Distance measures in cosmology

Summary

In an expanding universe there is no single notion of distance to a far galaxy, so cosmology uses several distance measures. The main ones are the comoving distance, the transverse comoving distance, the angular diameter distance and the luminosity distance. Each is defined so that a familiar formula of flat static space stays true. Examples are the size of an object versus its angle on the sky, and the brightness of a source versus its power. They are all computed from the scale factor as functions of the cosmological redshift.