Distant Supernovae and Cosmic Deceleration

Abstract

Distant supernovae can now be detected routinely. To date 34 supernovae at $z > 0.1$ have been discovered. Among them are 12 Type~Ia supernovae confirmed spectroscopically and suited to measure the cosmic deceleration when appropriately employed as standard candles. However, peak magnitudes have been determined for only two objects so far and a determination of $q_0$ is not yet possible. We describe the current status of the searches and possible pitfalls of the method which rests on few basic assumptions. The importance of sufficient information on the distant events is stressed and the observations of SN~1995K are used as an example of the detailed procedures employed in the analysis. Only spectroscopic classification and light curves in at least two filter bands provide the basis to use correction schemes for the luminosity which have successfully been established in nearby samples. Time dilation has been detected acting on the light curve of SN~1995K at a redshift of 0.478, providing clear evidence of universal expansion. The observations are fully consistent with local Type Ia supernovae in an expanding universe but incompatible with the expectations from a static universe. The contributions of the new, large telescopes to this research area are described. The extension of the observations to even more distant objects will provide a better leverage to distinguish between the possible decelerations and the inclusion of Type II supernovae into the sample add an independent check on the cosmological distances.