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

TL;DR

This paper investigates unitarity constraints in effective field theories describing early universe cosmology, highlighting bounds on inflationary energy scales to ensure consistent quantum evolution.

Contribution

It identifies conditions under which unitarity is preserved in effective field theories of early universe models, especially constraining inflationary scenarios.

Findings

No unitarity constraints for bouncing and emergent cosmologies below UV cutoff.

Upper bounds on inflationary energy scale to maintain unitarity.

Effective field theory viability depends on energy scales relative to UV cutoff.

Abstract

In the context of Effective Field Theory, the Hilbert space of states increases in an expanding universe. Hence, the time evolution cannot be unitary. The formation of structure is usually studied using effective field theory techniques. We study the constraints on effective field theory analyses of early universe models which come from demanding that the factor of the space of states corresponding to length scales where the primordial fluctuations are manifest does not suffer from the unitarity problem. For bouncing and emergent cosmologies, no constraints arise provided that the energy scale of the bounce or emergent phases is smaller than the ultraviolet (UV) cutoff scale. On the other hand, in the case of the inflationary scenario, non-trivial upper bounds on the energy scale of inflation arise.