The HST Key Project on the Extragalactic Distance Scale: A Search for Three Numbers

Abstract

We present a review of the main results of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale with emphasis on the new techniques that were developed in order to undertake the observations, and the methods that were adopted in both reporting the results and quantifying especially the associated statistical and systematic errors. The three numbers (the cosmological expansion rate H _o , its statistical error and the systematic uncertainty, respectively) are H _o = 72 ±(3) ±[7] km/sec/Mpc.