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In contemporary scientific cosmology, the age of the universe is the time that has elapsed since the Big Bang, which in standard cosmological models is the past limit to the hotter, denser phases that are encountered as one goes farther and farther back into the past. In these models the Big Bang is a singularity, a region characterized by infinite density, temperature, and curvature. Quantum gravitational and quantum cosmological treatments of the Big Bang, using concepts like superstrings, are beginning to provide a more adequate description of this primordial cosmological epoch, which is often referred to as the Planck era, during which the temperature of the universe was above 10³² K (kelvin). Here, classical relativistic gravitational theory (Albert Einstein's General Relativity) breaks down. It is from this extremely hot Planck era that the universe emerges with its three spatial dimensions, its one time dimension, its four basic physical interactions, and its matter and radiation. Before that emergence they were all unified in ways that are not yet completely understood.

A rough upper limit on the age of the universe, t_H, is given by the reciprocal of the Hubble parameter now, H₀, which gives the rate of expansion of the universe per unit distance. Thus, t_H = 1/H₀. Using the currently measured range of values of H₀, t_H is between twelve to sixteen billion years. Compare this to the very reliable age of the Earth and the sun, which is about 4.8 billion years. These ages have been confirmed by a variety of astronomical and isotopic techniques, including the measurement of the ages of stars in globular clusters (which are very old), and the estimation of how much uranium has decayed to lead and how much rubidium has decayed to strontium.

From the point of view of prescientific cultural and religious traditions, the age of the universe is the time that has elapsed since the world or the universe was created. In many traditions the creation is also taken to be the "event" in which time itself began. Some of those who interpret the Genesis creation and pre-Abraham historical accounts literally—as scientifically and historically reliable documents describing the formation of the universe and of the world, and earliest human history—have calculated the age of the world and of created reality (the universe) to be about 6,000 years, having begun in 4004 B.C.E. This has been done by counting the generations listed in Genesis from Adam and Eve to Abraham, and then estimating the number of years from Abraham to Moses, both of which are fairly well known, to the present. Experts have disputed this literal approach, of course, particularly because it is strongly contradicted by independent bodies of evidence from both the natural and the human sciences. It also fails to recognize the mythological and legendary character of the relevant Genesis sources. This does not mean that the Genesis sources are not revealing and expressive of important truths, but it does mean that those truths are neither scientific nor directly historical, but rather religious and theological truths.

The cosmological age of the universe since the Big Bang, although it certainly has important theological significance, cannot be interpreted as the time since the creation of the universe, if universe is understood to mean all that exists and not God. There could have been and there could be many other regions of reality, either completely separate from or linked with ours only at the Big Bang itself, which preceded or are older than our observable universe. Furthermore, it is unclear whether "creation" or "the first moment of creation" took place at any definite time. However, it does make some sense to date the beginning of the observable universe at the Big Bang, even though the coordinated manifold of primordial quantum events is not adequately understood.

See also BIG BANG THEORY; COSMOLOGY, PHYSICAL ASPECTS; SINGULARITY; STRING THEORY

WILLIAM R. STOEGER