Wendy Freedman

"The , which measures the , together with the total energy density of the Universe, sets the size of the observable Universe, its age, and its radius of curvature. Excellent progress has been made recently toward the measurement of the Hubble constant: a number of different methods for measuring distances have been developed and refined, and a primary project of the has been the accurate calibration of this difficult-to-measure parameter. The recent progress in these measurements is summarized, and areas where further work is needed are discussed. Currently, for a wide range of possible s, the Universe appears to have a . Combined with current estimates of stellar ages, the results favor a . They are consistent with either an , or a with a non-zero value of the ."

"We are at an interesting juncture in cosmology. With new methods and technology, the accuracy in measurement of the has vastly improved, but a recent tension has arisen that is either signaling or as-yet unrecognized uncertainties. Just under a century ago, Edwin Hubble revolutionized cosmology with his discovery that the . Hubble found a relationship between radial velocity and the distance to nearby galaxies, determining the proportionality constant Ho (=v/r), that now bears his name. The Hubble constant remains one of the most important parameters in cosmology. An accurate value of Ho can provide a powerful constraint on the cosmological model describing the evolution of the universe. In addition, it characterizes the expansion rate of the Universe at the current time, defines the observable size of the Universe, and its inverse sets the ."

"... after finishing my postdoc, I became a faculty member at , and then ended up being the scientific leader of this project to measure the rate at which the universe is expanding. ... when that project finished, .. we .. resolved the issue. We went from a factor of 2 uncertainty — we measured an uncertainty of 10%."