Cosmologists use telescopes, astronomical satellites, and other instruments to study the universe. The data that these instruments provide allow scientists to evaluate current theories and to come up with ideas to better explain the universe. Modern cosmologists are continuously calculating the age, density, and rate of expansion of the universe.

The universe’s density, expansion rate, and age are all related. The density of the universe’s matter determines how much the gravitational force will slow the expansion rate. The rate of expansion depends on the age and density of the universe. If cosmologists measure the rate of expansion by examining galactic redshifts and estimate the density of the universe, they can calculate an estimate of the universe’s age. Cosmologists calculate the expansion rate of the universe by finding the relationship between the distance of an object from Earth and the rate at which it is moving away from Earth. This relationship is represented by Hubble’s constant (H) in the formula v = H × d, where v is velocity (or the speed of the object) and d is the distance between the object and Earth. If Hubble's constant is relatively large, the universe is expanding relatively rapidly. A measure of the distance scale in a universe that is rapidly expanding would be larger than a measure of the distance scale in a universe of the same age with a smaller value of Hubble's constant.

For a universe with very low density, the age of the universe would be directly related to its expansion rate. This universe would expand forever; this eternal expansion defines an open universe. If, on the other hand, the density of a universe is sufficiently high, the expansion rate is changing—slowing down as the universe ages. This universe would eventually stop expanding and begin contracting, which defines it as a closed universe. Astronomers and cosmologists have been able to estimate the density of the universe, but until the Wilkinson Microwave Anisotropy Probe (WMAP) results were released the density estimates covered a wide range of values. Some estimates of density fell in the range for an open universe, others in the range for a closed universe, and still others near the boundary between the two. Age calculations for the higher densities are about two-thirds of those for the lower densities.

Estimates of the age, density, and expansion rate of the universe include many possible sources of uncertainty. For example, many galaxies orbit each other as members of clusters of galaxies. The velocity of any one galaxy in the cluster as seen from Earth varies over time as it circles the cluster, moving toward Earth through part of its orbit and away through the remainder. Cosmologists, therefore, must find the average expansion velocity of the entire cluster. Recent studies drawing on data collected by the Hubble Key Project, the Hipparcos satellite, and WMAP have helped reduce the uncertainty of estimates for age, density, and expansion rate.