The Calan/Tololo Project
Ever since E. Hubble in 1929 identified Cepheid variables in nearby
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.