Evidence for an accelerating universe from type IA supernovae

Measuring accurate distances allows us to map the expansion history of the universe. In effect, this is a measurement of the Hubble parameter at different cosmological ages. Since the various energy components in a Friedmann universe affect the scale parameter, i.e. redshift, differently, it is possible to disentangle their importance as a function of time. Currently, the only distance indicator stretching over a large enough redshift range is provided by Type Ia Supernovae. Many open questions concerning the systematics of Type la Supernovae remain, however, their properties as distance indicators are securely established. The most critical issue is potential evolution of the normalisation of the peak luminosity of the supernovae. In the future, the equation of state of the dominant form of energy in the universe can be established with observations of supernovae. The first experiments have already started and will prepare the ground for possible future satellite missions.