Speaker
Description
Cosmic inflation may exhibit stochastic periods during which quantum fluctuations dominate over the semi-classical evolution. Extracting observables in these regimes is a notoriously difficult program as quantum randomness makes them fully probabilistic.
We propose a new way of framing Stochastic Inflation by enforcing the point of view of a local observer for which inflation has ended i.e. among all possible quantum histories, it only selects the ones which are relevant for Cosmology.
Formally, it amounts to redefine the usual stochastic-$\delta N$ formalism by a time-reversed version where curvature fluctuations are computed over all stochastic realisations producing the same lifetime, for all lifetimes (including the infinite limit).
As illustrative examples, I will show that for a flat semi-infinite potential, the time-reversed approach cures the divergences obtained in the conventional "forward" approach: the curvature probability distribution is finite with heavy tails. When adding a constant drift, we obtain exponential tails, which, in the classical-like limit of very large drift, become Gaussian, a feature which is not present in the "forward" approach. Eventually, for a flat bounded potential, the tail behaviour matches the "forward" picture up to a factor 2 in the decay which may have consequences on Primordial Black Hole abundances.
All these discrepancies end up being related to the very definition of the background which is ambiguous when a genuine classical trajectory does not exist.
Based on JCAP 11 (2025) 032, ArXiv:2511.21388 and forthcoming article