Determination of the Hubble con

Establishing accurate extragalactic dis- tances has provided an immense challenge to astronomers since the 1920s. The situation has improved dramatically as better detectors have become available, and as several new, promising techniques have been developed. For the first time in the history of this difficult field, relative distances to galaxies are being compared on a case-by-case basis, and their quantitative agreement is being established. New instrumen- tation, the development of new techniques for measuring distances, and recent measurements with the Hubble Space telescope all have resulted in new distances to galaxies with precision at the ?5-20% level. The current statistical uncer- tainty in some methods for measuring Ho is now only a few percent; with systematic errors, the total uncertainty is ap- proaching ?+10%. Hence, the historical factor-of-two uncer- tainty in the value of the Ho is now behind us. Though there has been remarkable progress in measuring the cosmological parameters, the accuracy of these quantities is determined by the available technology and measurement techniques and is still not sufficiently high to discriminate among the various existing world models (1). Because of the fundamental dependence on the cosmological parameters in all of the models, accurate determinations are critical to make reliable predictions based on the current models. For instance, a reliable value of the Hubble constant is required to constrain the density of baryons from nucleosynthesis at an early epoch of the universe. The Hubble constant sets the time and length scale at the epoch of equality of the energy density of matter and radiation. In the structure formation paradigm based on gravitational instability, the horizon scale at matter-radiation equality specifies the critical range of the density perturbation spectrum turnover, and an accurate knowledge of the Hubble constant allows a quantitative comparison of the anisotropies in the cosmic background radiation and theories of the large- scale structure of the universe. In addition, in the issue addressed in this session, that of the age of the universe, there is a direct confrontation between the expansion age inferred from the Hubble constant in the standard model and age dating of the oldest objects in the universe. The reason for testing the cosmological model by using the age of the universe is obvious: there should be no astronomical object in the universe older than the universe itself. Consequently, the oldest objects known provide the minimum age of the universe. What is required to measure an accurate value of Ho? According to the Hubble law, what is needed are measure- ments of both redshifts of galaxies (via spectral lines), and distances to galaxies (at sufficiently large distances where