**Title:**
Is the Universe Accelerating?

**Speaker:**
Robert Kirshner

**Abstract:**
Type Ia Supernovae, thought to be exploding white dwarfs,
can be made into the best yardsticks for measuring distances in the
Universe. By detecting and measuring supernovae at redshifts up to
0.97, the High-Z Supernova Team has constructed a Hubble diagram that
is precise enough to reveal the effects of cosmic deceleration due to
matter in any form, dark or bright. Instead of seeing signs of cosmic
deceleration, the observations favor acceleration, as would be
produced by a significant amount of vacuum energy (for example as a
cosomological constant.) When combined with cosmic microwave
background fluctuations, the supernova results favor a flat cosmology
with Omega = 0.94 +/- 0.26, a dimensionless age of the universe,H t =
0.97 (14 Gyr for H= 65 km/sec/Mpc), and a energy component of the
Universe which dominates the dynamics at the present time, and which
has an equation of state that is not too different from that expected
for the cosmological constant. All these conclusions rest on our
understanding of the way supernova light curves depend on cosmic
epoch, which is not complete.