The universe right after the Big Bang was like a boiling cauldron: bubbles of a new state of matter were bursting and growing everywhere. These were first-order phase transitions — exactly like water boiling, only at temperatures reaching trillions of degrees. According to the Standard Model, one such transition could have spawned the very matter that makes up everything around us.
Calculations based on pure Yang–Mills theory (a simplified version of the strong interaction) have revealed an unpleasant surprise. It turns out that the motion of bubble walls hits hydrodynamic barriers — as if a bubble in boiling water suddenly runs into an invisible wall. Some solutions simply vanish. For physics close to quantum chromodynamics, explosive expansion modes — detonations — are completely forbidden. Instead, 'shocked detonations' can arise: an additional shock wave is born inside a rarefaction wave.
All this dramatically changes the energy balance inside the bubbles and, consequently, predictions about the gravitational-wave background. If detonations are forbidden, the most powerful waves simply do not occur. On the other hand, knowing about such invisible barriers helps to better separate extraneous bursts from the sought-after ripples. The code for constructing solutions has been made publicly available — now anyone can check how the bubbles of the early universe conceal their waves.
The walls of such bubbles can accelerate to nearly the speed of light, but sometimes slow down to the speed of sound — like a speedboat suddenly dropping to a gentle current.