big-bang model,widely held theory of the evolution of the
universe. Its essential feature is the emergence of the universe
from a state of extremely high temperature and density—the so-
called big bang that occurred at least 10,000,000,000 13.8
billion years ago. Although this type of universe was proposed
by Alexander Russian mathematician Aleksandr Friedmann and Abbé
Belgian astronomer Georges Lemaître in the 1920s, the modern
version was developed by Russian-born American physicist George
Gamow and colleagues in the 1940s.  The big-bang model is based
on two assumptions. The first is that Albert Einstein’s general
theory of relativity correctly describes the gravitational
interaction of all matter. The second assumption, called the
cosmological principle, states that an observer’s view of the
universe depends neither on the direction in which he looks nor
on his location. This principle applies only to the large-scale
properties of the universe, but it does imply that the universe
has no edge, so that the b