Safely smoothing spacetime: backreaction in relativistic cosmological
  simulations
  A persistent theme in the study of dark energy is the question of whether it
really exists or not. It is often claimed hat we are mis-calculating the
cosmological model by neglecting the effects associated with averaging over
large-scale structures. In the Newtonian approximation this is clear: there is
no effect. Within the full relativistic picture this remains an important open
question however, owing to the complex mathematics involved. We study this
issue using particle numerical simulations which account for all relevant
relativistic effects without any problems from shell crossing. In this context
we show for the first time that the backreaction from structure can differ by
many orders of magnitude depending upon the slicing of spacetime one chooses to
average over. In the worst case, where smoothing is carried out in synchronous
spatial surfaces, the corrections can reach ten percent and more. However, when
smoothing on the constant time hypersurface of the Newtonian gauge,
backreaction contributions remain 3-5 orders of magnitude smaller.
