On the Green and Wald formalism

TL;DR

This paper discusses the Green and Wald formalism for backreaction in cosmology, highlighting its limitations and misconceptions, in the context of precise future observations and the complexity of averaging in an inhomogeneous universe.

Contribution

It provides a critical analysis of the Green and Wald formalism, clarifying misconceptions and discussing its drawbacks in modeling cosmological backreaction effects.

Findings

The Green and Wald formalism has limitations in accurately describing backreaction.

Misconceptions about the formalism have influenced the backreaction debate.

The framework's drawbacks suggest the need for alternative approaches.

Abstract

Backreaction in the cosmological context is a longstanding problem that is especially important in the present era of precise cosmology. The standard model of a homogeneous background plus density perturbations is most probably oversimplified and is expected to fail to fully account for the near-future observations of sub-percent precision. From a theoretical point of view, the problem of backreaction is very complicated and deserves careful examination. Recently, Green and Wald claimed in a series of papers to have developed a formalism to properly describe the influence of density inhomogeneities on average properties of the Universe, i.e., the backreaction effect. A brief discussion of this framework is presented, focussing on its drawbacks and on misconceptions that have arisen during the "backreaction debate".