The Hubble Constant
Abstract
I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. There are two broad categories of measurements. The first uses individual astrophysical objects which have some property that allows their intrinsic luminosity or size to be determined, or allows the determination of their distance by geometric means. The second category comprises the use of allsky cosmic microwave background, or correlations between large samples of galaxies, to determine information about the geometry of the Universe and hence the Hubble constant, typically in a combination with other cosmological parameters. Many, but not all, objectbased measurements give H_{0} values of around 7274 km s^{1} Mpc^{1}, with typical errors of 23 km s^{1} Mpc^{1}. This is in mild discrepancy with CMBbased measurements, in particular those from the Planck satellite, which give values of 6768 km s^{1} Mpc^{1} and typical errors of 12 km s^{1} Mpc^{1}. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the objectbased methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the allsky methods.
 Publication:

Living Reviews in Relativity
 Pub Date:
 December 2015
 DOI:
 10.1007/lrr20152
 Bibcode:
 2015LRR....18....2J
 Keywords:

 Cosmology;
 Hubble constant