Expanding Universe: slowdown or speedup?
  The kinematics and dynamic interpretation of the cosmological expansion is reviewed in a widely accessible manner with emphasis on the acceleration aspect. Virtually all the approaches that can in principle account for the accelerated expansion of the Universe are reviewed, including dark energy as an item in the energy budget of the Universe; modified Einstein equations; and, on a fundamentally new level, the use of the holographic principle. 
Yu.L. Bolotin, O.A. Lemets and D.A. Yerokhin
astro-ph.CO gr-qc hep-ph hep-th