What's the Matter in Cosmology?
  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.
