Soft-Collinear Gravity and Soft Theorems

TL;DR

This paper develops an effective field theory called soft-collinear gravity to systematically analyze soft graviton interactions, deriving gravitational soft theorems and explaining their universality through emergent symmetries.

Contribution

It introduces the soft-collinear gravity framework, revealing gauge symmetries and providing a systematic derivation of soft theorems at all orders in the power expansion.

Findings

Soft-collinear gravity captures graviton interactions with matter and themselves.

Soft theorems are derived from the effective theory at the Lagrangian level.

Symmetries explain the universality of soft graviton emissions.

Abstract

This chapter reviews the construction of ``soft-collinear gravity'', the effective field theory which describes the interaction of collinear and soft gravitons with matter (and themselves), to all orders in the soft-collinear power expansion, focusing on the essential concepts. Among them are an emergent soft background gauge symmetry, which lives on the light-like trajectories of energetic particles and allows for a manifestly gauge-invariant representation of the interactions in terms of a soft covariant derivative and the soft Riemann tensor, and a systematic treatment of collinear interactions, which are absent at leading power in gravity. The gravitational soft theorems are derived from soft-collinear gravity at the Lagrangian level. The symmetries of the effective theory provide a transparent explanation of why soft graviton emission is universal to sub-sub-leading power, but gauge boson emission is not and suggest a physical interpretation of the form of the universal soft factors in terms of the charges corresponding to the soft symmetries. The power counting of soft-collinear gravity further provides an understanding of the structure of loop corrections to the soft theorems.