Soft gravitons as Goldstone modes of spontaneously broken asymptotic symmetries in de Sitter spacetimes

TL;DR

This work identifies soft graviton modes in de Sitter space as the Goldstone bosons of spontaneously broken asymptotic diffeomorphism symmetries, providing a unified symmetry-based view of infrared gravitons. The analysis clarifies the correct counting of Goldstone modes in the presence of spacetime symmetries, showing only two physical modes in four dimensions and a generalization to arbitrary dimsensions, with a Goldstone EFT that reproduces the standard graviton action. It differentiates global broken-phase physics from local symmetric-state descriptions via the role of measurements and the Page time, illustrating how environments select a broken vacuum in large volumes. Extending to inflation, the framework includes the dilatation-induced scalar mode alongside the tensor modes, recovers Maldacena’s consistency relations through symmetry charges, and links to infrared loop results and memory effects, with promising directions toward EFT extensions, modified gravity, CFT duals, and laboratory analogues.

Abstract

I demonstrate that soft graviton modes in de Sitter spacetimes are the Goldstone modes of the spontaneously broken asymptotic symmetry group of de Sitter space. I then show that any local measurement, including the effects of the environment, will collapse the symmetric state onto the broken state in the large volume limit. In any discussion involving observers, de Sitter spacetimes are, therefore, best described globally by the broken phase, while local observers, in the small volume limit, can not discriminate between different degenerate global vacuum states and are therefore best described by the symmetric state. As a consequence, a small Hubble-sized local region initially in the symmetric state will, after a time scale corresponding to the Page time of de Sitter space, have expanded to a large region in the broken state. This illuminates the physical nature of soft graviton modes in de Sitter spacetimes.