Acceleration of the Universe
  The cosmological model best capable of fitting current observational data features two separate epochs during which the Universe is accelerating. During the earliest stages of the Universe, such acceleration is known as cosmological inflation, believed to explain the global properties of the Universe and the origin of structure. Observations of the present state of the Universe strongly suggest that its density is currently dominated by dark energy with properties equivalent or similar to a cosmological constant. In these lecture notes, I provide an introductory account of both topics, including the possibility that the two epochs may share the same physical description, and give an overview of the current status. 
Andrew R Liddle