big bang theory

noun Astronomy.
a theory that deduces a cataclysmic birth of the universe (big bang) from the observed expansion of the universe, cosmic background radiation, abundance of the elements, and the laws of physics.
Also called big-bang model.


Origin:
1950–55

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World English Dictionary
big bang
 
n
1.  any sudden forceful beginning or radical change
2.  (modifier) of or relating to the big-bang theory
3.  (sometimes capitals) the major modernization that took place on the London Stock Exchange on Oct 27 1986, after which the distinction between jobbers and brokers was abolished and operations became fully computerized

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Etymonline
Word Origin & History

big bang
hypothetical explosive beginning of the universe, developed from the work of Monsignor Georges Henri Joseph Édouard Lemaître and George Gamow, the name first attested 1950 (said to have been used orally 1949) by British astronomer Fred Hoyle (1915-2001) in an attempt to explain the idea
in laymen's terms.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
big bang  [%PREMIUM_LINK%]     (bĭg)  Pronunciation Key 
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.

Our Living Language  : 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.

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Cultural Dictionary

Big Bang theory definition


In astronomy, a theory according to which the universe began billions of years ago in a single event, similar to an explosion. There is evidence for the Big Bang theory in the observed red shift of distant galaxies, which indicates that they are moving away from the Earth, in the existence of cosmic microwave background, and from other data. The Big Bang theory of the origin of the universe is accepted by most astronomers today.

Note: Scientists have recently found that the expansion of the universe is actually speeding up. This effect is attributed to the presence of dark energy.
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Example sentences
By that, they mean that each cycle ends with a big bang that starts the next
  cycle.
It would also mean that possibly trillions of galaxies were already formed and
  in close proximity to the big bang.
Besides, the cosmological evidence suggests a beginning: the big bang.
These, he calculated, had been created within the first hundred-quintillionth
  of a second after the big bang.
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