Local Distance Ladder Measurements and Determination of the Hubble Constant

Our ability to measure the current expansion rate of the universe, or Hubble constant, H0, continues to improve with the development of new techniques, instrumentation, and both ground- and space-based telescopes. An early program of the Hubble Space Telescope (HST) was a Key Project to measure H0, using Cepheid variables to calibrate the extragalactic distance scale. In 2001 the Key Project resolved a factor-of-two debate, and yielded a value of H0 = 72 km / sec / Megaparsec with a combined statistical and systematic uncertainty of 10%. The value of the Hubble constant measured using Cepheid variables has remained stable (in the low to mid-70s) for about two decades, but the precision of the measurements (using HST and Spitzer) has increased significantly. A new value of H0 based on the Tip of the Red Giant Branch (TRGB) results in a value of H0 = 70 km / sec / Megaparsec, with an uncertainty of 3%, in good agreement with that obtained using Cepheids. However, a tension has arisen between the value inferred from measurement of the cosmic background radiation (assuming a standard cosmological model with a cosmological constant and cold dark matter), and those measured locally. If the tension is real, it may be signaling new physics beyond the standard model.