The Hubble Constant

Neal Jackson

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. A variety of measurements are now giving consistent answers. These measurements range from local determinations of distances to nearby galaxies to distances in the far universe, and also comprise one-step distance measures as well as conventional ``ladder'' methods in which one measurement is used to calibrate another, and methods in which several cosmological parameters are constrained together. Estimates have now converged to about, or possibly just less than, 70 km s^{-1} Mpc^{-1}, although there is scope for improvement in many methods.