Group Averaging for de Sitter free fields

TL;DR

This paper investigates the construction of de Sitter-invariant quantum states for free fields using group-averaging, demonstrating convergence for states with enough particles across various field types and dimensions.

Contribution

It extends the group-averaging technique to free scalar, vector, and tensor fields in de Sitter space, showing convergence for multi-particle states with smooth wavefunctions.

Findings

Group averaging converges for states with sufficient particles.

Explicit boost matrix elements are derived.

Results apply to scalar, vector, and tensor fields in any dimension.

Abstract

Perturbative gravity about global de Sitter space is subject to linearization-stability constraints. Such constraints imply that quantum states of matter fields couple consistently to gravity {\it only} if the matter state has vanishing de Sitter charges; i.e., only if the state is invariant under the symmetries of de Sitter space. As noted by Higuchi, the usual Fock spaces for matter fields contain no de Sitter-invariant states except the vacuum, though a new Hilbert space of de Sitter invariant states can be constructed via so-called group-averaging techniques. We study this construction for free scalar fields of arbitrary positive mass in any dimension, and for linear vector and tensor gauge fields in any dimension. Our main result is to show in each case that group averaging converges for states containing a sufficient number of particles. We consider general $N$-particle states with smooth wavefunctions, though we obtain somewhat stronger results when the wavefunctions are finite linear combinations of de Sitter harmonics. Along the way we obtain explicit expressions for general boost matrix elements in a familiar basis.