The universe sometimes resembles a pot of boiling water. With enough heat, liquid turns to vapor: that's a phase transition. Something similar could have happened in the early universe: a field filling space suddenly changed its state, like water becoming gas. And from such an event, ripples spread across the fabric of spacetime — gravitational waves.
The authors of the study suggest that this ripple could have been born from a mechanism explaining the tiny masses of neutrinos. In the inverse seesaw mechanism, the lightness of neutrinos is achieved through a very weak interaction with heavy partners, and the key process here violates lepton number — a characteristic related to particle families (electron, muon, tau neutrinos). If this process didn't occur immediately but switched on as the universe cooled, like a crack in glass when temperature drops, it was accompanied by a first-order phase transition — a sharp one, with energy release.
They propose to look for this ripple using pulsars — rapidly rotating neutron stars whose radio pulses reach us with clock-like precision. If the space between us and a pulsar wobbles slightly, the rhythm is disrupted. Pulsar timing arrays are already probing the low-frequency gravitational wave background, and a signal from a phase transition could show up in these data.
Thus, the trembling of space becomes a window into a world almost invisible to ordinary instruments. The universe's pot has long cooled, but its boiling could have left a mark in the rhythm of distant stellar beacons.
Pulsars are so stable that a deviation of one billionth of a second per year is already considered a signal — roughly like noticing that your wristwatch has lost a thousandth of a second over a year.
In Carl Sagan's novel "Contact," a signal from an extraterrestrial civilization comes precisely through pulsars, but here the universe itself "telegraphs" to us about its past.