Explaining the observed cosmic acceleration of the background expansion of
the Universe will likely require yet unknown physics. Assuming General Relativity
is the correct gravity theory up to the largest scales, one is forced to introduce
a new form of energy with negative pressure, dubbed dark energy, in order to
create the gravitational repulsion necessary to produce acceleration.

On the other hand, if we allow for General Relativity to be modified on cosmologically
interesting scales, it may be possible to explain cosmic acceleration from such modification.
The simplest modification of gravity is the cosmological constant, which can also be
viewed as a form of dark energy, depending on which side of the Einstein's equations it
is introduced. Other possibilities include the addition of extra dimensions to which
only gravity leaks, therefore becoming weaker at large scales, as well as more
phenomenological modifications to the Einstein-Hilbert action.

With Hiro Oyaizu, Fabian Schmidt and Wayne Hu, I have worked on producing and analysing
N-body simulations of f(R) and DGP models of modified gravity, exploring the
effects of modified gravity on the matter power spectrum, spherical collapse and
the properties of dark matter halos. With Mark Wyman and Elise Jennings I have
studied similar effects, including now redshift distortions, for Galileon models.