The LIGO and Virgo detectors pick up gravitational waves — ripples in spacetime from distant cosmic catastrophes. For more than a year they recorded data, and among the loud signals from ordinary black holes, a barely audible whisper slipped through. That's what collisions of very light objects — lighter than the Sun — look like. But how do you know if it's a real signal or random noise?

That's where a special mathematical method comes in: it refines the signal's parameters, step by step cleaning the record from interference. An analysis of seven of the most suspicious cases showed that three of them were most likely mergers involving a black hole with a mass of about a quarter of the Sun. It's like hearing a whisper and making out the words. Because ordinary stars don't turn into such light black holes. Such objects, if they exist, could only have been born in the first moments after the Big Bang — they are called primordial black holes.

There's no confirmation yet, but these 'whispers' are the most real candidates we have. The next generation of detectors might turn the whisper into a coherent story about the early days of the universe.