Minimal decoherence from inflation

TL;DR

This paper calculates the minimal rate at which inflationary cosmological fluctuations decohere due to gravitational interactions, revealing that decoherence is efficient at high inflation scales and providing insights into quantum effects in the early universe.

Contribution

It introduces a controlled Open Effective Field Theory approach to compute inflationary decoherence, considering only self-interactions from General Relativity in single-clock models.

Findings

Decoherence is suppressed by slow-roll parameters and energy density.

Decoherence is enhanced by the volume within the Hubble scale.

At GUT-scale inflation, decoherence is nearly complete for most scales.

Abstract

We compute the rate with which super-Hubble cosmological fluctuations are decohered during inflation, by their gravitational interactions with unobserved shorter-wavelength scalar and tensor modes. We do so using Open Effective Field Theory methods, that remain under control at the late times of observational interest, contrary to perturbative calculations. Our result is minimal in the sense that it only incorporates the self-interactions predicted by General Relativity in single-clock models (additional interaction channels should only speed up decoherence). We find that decoherence is both suppressed by the first slow-roll parameter and by the energy density during inflation in Planckian units, but that it is enhanced by the volume comprised within the scale of interest, in Hubble units. This implies that, for the scales probed in the Cosmic Microwave Background, decoherence is effective as soon as inflation proceeds above $\sim 5\times 10^{9}$ GeV. Alternatively, if inflation proceeds at GUT scale decoherence is incomplete only for the scales crossing out the Hubble radius in the last ~ 13 e-folds, of inflation. We also compute how short-wavelength scalar modes decohere primordial tensor perturbations, finding a faster rate unsuppressed by slow-roll parameters. Identifying the parametric dependence of decoherence, and the rate at which it proceeds, helps suggest ways to look for quantum effects.