The Cosmic Quartet - Cosmological Parameters of a Smoothed Inhomogeneous Spacetime

TL;DR

This paper explores how inhomogeneities in the universe affect the interpretation of cosmological parameters, distinguishing between true average values and those inferred under homogeneous assumptions, introducing concepts like curvature backreaction and volume effect.

Contribution

It introduces the concepts of 'curvature backreaction' and 'volume effect' to quantify differences between true and observed cosmological parameters in inhomogeneous spacetimes.

Findings

Identification of curvature backreaction and volume effect as key factors

Estimate of volume effect using a simple geometrical example

Clarification of the distinction between 'bare' and 'dressed' parameters

Abstract

We discuss the relation between `bare' cosmological parameters as the true spatial average characteristics that determine the cosmological model, and the parameters interpreted by observers with a `Friedmannian bias', i.e., within a homogeneous space geometry. We may say that the latter are `dressed' by the smoothed-out geometrical inhomogeneities of the surveyed spatial region. We identify two effects that quantify the difference between `bare' and `dressed' parameters: `curvature backreaction' and `volume effect'. An estimate of the latter is given in terms of a simple geometrical example.