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.
as written by the authors arXiv:2606.12636
We formulate stochastic inflation in an open quantum system framework. The field coarse-grained in a patch of fixed physical size, and the total momentum of that patch, form a canonical pair and act on a one-mode Fock space which we identify as the "bulk". At each time step, new comoving modes join the coarse-grained patch and the bulk has to be redefined. This redefinition produces an entangled mode that is traced over, yielding a non-unitary evolution equation for the bulk's density matrix. For a free test field in de Sitter, one obtains GKLS dynamics, generated by an effective Hamiltonian and a single non-Hermitian Lindblad operator, hence diffusion and Hubble friction originate from the same quantum channel. The Wigner-Weyl transform of the GKLS equation leads to a Fokker-Planck equation for the Wigner function, which matches the one that applies to the classical phase-space distribution of stochastic inflation. We also provide several schemes under which one can unravel the GKLS dynamics into stochastic Schrodinger equations when continuous measurements of the decoupled mode are performed, making contact with Langevin formulations of stochastic inflation. In the light-field regime, an additional overdamped reduction can be performed by integrating out the momentum variable in the Wigner distribution, leading to Starobinsky's slow-roll Fokker-Planck equation. In that regime, the purity of the patch is strongly suppressed. In contrast, for heavy fields, field diffusion is suppressed and the coarse-grained patch remains close to a pure underdamped oscillator, which prevents a classical stochastic treatment.
In the slow-roll regime, the purity of the state drops so much that the quantum description becomes indistinguishable from classical probability theory, as if a coin were worn down until heads and tails were completely indistinguishable.
In Greg Egan's novel 'Schild's Ladder,' quantum vacuum fluctuations in an inflating universe spawn new universes with different laws of physics — an idea echoing how random kicks to the field during inflation could create regions with different densities.