A low rumble coming from underground could be the noise of a distant construction site, or it could be the echo of an ancient earthquake. Astronomers faced a similar puzzle in 2023 when they detected ultra-low-frequency gravitational waves — the trembling of spacetime itself, rippling across the universe.
These waves, with frequencies in the nanohertz range (one period lasting years), were discovered using pulsar timing arrays — observations of dozens of neutron stars that spin with the precision of atomic clocks. A gravitational wave slightly disrupts the rhythm of these cosmic lighthouses. After gathering 15 years of data, researchers saw a common "hum" but cannot determine where it comes from. Two main suspects: pairs of supermassive black holes at the centers of galaxies, slowly spiraling toward each other, and primordial gravitational waves born in the first moments after the Big Bang.
The new study proposes a "translator" that, based on barely noticeable features in the noise, can tell a construction site from an earthquake. The authors developed a hierarchical Bayesian analysis — a method that simultaneously looks for signs of non-Gaussianity (deviations from simple random noise) in the signal. If the hum is created by individual bright sources, such as nearby pairs of black holes, characteristic bursts appear in the data. If it is primordial waves, the noise is more uniform. The method not only separates the hypotheses but also allows measuring the properties of the binary black hole population: their masses, distances, and merger rates.
Thus, the cosmic hum ceases to be just noise and becomes a story about invisible giants inhabiting the heart of every galaxy.
Pulsars are so stable that a deviation in their signal of one millionth of a second over 10 years is already considered an event. Such precision is comparable to measuring the distance to the Moon with an error of the thickness of a human hair.
In Carl Sagan's novel "Contact," a signal from space received on ultra-long waves turned out to be a message from another civilization. Real nanohertz waves are still just noise, but deciphering them could be the first step toward "listening" to the most hidden processes of the universe.