BMS in Cosmology

TL;DR

This paper explores BMS symmetries at null infinity in cosmology, revealing new scalar and tensor Goldstone modes, and discusses their implications for vacuum structure and gravitational memory effects in an expanding universe.

Contribution

It extends the concept of BMS symmetry to cosmological settings, identifying new physical degrees of freedom and their role in vacuum transitions and memory effects.

Findings

BMS transformations generate scalar and tensor Goldstone modes.

Null infinity exists in decelerating FRW backgrounds.

Gravitational memory includes contributions from the interior of the light cone.

Abstract

Symmetries play an interesting role in cosmology. They are useful in characterizing the cosmological perturbations generated during inflation and lead to consistency relations involving the soft limit of the statistical correlators of large-scale structure dark matter and galaxies overdensities. On the other hand, in observational cosmology the carriers of the information about these large-scale statistical distributions are light rays traveling on null geodesics. Motivated by this simple consideration, we study the structure of null infinity and the associated BMS symmetry in a cosmological setting. For decelerating Friedmann-Robertson-Walker backgrounds, for which future null infinity exists, we find that the BMS transformations which leaves the asymptotic metric invariant to leading order. Contrary to the asymptotic flat case, the BMS transformations in cosmology generate Goldstone modes corresponding to both scalar and tensor degrees of freedom which may exist at null infinity and perturb the asymptotic data. Therefore, BMS transformations generate physically inequivalent vacua as they populate the universe at null infinity with these physical degrees of freedom. We also discuss the gravitational memory effect when cosmological expansion is taken into account. In this case, there are extra contribution to the gravitational memory due to the tail of the retarded Green functions which are supported not only on the light-cone, but also in its interior. The gravitational memory effect can be understood also from an asymptotic point of view as a transition among cosmological BMS-related vacua.