The Magnetic Part of the Weyl Tensor, and the Expansion of Discrete
  Universes
  We examine the effect that the magnetic part of the Weyl tensor has on the
large-scale expansion of space. This is done within the context of a class of
cosmological models that contain regularly arranged discrete masses, rather
than a continuous perfect fluid. The natural set of geodesic curves that one
should use to consider the cosmological expansion of these models requires the
existence of a non-zero magnetic part of the Weyl tensor. We include this
object in the evolution equations of these models by performing a Taylor series
expansion about a hypersurface where it initially vanishes. At the same
cosmological time, measured as a fraction of the age of the universe, we find
that the influence of the magnetic part of the Weyl tensor increases as the
number of masses in the universe is increased. We also find that the influence
of the magnetic part of the Weyl tensor increases with time, relative to the
leading-order electric part, so that its contribution to the scale of the
universe can reach values of ~1%, before the Taylor series approximation starts
to break down.
