||The Hubble parameter for the present epoch. It is the constant of proportionality
between the recession velocities of galaxies and their distances from each other.
The latest determinations using the Hubble Space Telescope observations of Cepheids
give H_0 = 72 ± 8 km s^-1 Mpc^-1, the WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1,
and the Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1. More recently,
the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble
Space Telescope, with a 2.4% accuracy. The new value, 73.2 km s^-1 Mpc^-1, suggests
that the Universe is expanding between five and nine percent faster than previously
calculated. The Hubble law is only applicable for large distances (> 20 Mpc), when
the proper motions of galaxies in groups and clusters cannot confuse the recession
due to expansion.