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We investigate quantum decoherence in a class of models which interpolatesbetween expanding (inflation) and contracting (ekpyrosis) scenarios. For thecases which result in a scale-invariant power spectrum, we find that ekpyroticuniverses lead to complete decoherence of the curvature perturbation before thebounce. This is in stark contrast to the inflationary case, where recoherencehas been previously observed in some situations. Although the purity can becomputed for couplings of all sizes, we also study the purity perturbativelyand observe that late-time (secular growth) breakdown of perturbation theoryoften occurs in these cases. Instead, we establish a simple yet powerfullate-time purity resummation which captures the exact evolution to a remarkablelevel, while maintaining analytical control. We conclude that the cosmologicalbackground plays a crucial role in the decoupling of the heavy fields duringinflation and alternatives.

Decoherence out of fire: Purity loss in expanding and contracting universes