1 INTRODUCTION2 EXPANSION OF THE UNIVERSE: THE COSMOLOGICAL CONTEXT3 MEASUREMENT OF DISTANCES3.1 Cepheid Distance Scale3.2 Tip of the Red Giant Branch (TRGB) Method3.3 Maser Galaxies3.4 Surface Brightness Fluctuation (SBF) Method3.5 Tully-Fisher Relation3.6 Type Ia Supernovae4 THE HUBBLE SPACE TELESCOPE (HST) KEY PROJECT4.1 Systematics on H at the End of the Key Project and a Decade Later5 OTHER METHODS FOR DETERMINING Ho5.1 Gravitational Lens Time Delays and Ho5.2 The Sunyaev-Zel'dovich (SZ) Effect and Ho5.3 Measurements of Anisotropies in the Cosmic Microwave Background6 AGE OF THE UNIVERSE7 WHY MEASURE Ho TO HIGHER ACCURACY?7.1 Constraints on Dark Energy7.2 Constraints on the Neutrino Mass7.3 Measuring Ho to 2%8 FUTURE IMPROVEMENTS9 SUMMARY POINTS10 DISCLOSURE STATEMENT11 ACKNOWLEDGEMENTSarXiv:1004.1856v1 [astro-ph.CO] 11 Apr 2010Annu. Rev. Astron. Astrophys. Vol. 48, 2010 1THE HUBBLE CONSTANTWendy L. Freedman and Barry F. MadoreCarnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101, USA;email: [email protected], [email protected] Words Cosmology, Distance Scale, Cepheids, S upernovae, Age of Uni-verseAbstract Considerable progress has been made in determining the Hubble constant over thepast two decades. We discuss th e cosmological context and importance of an accurate measure-ment of the Hubble constant, and focus on six high-precision distance-determination methods:Cepheids, tip of the red giant branch, maser galaxies, surface-brightnes fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in themeasurement of galaxy distances and how t o minimize them. Our best current estimate of theHubble constant is 73 ±2 (random) ±4 (systematic) k m s−1Mp c−1. The importance of im-proved accuracy in the Hubble constant will increase over the next decade with new missionsand experiments designed to increase the precision in other cosmological parameters. We outlinethe steps that will be required to d eliver a value of the Hubble constant to 2% systematic u ncer-tainty and discuss the constraints on other cosmological parameters th at will then be possiblewith such accuracy.CONTENTSINTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Annu. Rev. Astron. Astrophys. 2010 48 1056-8700/97/0610-00EXPANSION OF THE UNIVERSE: THE COSMOLOGICAL CONTEXT . . . . 5MEASUREMENT OF DISTANCES . . . . . . . . . . . . . . . . . . . . . . . . . . 8Cepheid Distance Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Tip of the Red Giant Branch (TRGB) Method . . . . . . . . . . . . . . . . . . . . . . . 22Maser Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Surface Brightness Fluctuation (SBF) Method . . . . . . . . . . . . . . . . . . . . . . . 29Tull y-Fisher Relation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Type Ia Supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32THE HUBBLE SPACE TELESCOPE (HST) KEY PROJECT . . . . . . . . . . . 37Systematics on H◦at the End of the Key Project and a Decade Later . . . . . . . . . . 38OTHER METHODS FOR DETERMINING Ho. . . . . . . . . . . . . . . . . . . 41Gravitational Lens Time Delays and Ho. . . . . . . . . . . . . . . . . . . . . . . . . . 41The Sunyaev-Zel’dovich (SZ) Effect and Ho. . . . . . . . . . . . . . . . . . . . . . . . 43Measurements of Anisotropies in the Cosmic Microwave Background . . . . . . . . . . 44AGE OF THE UNIVERSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47WHY MEASURE HoTO HIGHER ACCURACY? . . . . . . . . . . . . . . . . . . 49Constraints on Dark Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Constraints on the Neutrino Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Measuring Hoto ±2% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52FUTURE IMPROVEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53SUMMARY POINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55DISCLOSURE STATEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562Annu. Rev. Astron. Astrophys. Vol. 48, 2010 31 INTRODUCTIONIn 1929 Carnegie astronomer, Ed win Hubble, published a linear correlation be-tween the apparent distances to galaxies and th eir recessional velocities. Thissimple plot provided evidence that our Universe is in a state of expansion, a dis-covery that still stands as one the most profound of the twentieth century (Hubble1929a). This result had been anticipated earlier by Lemaˆıtre (1927), who firs tprovided a mathematical solution for an expanding universe, and noted that itprovided a n atural explanation for the observed receding velocities of galaxies.These results were published in the Ann als of the Scientific Society of Brussels(in French), and were not widely known.Using photographic data obtained at the 100-inch Hooker telescope situatedat Mount Wilson CA, Hubble measured the distances to six galaxies in the Lo-cal Group using the Period-Luminosity relation (hereafter, the Leavitt Law) forCepheid variables. He then extended the sample to an …