Evidence for an Accelerating Universe from Type Ia Supernovae

Abstract.

The expansion history of the universe is revealed by measurements of cosmic distances. Since the various energy components in a Friedmann universe contribute differently to the evolution of the scale parameter it is possible to disentangle their importance as a function of time. Type Ia Supernovae are the only distance indicator available over a large enough range of the universal history. All other observations are based either on the assumption of a uniform evolution within a given cosmological model (e.g. Hubble constant, age of the universe, current matter density) or the comparison of exactly two epochs, the time of matter/radiation decoupling and now (CMB fluctuations).

Many questions concerning the systematics of type Ia supernovae remain unanswered, although their properties as distance indicators have been established securely. The small scatter around the expansion line in the Hubble diagram for supernovae in the nearby universe is evidence for the accurate distances. For objects in the distant universe this check does not work any longer and is replaced by the assumption of uniform peak luminosity. The most critical issue is evolution of the peak luminosity of the supernovae.

In the future, the equation of state parameter of the dominant form of energy in the universe can be established through observations of supernovae. The first experiments have started and will prepare the ground for possible future satellite missions.

Partly based on the results obtained by the High-z Supernova Search Team. More information is available at http://cfa-www.harvard.edu/cfa/oir/Research/supernova/HighZ.html