Approaches to Understanding Cosmic Acceleration
  Theoretical approaches to explaining the observed acceleration of the
universe are reviewed. We briefly discuss the evidence for cosmic acceleration,
and the implications for standard General Relativity coupled to conventional
sources of energy-momentum. We then address three broad methods of addressing
an accelerating universe: the introduction of a cosmological constant, its
problems and origins; the possibility of dark energy, and the associated
challenges for fundamental physics; and the option that an infrared
modification of general relativity may be responsible for the large-scale
behavior of the universe.
