Acceleration from Modified Gravity: Lessons from Worked Examples
  I examine how two specific examples of modified gravity explanations of
cosmic acceleration help us understand some general problems confronting
cosmological tests of gravity: how do we distinguish modified gravity from dark
energy if they can be made formally equivalent? how do we parameterize
deviations according to physical principles with sufficient generality, yet
focus cosmological tests into areas that complement our existing knowledge of
gravity? how do we treat the dynamics of modifications which necessarily
involve non-linearities that preclude superposition of forces? The modified
action f(R) and DGP braneworld models provide insight on these question as
fully-worked examples whose expansion history, linear perturbation theory, and
most recently, non-linear N-body and force-modification field dynamics of
cosmological simulations are available for study.
