  Accurate distances to celestial objects are key to establishing the age and
energy density of the Universe and the nature of dark energy. A distance
measure using active galactic nuclei (AGN) has been sought for more than forty
years, as they are extremely luminous and can be observed at very large
distances. We report here the discovery of an accurate luminosity distance
measure using AGN. We use the tight relationship between the luminosity of an
AGN and the radius of its broad line region established via reverberation
mapping to determine the luminosity distances to a sample of 38 AGN. All
reliable distance measures up to now have been limited to moderate redshift --
AGN will, for the first time, allow distances to be estimated to z~4, where
variations of dark energy and alternate gravity theories can be probed.
