The Direction of Gravity
  Gravity directs the paths of light rays and the growth of structure.
Moreover, gravity on cosmological scales does not simply point down: it
accelerates the universal expansion by pulling outward, either due to a highly
negative pressure dark energy or an extension of general relativity. We examine
methods to test the properties of gravity through cosmological measurements. We
then consider specific possibilities for a sound gravitational theory based on
the Galileon shift symmetry. The evolution of the laws of gravity from the
early universe to the present acceleration to the future fate -- the paths of
gravity -- carries rich information on this fundamental force of physics and on
the mystery of dark energy.
