Minimal decoherence from inflation

We compute the rate with which super-Hubble cosmologicalfluctuations are decohered during inflation, by their gravitationalinteractions with unobserved shorter-wavelength scalar and tensormodes. We do so using Open Effective Field Theory methods, thatremain under control at the late times of observational interest,contrary to perturbative calculations. Our result is minimal in thesense that it only incorporates the self-interactions predicted byGeneral Relativity in single-clock models (additional interactionchannels should only speed up decoherence). We find that decoherenceis both suppressed by the first slow-roll parameter and by theenergy density during inflation in Planckian units, but that it isenhanced by the volume comprised within the scale of interest, inHubble units. This implies that, for the scales probed in the CosmicMicrowave Background, decoherence is effective as soon as inflationproceeds above ∼ 5 × 10$^{9}$ GeV. Alternatively, ifinflation proceeds at GUT scale decoherence is incomplete only forthe 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 atwhich it proceeds, helps suggest ways to look for quantumeffects.