The Calan/Tololo Project

Ever since E. Hubble in 1929 identified Cepheid variables in nearby galaxies, we have known that the universe is expanding. In the ensuing decades since this revolutionary discovery, astronomers have devoted significant effort to measuring the deceleration parameter, since this measurement promises to reveal the geometry and fate of the universe. As of a decade ago, this measurement was not feasible, due to the lack of a precise distance indicator that could be applied to distant objects.

Given this problem, in 1989 I initiated a survey of supernovae (SNe) with the aim of studying their usefulness as distance indicators. The Calan/Tololo project (a collaboration with CTIO staff members M. Phillips, N. Suntzeff, R. Schommer, L. Wells, and J. Maza of the University of Chile) led to the discovery of 50 SNe (in the range z=0.01-0.1) in four years.

A key result of this survey was that Type Ia SNe (exploding white dwarfs) are not perfect standard candles, but have an intrinsic scatter of approximately 0.3-0.4 mag. However, following on the initial study by Phillips (1993), we demonstrated that the peak luminosities of SNe Ia are highly correlated with their decline rate from maximum light. We found that, based on the photometric evolution within three weeks of maximum light, it is possible to correct for this intrinsic luminosity spread and use SNe Ia as high precision distance indicators (sigma=0.15 mag).

This result permitted us to solve for a value of 65+/-4 for the Hubble constant and made possible the measurement of the elusive deceleration parameter. Recently, two groups of astronomers applied this method to distant SNe Ia. By comparing the distances of high-z SNe with those in the Calan/Tololo sample, these groups independently reported a remarkable finding: contrary to expectations, the universe is presently accelerating (Riess et al. 1998, Perlmutter et al. 1999)! Moreover, these observations seem to provide evidence that this acceleration is due to a vacuum energy that permeates space which acts as a repulsive force on large scales.