Hot tea in a freezer doesn't freeze all at once. First, tiny ice needles dart around — forming and melting until the temperature drops enough for the ice to set solid. The early universe behaved similarly as it cooled and changed its internal structure. Physicists call this a first-order phase transition — an abrupt change of state, like water freezing.
For a long time, it was thought that the main events, the birth of large bubbles of the new phase, occur against a clean, uniform background. But the authors of the new study wondered: what if by that time the background isn't clean but ripples with tiny 'ghost bubbles'? These bubbles are called subcritical: they are too small to grow and quickly collapse. However, if there are many of them, they can change the overall picture.
The researchers tracked the kinetics of these bubbles and compared it with the classical picture of critical bubble nucleation: those that are capable of growing. It turned out that under certain conditions, when the barrier between phases is low and the free energy difference is moderate, the background becomes noticeably mixed. In simple terms, by the time of the main transition, space is no longer clean but half-filled with bubbles of the new phase.
Such a 'dirty' background changes how the walls of large bubbles form and collide, and thus the characteristics of gravitational waves that might have reached us from those times. Future detectors might hear not just the echo of one big transition, but a whole chorus of bubble boiling that began long before the main event, just as ice needles in tea herald the formation of a solid crust.
Subcritical bubbles are like steam bubbles in a pot of water that hasn't boiled yet: they appear at the bottom but collapse before reaching the surface. Only in space, their size could be billionths of a meter, and they lived for unimaginably short times.
In Charles Stross's novel 'Accelerando', bubbles of new vacuum physics spread at the speed of light, rewriting the laws of reality, but there they are born in a 'clean' background, not in an already boiling soup as in this work.