The study of the universe on the largest scales of length and time, particularly the propounding of theories concerning the origin, nature, structure, and evolution of the universe. A cosmology is any model said to represent the observed universe. Western cosmology is entirely scientific in its approach, and has produced two famous models in modern times: the 'big bang' and the steady state hypotheses. The study of the origin and mode of formation of various celestial objects is known as cosmogony.

The universe is everything there is that can be detected by physical effects, from the Earth beneath our feet, to the Sun in our sky, to other worlds in the farthest reaches of Space. It contains billions of 'island universes' called Galaxies, which are huge clusters of stars. The galaxies are all moving away from each other, because the universe is expanding as a result of the Big Bang - a kind of 'explosion' which happened some 15 billion years ago. This singular event created space, time, and matter/energy. There are at least 100 billion galaxies, each with approximately 1 billion stars. How many of those stars have planets? How many of those planets have intelligent life? These are some of the questions of astronomy and SETI. The scientific study of the universe is called cosmology. Some scientists speculate that there is more than one universe; this concept of the multiverse (or parallel universes) arises from quantum physics.

The expansion of the universe over most of its history has been relatively gradual. The notion that a rapid period "inflation" preceded the Big Bang expansion was first put forth 25 years ago. The new WMAP observations favor specific inflation scenarios over other long held ideas. Credit: NASA/WMAP