An Etymological Dictionary of Astronomy and Astrophysics

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 H₀ = 72 ± 8 km s^-1 Mpc^-1 (W. L. Freedman et al., 2001, ApJ 553, 47, arXiv:astro-ph/0012376), the
→ WMAP observations yield
70.4 ± 1.3 km s^-1 Mpc^-1 (N. Jarosik et al., 2011, ApJS 192, 14, arXiv:1001.4744), and the → Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1 (Planck Collaboration, 2014, A&A 571, A16, arXiv:1303.5076). More recently, the Hubble constant
was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy (Adam G. Reiss et al., 2016, arXiv:1604.01424). 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.

See also: → Hubble; → Friedmann-Lemaitre Universe; → constant.