What is a photon in de Sitter spacetime?

TL;DR

This paper explores the nature of photon states in de Sitter spacetime, revealing their emergence without gauge symmetry, their unique infrared behavior, and implications for primordial magnetogenesis.

Contribution

It demonstrates that photon states in de Sitter arise generically in quantum field theories without gauge symmetry and analyzes their late-time behavior and implications.

Findings

Photon states form a UIR of SO(1,4) in de Sitter.

Photon states can be created by composite operators from massive fields.

Photon two-point functions decay slower than electromagnetic fields at late times.

Abstract

The states of a single photon in four-dimensional de Sitter (dS) spacetime form a Unitary Irreducible Representation (UIR) of SO(1,4), which we call the photon UIR. While in flat spacetime photons are intimately tied to gauge symmetry, we demonstrate that in de Sitter, photon states emerge generically in any quantum field theory, even without an underlying U(1) gauge field. We derive a K\"all\'en-Lehmann representation for antisymmetric tensor two-point functions and show that numerous composite operators constructed from massive free fields can create states in the photon UIR. Remarkably, we find that some of these operators exhibit two-point functions with slower late-time and large-distance decay than the electromagnetic field strength itself, challenging the conventional notion that photons dominate the infrared regime. Using our spectral representation, we establish non-perturbative bounds on the late-time behavior of electric and magnetic fields in de Sitter, with potential implications for primordial magnetogenesis. Through one-loop calculations, we demonstrate that both the creation of photon states and the enhanced late-time large-distance behavior persist in weakly interacting theories.