Systematics of Adiabatic Modes: Flat Universes

TL;DR

This paper systematically derives adiabatic modes in flat universe cosmologies, revealing new modes and clarifying their classical and quantum distinctions, with implications for cosmological soft theorems.

Contribution

It introduces several new adiabatic modes in flat universes and clarifies their gauge dependence and quantum-classical differences.

Findings

New vector, tensor, and scalar adiabatic modes identified.

Decay behavior of modes in standard versus contracting universes.

Preliminary soft theorem derivations related to these modes.

Abstract

Adiabatic modes are cosmological perturbations that are locally indistinguishable from a (large) change of coordinates. At the classical level, they provide model independent solutions. At the quantum level, they lead to soft theorems for cosmological correlators. We present a systematic derivation of adiabatic modes in spatially-flat cosmological backgrounds with asymptotically-perfect fluids. We find several new adiabatic modes including vector, time-dependent tensor and time-dependent scalar modes. The new vector and tensor modes decay with time in standard cosmologies but are the leading modes in contracting universes. We present a preliminary derivation of the related soft theorems. In passing, we discuss a distinction between classical and quantum adiabatic modes, we clarify the subtle nature of Weinberg second adiabatic mode and point out that the adiabatic nature of a perturbation is a gauge dependent statement.