Beyond Einstein's General Relativity
  Modern astrophysical and cosmological models are plagued with two severe
theoretical difficulties, namely, the dark energy and the dark matter problems.
Relative to the former, high-precision observational data have confirmed with
startling evidence that the Universe is undergoing a phase of accelerated
expansion. This phase, one of the most important and challenging current
problems in cosmology, represents a new imbalance in the governing
gravitational equations. Several candidates, responsible for this expansion,
have been proposed in the literature, in particular, dark energy models and
modified gravity, amongst others. Outstanding questions are related to the
nature of this so-called "dark energy" that is driving the acceleration of the
universe, and whether it is due to the vacuum energy or a dynamical field. On
the other hand, the late-time cosmic acceleration may be due to modifications
of General Relativity, which introduce new degrees of freedom to the
gravitational sector itself. We analyze some of the modified theories of
gravity that address these intriguing and exciting problems facing modern
physics, and explore the foundations of gravitation theory, essential for the
construction of modified theories of gravity.
