A hum resonates across the universe. It is not sound, but a trembling of space itself — the gravitational wave background. It is produced by pairs of supermassive black holes circling each other at the centers of distant galaxies. Each pair is like a bass instrument in a vast orchestra. Catching the solo of one instrument against the general drone is the task for pulsar timing arrays.
Pulsars are neutron stars that emit radio pulses with incredible precision. Arrays of radio telescopes have been monitoring dozens of pulsars for decades, looking for glitches in their rhythm — the imprint of a passing gravitational wave. But so far, it has been impossible to say with confidence which black holes are playing the loudest. A team of scientists has proposed a new approach: they introduced the number Nc, a characteristic of how much individual sources stand out above the background. It is like counting how many musicians are playing particularly loudly.
Applying the method to simulated data from the 15-year NANOGrav survey, the authors tested 114 candidate black hole binaries from observations of active galactic nuclei. It turned out that 21 of them produce too strong a signal: their existence does not fit well with actual data. Previous analysis had found only one such tension case. The new method is more sensitive.
The new tool not only filters out unrealistic models but also helps probe the properties of the entire population of supermassive black hole binaries. Within a few years, the pulsar network may transform from a background detector into a true telescope for observing the dances of giants.
Pulsars are so stable that their 'ticking' rivals atomic clocks in precision. A shift of one billionth of a second can already indicate the passage of a gravitational wave.