Gravitational energy as dark energy: Cosmic structure and apparent
  acceleration
  Below scales of about 100/h Mpc our universe displays a complex inhomogeneous
structure dominated by voids, with clusters of galaxies in sheets and
filaments. The coincidence that cosmic expansion appears to start accelerating
at the epoch when such structures form has prompted a number of researchers to
question whether dark energy is a signature of a failure of the standard
cosmology to properly account, on average, for the distribution of matter we
observe. Here I discuss the timescape scenario, in which cosmic acceleration is
understood as an apparent effect, due to gravitational energy gradients that
grow when spatial curvature gradients become significant with the nonlinear
growth of cosmic structure. I discuss conceptual issues related to the
averaging problem, and their impact on the calibration of local geometry to the
solutions of the volume-average evolution equations corrected by backreaction,
and the question of nonbaryonic dark matter in the timescape framework. I
further discuss recent work on defining observational tests for average
geometric quantities which can distinguish the timescape model from a
cosmological constant or other models of dark energy.
