Interacting Field Theories in de Sitter Space are Non-Unitary

TL;DR

This paper demonstrates that the expected cancellation of IR divergences in interacting field theories, which is linked to unitarity, does not occur in de Sitter space, indicating potential non-unitarity issues.

Contribution

It reveals that IR divergence cancellations, essential for unitarity in flat space theories, fail in de Sitter space, highlighting fundamental differences in quantum field theory behavior.

Findings

IR divergences do not cancel in de Sitter space

Unitarity of the S-matrix is compromised in de Sitter space

Differences in IR behavior suggest non-unitarity in de Sitter QFTs

Abstract

It is well known that there should be a total cancellation of the IR divergences in unitary interacting field theories, such as QED and gravity. The cancellation should be at all orders between loop and tree level contributions to cross--sections. This is the crucial fact related to the unitarity of the evolution operator (S--matrix) of the underlying interacting field theory. In this note we show that such a cancellation does {\it not} happen in de Sitter space.