September 24, 1998 CfA Colloquium

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.

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