For decades, astronomers have listened to the cosmos through pulsars — rapidly spinning neutron stars that emit perfectly timed radio pulses.
These “cosmic clocks” are so precise that even a tiny disturbance in spacetime can be detected by watching how their ticks shift.
Now, new research suggests that these tiny changes might let us “hear” a new kind of cosmic rhythm — the beats of gravitational waves.
In 2023, several global collaborations, including NANOGrav in the United States and the European Pulsar Timing Array, announced strong evidence for ultra–low-frequency gravitational waves.
These waves, with periods stretching from months to years, may come from titanic cosmic events.
The findings weren’t yet at the gold-standard 5-sigma confidence level used in physics, but scientists believe we are on the brink of confirming their existence.
Two possible explanations exist for these slow waves. One is that they were born in the very early universe, during the rapid expansion known as cosmic inflation.
The other is that they come from pairs of supermassive black holes — each billions of times the mass of our Sun — orbiting each other after their host galaxies merged.
Both possibilities would produce similar-looking ripples in pulsar timing data, making them difficult to tell apart.
That’s where Hideki Asada, a theoretical physicist at Hirosaki University, and his colleague Shun Yamamoto come in.
In a recent paper published in the Journal of Cosmology and Astroparticle Physics, they propose a clever new way to distinguish the sources — by looking for gravitational “beats.”
In music, beats occur when two notes are almost the same pitch.
Their sound waves interfere, creating a pattern of rising and falling volume — a slow throb that musicians recognize instantly. Asada and Yamamoto suggest a similar effect could happen with gravitational waves.
If two pairs of supermassive black holes produce waves at nearly the same frequency, the ripples could overlap, creating beats in spacetime itself.
These beats wouldn’t be audible, but their rhythm might show up in the arrival times of pulsar radio pulses.
By studying how the pulses from many pulsars shift together, astronomers could detect the telltale pattern of these cosmic beats. Finding such a signal would point to nearby black hole binaries, rather than a diffuse background from the early universe.
Asada believes that once the gravitational-wave signal is confirmed beyond doubt, perhaps within a few years, this method could help solve one of astronomy’s most profound mysteries: Are we hearing the echoes of the universe’s birth — or the slow dance of colossal black holes?
