scales, we rely on distant, stable pulsars hundreds to thousands of light-years away. "The signal was statistically reliable but below the 5-sigma threshold that particle physicists usually require," he continues. "It's 'strong evidence' but not yet a confirmed detection, but the cosmology and astrophysics community believes we are approaching the first detection of nanohertz gravitational waves."
Although the evidence is promising, it still falls short of absolute confirmation. Asada notes that if future data strengthen the result, the next step will be to pinpoint the origin. "There are two main candidate sources for nanohertz gravitational waves," he explains. "One is cosmic inflation, which would have created spacetime fluctuations in the very early universe, later stretched to cosmic scales. The other is supermassive black hole binaries, which form when galaxies merge. Both scenarios could generate nanohertz gravitational waves."
Distinguishing between these possibilities has been difficult because the correlation patterns seen in pulsar data -- the way timing differences between pulsars relate to each other -- were once thought to appear similar in both cases. "In our paper, we explored the situation where a nearby pair of supermassive black holes produces a particularly strong signal," Asada says. "If two such systems have very similar frequencies, their waves can interfere and create a beat pattern, like in acoustics. That feature could, in principle, allow us to distinguish them from the stochastic background of inflation."
Asada and Yamamoto therefore leverage a familiar acoustic effect: beats. When two waves have almost -- but not exactly -- the same frequency, their superposition produces periodic strengthening and weakening. Applied to gravitational waves, two supermassive-black-hole binaries with similar frequencies would imprint a characteristic modulation in the pulsar-timing signal. The method is to look for this modulation -- the "beat" -- in the pulsar correlation patterns. If it's present, that strongly suggests the signal is not a diffuse background but arises from specific, relatively nearby binaries.
We now await stronger confirmation of the pulsar signal's nature. "I think once a confirmed detection at 5-sigma is achieved, maybe within a few years, the next step will be to ask: what is the origin of the waves? At that point, our method could be useful to distinguish whether they come from inflation or from nearby supermassive black hole binaries," Asada concludes.
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Journal Reference:
- Shun Yamamoto, Hideki Asada. Can we hear beats with pulsar timing arrays? Journal of Cosmology and Astroparticle Physics JCAP, 14 October 2025 DOI: 10.48550/arXiv.2501.13450
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