Cosmology is, simply put, the study of the universe. Relativistic cosmology is the study of solutions of Einstein's equation which may represent the broad features of the universe as a whole.

One constraint on such models is the seemingly innocuous statement that the sky is dark at night. Why is this surprising? In a Euclidean universe with a uniform distribution of stars, both the density of matter and the apparent luminosity fall off as $1/r^2$. Thus, the perceived brightness of every thin spherical shell of stars, as seen at the center of the sphere, is the same; an infinite universe would therefore lead to an infinitely bright night sky! This seeming contradiction is known as Olbers' paradox.

How can we avoid Olbers' paradox? Perhaps the universe is finite. Or perhaps the spatial distribution of stars is not uniform. Or perhaps the universe is not Euclidean. Or perhaps the universe is not constant in time.

Any reasonable theory of cosmology must provide a resolution to this conundrum.