Distance measures are used in
physical cosmology to give a natural notion of the
distance between two objects or events in the
universe. They are often used to tie some
observable quantity (such as the
luminosity of a distant
quasar, the
redshift of a distant
galaxy, or the angular size of the acoustic peaks in the
CMB power spectrum) to another quantity that is not
directly observable, but is more convenient for calculations (such as the
comoving coordinates of the quasar, galaxy, etc.). The distance measures discussed here all reduce to the common notion of
Euclidean distance at low redshift.