Recent observations of Type Ia supernovae, cosmic microwave background
radiation and large scale structure indicate that the expansion rate
of the universe is increasing. A number of models describing exotic
forms of matter, generically referred to as dark energy (not to be
confused with dark matter), have been proposed to explain this
acceleration. For example, the dark energy might be Einstein's
cosmological constant. In this talk I will give an introduction to big
bang cosmology and dark energy, with emphasis on the dark energy
equation of state and how it determines the future (large time
evolution) of the universe.