The cosmological constant \Lambda presents one of the most
difficult puzzles in astronomy and physics. Although its
status as a logically possible contributor to the dynamics
of the universe has long been appreciated, various
naturalness arguments have suggested to many that it must be
exactly zero. This attitude has begun to change, as
observations of Type Ia supernovae at high redshifts have
provided strong evidence for a positive \Lambda --- a
possibility which provides an excellent fit with a variety
of independent cosmological observations. I will briefly
review the physics underlying the cosmological constant, the
observational evidence bearing on its value, and its
potential consequences for cosmology.
[EB22.02] Cosmological Background Radiation
Max Tegmark (Institute for Advanced Studies)
This abstract not available.
[EB22.03] Omega
Michael Strauss (Princeton University)
This abstract not available.
[EB22.04] The Hubble Constant
Robert Kennicutt (University of Arizona)
Accurately measuring the expansion rate of the universe, the Hubble constant, has proven to be one of the most difficult and frustrating challenges in extragalactic astronomy. Now thanks to a multi-year Key Project with the Hubble Space Telescope (HST), combined with groundbased observations by several groups, the Hubble constant (H_0) has been measured to an accuracy approaching \pm10%. This talk will review recent progress in calibrating H_0, with emphasis on the final results from the HST Key Project on the Extragalactic Distance Scale. HST observations of Cepheid variable stars in 25 galaxies now firmly anchor the local distance scale within the Virgo supercluster. These distances have been used in turn to calibrate several secondary distance methods, which yield independent determinations of H_0 and an external measure of its uncertainty. In parallel with this traditional approach, several direct physical probes of extragalactic distances, including gravitational lensing of multiply imaged quasars, the Sunyaev-Zeldovich effect in galaxy clusters, expansion parallaxes of extragalactic supernovae, and interferometric parallaxes of extragalactic masers can provide reliable checks on the zeropoint of the cosmological distance scale. The convergence of these independent approaches now enables us to place meaningful constraints on the cosmic age scale, and test the consistency of the observations with cosmological models.
[EB22.05] The Age and Fate of the Universe
John Huchra (Harvard-Smithsonian Center for Astrophysics)
As seen in the previous talks, new cosmological observations are closing in fast on a model for the Universe that is of surprise and perhaps delight to both theorists and observers. This is the result of a combination of long term programs to determine the cosmic expansion rate, the masses of large structures and the ages of its oldest constituents and startling new observations of the Universe's global geometry using high redshift supernovae and steadily improving constraints from Cosmic Microwave Background observations. The debate about the cosmological mass density now strongly favors a low value, about 1/4 to 1/3 of that needed to close the Universe, made up of a mix of baryons, neutrinos and some as yet to be determined cold dark matter. Evidence for a cosmological constant is improving, and it appears as if the debate on the total value of Ømega is being resolved in favor of the favored theoretical value of unity, with Ømega_matter \sim 0.3 and Ømega_\Lambda \sim 0.7. Multiple determinations of the value of the Hubble Constant are converging to a value in the range of 55-75 km/s/Mpc. Taken together, these parameters yield a Universe with an age of 16-17 Gyr, fully consistent with the latest age determinations of the oldest stars. If these are indeed the correct model and nearly the correct parameters, the Universe will continue to expand forever --- to paraphrase Robert Frost, ice is likely to suffice.