| Term: | Hubble constant |
| Definition: |
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. |