What's the Matter in Cosmology?

TL;DR

This paper explores the implications of matter clumping and interaction energies in cosmological models, questioning the validity of the dust approximation and how interaction energies influence the universe's large-scale dynamics.

Contribution

It discusses recent results on whether interaction energies between masses should gravitate and how they affect cosmological equations.

Findings

Interaction energy may contribute to gravitational effects.

The dust model might be insufficient for realistic matter distributions.

Recent results shed light on averaging procedures in cosmology.

Abstract

Almost all models of the universe start by assuming that matter fields can be modelled as dust. In the real universe, however, matter is clumped into dense objects that are separated by regions of space that are almost empty. If we are to treat such a distribution of matter as being modelled as a fluid, in some average or coarse-grained sense, then there a number of questions that must be answered. One of the most fundamental of these is whether or not the interaction energy between masses should gravitate. If it does, then a dust-like description may not be sufficient. We would then need to ask how interaction energies should be calculated in cosmology, and how they should appear in the Friedmann-like equations that govern the large-scale behaviour of the universe. I will discuss some recent results that may shed light on these questions.