Picture a chain of underground passages. If they are all the same, you walk through each at the same speed. But what if each tunnel is slightly narrower than the previous one? Then your movement slows down. That is what happens with quantum transitions in the very early universe, when the field responsible for rapid expansion feels the curvature of space. Previously, it was thought that all steps were uniform. But then the energy of the process is strictly determined by the pattern of fluctuations seen on the cosmic microwave background map. Measurements say this energy must be tiny — no more than 3 GeV, just a few times the mass of a proton. With such energy, the expansion could not have produced heavy particles or detectable gravitational waves.

When physicists added this interaction (denoted as ξRφ²), the picture changed. At sufficiently high energy — say $10^{11}$ GeV, ten million times hotter than at the Large Hadron Collider — the unevenness becomes strong. The transitions slow down non-uniformly, and the observed pattern of spots in the sky is exactly what telescopes see. Moreover, colliding bubbles of new vacuum generate gravitational waves at frequencies around a kilohertz. Future detectors could pick them up, confirming that the tunnels at the dawn of time were indeed uneven.