Cosmology is concerned with the large-scale structure, origin, evolution, and ultimate fate of the universe. The accurate measurement of the distances of galaxies is an essential prerequisite to determining the scale of the universe in space and time and to mapping the distribution of matter within. Even though the first successful measurement of a galaxy's distance was achieved in 1923 by Edwin Hubble (see Chapter 13) and modern techniques have greatly extended the range to which such measurements can be made, the distance scale of the universe is still shrouded in a degree of uncertainty.

THE COSMOLOGICAL DISTANCE LADDER

In order to measure the distances of galaxies, astronomers need to be able to identify objects of known luminosity or diameter, which can be used as distance indicators. By comparing the observed brightness to the assumed luminosity or the observed apparent diameter to the assumed physical diameter, it is possible (see Chapter 13) to calculate the distance at which a chosen indicator must be in order to appear as faint (or as small) as it does. An ideal indicator would be highly luminous (or very large) in order to be visible at great distances, have a very small spread of luminosities (or diameters), be easy to identify, and be relatively common, so that examples can readily be found in remote galaxies. In practice, few indicators come close to matching all of these criteria.