The explosion of an extremely small, hot, and dense body of matter that, according to some cosmological theories, gave rise to the universe between 12 and 20 billion years ago. Compare
big crunch, steady state theory. See also
open universe.
In the 1920s astronomer Edwin Hubble discovered that wherever one looked in space, distant galaxies were rapidly moving away from Earth, and the more distant the galaxy the greater its speed. Through this observation he determined that the universe was becoming larger. Hubble also found that the ratio between a galaxy's distance and velocity (speed and direction of travel) was constant; this value is called the
Hubble constant. By calculating the distance and velocity of various galaxies and working backward, astronomers could determine how long ago the expansion began—in other words, the age of the universe. The figure, which scientists are constantly refining, is currently thought to be between 12 and 20 billion years. According to the widely accepted theory of the
big bang, the universe was originally smaller than a dime and almost infinitely dense. A massive explosion, which kicked off the expansion, was the origin of all known space, matter, energy, and time. Scientists are also attempting to calculate how much mass the universe contains in order to predict its future. If there is enough mass, the gravity attracting all its pieces to each other will eventually stop the expansion and pull the universe back together in a
big crunch. There may not be enough mass, however, to result in an eventual collapse. If that is the case, then the universe will expand forever, and all galaxies and matter will drift apart, eventually becoming dark and cold.