Modified gravity models of dark energy
  We review recent progress of modified gravity models of dark energy--based on
f(R) gravity, scalar-tensor theories, braneworld gravity, Galileon gravity, and
other theories. In f(R) gravity and Brans-Dicke theory it is possible to design
viable models consistent with local gravity constraints under a chameleon
mechanism, while satisfying conditions for the cosmological viability. The
Dvali-Gabadazde-Porrati braneworld model can be compatible with local gravity
constraints through a nonlinear field self-interaction arising from a
brane-bending mode, but the self-accelerating solution contains a ghost mode in
addition to the tension with observational data about the cosmic expansion
history. The extension of the field self-interaction to more general forms
satisfying a Galilean symmetry in the flat space-time allows a possibility to
avoid the appearance of ghosts and Laplacian instabilities, while the late-time
cosmic acceleration can be realized by the field kinetic energy. We study the
evolution of cosmological perturbations in those models to place constraints on
model parameters from the observations of large-scale structure, cosmic
microwave background, and weak lensing. We also briefly review other modified
gravitational models of dark energy-- such as those based on Gauss-Bonnet
gravity and Lorentz-violating theories.
