Evidence for dark energy from Type Ia supernovae

By combining distances and redshifts the evolution of the cosmic scale factor can be traced. The accurate relative distances required for this measurement can currently only be provided by Type Ia supernovae. Their ability to yield precise distances is checked empirically by the linear expansion of the local universe. At higher redshifts this characteristic has to be assumed. Various checks of the observable supernova signatures are used to assess the similarity of the local and the distant objects. It is now time to test the various proposed models for dark energy. Ongoing projects for the next few years have the potential, in combination with accurate determinations of the matter density, to measure the integrated value of the equation of state parameter to better than 10%.