Mairi Sakellariadou

Road noise is a great informant. Rare cars sound as individual signals; dense traffic merges into a steady hum. Just the same, mergers of black holes produce either single bursts or a continuous background of gravitational waves.

Two sensitive microphones are listening to the Universe. One hears a low hum, the other hears almost nothing. It's not a malfunction: it's about how they are oriented. Gravitational-wave observatories like LIGO don't just catch loud bursts from black hole mergers.

For a whole year, LIGO, Virgo, and KAGRA listened to the trembling of spacetime. Thousands of mergers of black holes and neutron stars left clear signatures. But hidden among them was another signal — a barely audible whisper.

A noisy café. You're trying to make out two quiet conversations: one from the table by the window (black hole mergers), the other a barely audible whisper from the far corner (echoes of the universe's birth). But the hiss of the espresso machine and street noise (detector jitter) drown everything out. New work teaches a computer to separate these sounds.