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In the context of Effective Field Theory, the Hilbert space of statesincreases in an expanding universe. Hence, the time evolution cannot beunitary. The formation of structure is usually studied using effective fieldtheory techniques. We study the constraints on effective field theory analysesof early universe models which come from demanding that the factor of the spaceof states corresponding to length scales where the primordial fluctuations aremanifest does not suffer from the unitarity problem. For bouncing and emergentcosmologies, no constraints arise provided that the energy scale of the bounceor emergent phases is smaller than the ultraviolet (UV) cutoff scale. On theother hand, in the case of the inflationary scenario, non-trivial upper boundson the energy scale of inflation arise.

Unitarity Problems for an Effective Field Theory Description of Early Universe Cosmology