Astrophysical Ages and Time Scales ASP Conference Series, Vol. TBD, 2001

The age of the Universe depends on both the present-day Hubble Constant and on the history of cosmic expansion. For decelerating cosmologies such as Ω m = 1, the dimensionless product H 0 t 0 < 1 and modestly high values of the Hubble constant H 0 > 70 would be inconsistent with a cosmic age t 0 larger than 12 Gyr. But if Ω Λ > 0, then H 0 t 0 can take on a range of values. Evidence from the Hubble diagram for high redshift Type Ia supernovae favors Ω Λ ∼ 0.7 and H 0 t 0 ∼ 1. Then, if H 0 lies in the range 65–73, the age of the Universe, t 0 , is 14 ± 1.6 Gyr. 1. It has been an interesting five years! Five years ago, the combination of deep seated belief in inflation, implying Ω = 1, and stellar age estimates near 15 Gyr seemed to require H 0 ∼ 40. Measurements of H 0 ∼ 60 and Ω m ∼ 0.3 in clusters notwithstanding, Bartlett, Blanchard, Silk, and Turner wrote a provocative paper entitled, " The Case for a Hubble Constant of 30 km/s/Mpc. " Persuaded by the power of theoretical reasoning, Joe Silk bet Brian Schmidt and me a case of Scotch that H 0 < 60. While Joe has not yet paid up, in the past 5 years he has moved closer to the source of Scotch while the Hubble constant has moved to 60 and beyond. The new element is that supernovae have made the connection between Ω = 1 and the cosmic age more flexible because of plausible evidence for cosmic acceleration. A Danish-English team (Norgaard-Neilsen et al. 1989) initiated a program to find supernovae in clusters of galaxies at redshifts of 0.3-0.5, with the idea that they could distinguish the effects of cosmic deceleration, as expected in an Ω m = 1 universe by measuring the peak apparent magnitudes of supernova light curves. The observational problem was to find these faint (m∼21-22) and distant supernovae near the peak of their light curves. But small detectors kept this pioneering effort from yielding significant results. The Supernova Cosmology Project (SCP) based at Lawrence Berkeley Lab forged ahead with further attempts to find distant supernovae by extending the methods of the Danes to bigger, faster telescopes. After abandoning attempts to instrument the AAT prime focus for …