The early universe expanded at an unimaginable rate — faster than light could cross it. In this boiling cauldron, any region of space resembled an open can of paint: its boundaries continuously captured new 'vapor' from outside, while some of its own contents irrevocably escaped. The authors of the new work built a mathematical model of exactly such an open can — a quantum system that is not isolated from its surroundings.

The key trick is to track not the entire field at once, but a fixed volume, as if we were looking only at one cubic centimeter of expanding dough. Inside this volume reside two quantities, the field strength and the total momentum of particles, which behave like communicating vessels. When new modes are added to the volume (like fresh paint being poured into the can), old and new parts become entangled, and this connection must be severed — much like cutting the string joining two balloons. The severing generates irreversibility: friction appears, akin to viscous resistance, and simultaneously random kicks that jostle the field.

For light fields that slowly roll down an energy slope, this model reduces to the classical equation first written by Alexei Starobinsky. The quantum fuzziness (a mixture of states) becomes enormous — the field behaves like warm jelly that is constantly trembling. Heavy fields, in contrast, barely feel the kicks and remain remarkably clean, like a taut string that is hard to set in motion. This means that for them, a classical description, with a definite trajectory and velocity, simply does not work, and a full quantum language is needed.

Thus is born a bridge between the quantum microworld and the large-scale structure of the cosmos. The open can losing drops turns out to be an apt metaphor for how the initially quantum fog transforms into the galaxies and voids we observe today.