  I consider some of the issues we face in trying to understand dark energy.
Huge fluctuations in the unknown dark energy equation of state can be hidden in
distance data, so I argue that model-independent tests which signal if the
cosmological constant is wrong are valuable. These can be constructed to remove
degeneracies with the cosmological parameters. Gravitational effects can play
an important role. Even small inhomogeneity clouds our ability to say something
definite about dark energy. I discuss how the averaging problem confuses our
potential understanding of dark energy by considering the backreaction from
density perturbations to second-order in the concordance model: this effect
leads to at least a 10% increase in the dynamical value of the deceleration
parameter, and could be significantly higher. Large Hubble-scale inhomogeneity
has not been investigated in detail, and could conceivably be the cause of
apparent cosmic acceleration. I discuss void models which defy the Copernican
principle in our Hubble patch, and describe how we can potentially rule out
these models.
