What’s the matter 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.

A quasi-static approach to structure formation in black hole universes

“…In contrast, we found that when we also included interaction energies between clusters that the change in scale of the cosmology was relatively insensitive to the value of λ. This is despite the fact that the inter-cluster interaction energies were found to be very large in previous studies [27,31], and can be attributed to the result that large-scale interactions are not particularly affected by the degree of clustering in the model (see Fig. 7).…”

“…This quantity is defined in analogy to the standard expression for N black holes in an asymptotically flat space [43], and is closely related to the expression that one would obtain when considering the total mass of all black holes and interaction energies in one of the asymptotically flat flange regions on the far side of an Einstein-Rosen bridge [31]. Using it to evaluate the inter-cluster gravitational interaction energies, we find…”

“…However, if we want to perform a similar analysis for lattice models with some fixed total bare mass im i then a comparison with FLRW is less instructive. This is because there is a large difference between lattice models with some im i and FLRW models that contain the same total bare mass in dust [27,31]. What we can do, however, is compare between lattice models that have the same total bare mass but different values of λ.…”

“…For further discussion of masses and interaction energies between black holes in cosmological models the reader is referred to Refs. [30] and [31].…”

“…[19][20][21][22][23][24][25][26], and using exact methods in Refs. [27][28][29][30][31][32][33][34]. The basic idea behind these models is to split the matter content of the Universe into discretised packets, which are then modelled as black holes.…”