# 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 all-sky 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,
object-based measurements give H_{0} values of around 72 – 74 km s^{–1} Mpc^{–1}, with typical
errors of 2 – 3 km s^{–1} Mpc^{–1}. This is in mild discrepancy with CMB-based measurements,
in particular those from the Planck satellite, which give values of 67 – 68 km s^{–1} Mpc^{–1} and
typical errors of 1 – 2 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 object-based methods, and also on the assumptions about
other cosmological parameters and which datasets are combined in the case of the all-sky
methods.