big bang theory

American  

noun

Astronomy.

1. 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.

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big-bang theory British  

noun

1. a cosmological theory postulating that approximately 12 billion years ago all the matter of the universe, packed into a small superdense mass, was hurled in all directions by a cataclysmic explosion. As the fragments slowed down, the galaxies and stars evolved but the universe is still expanding Compare steady-state theory

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Big Bang theory Cultural  

1. 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.

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Discover More

Scientists have recently found that the expansion of the universe is actually speeding up. This effect is attributed to the presence of dark energy.

Etymology

Origin of big bang theory

First recorded in 1950–55

Example Sentences

When we first pitched the show to Casey Bloys and the creative team at HBO, we said it would be “glacially slow, with no big bang theory of dramatics.”

From Los Angeles Times • Jun. 17, 2025

We now possess an excellent description of how it all started: the big bang theory has successfully explained the features of our universe.

From Scientific American • Jul. 1, 2022

He says the big bang theory is a “paradigm-smashing concept to end all paradigm-smashing concepts” because it demonstrates that the universe had a beginning.

From Washington Times • Oct. 13, 2021

In the future, new observations will surely force cosmologists to tweak the big bang theory.

From Scientific American • Apr. 11, 2020

But Gran didn’t tolerate ideas like evolution or the big bang theory, despite the evidence.

From "Sir Fig Newton and the Science of Persistence" by Sonja Thomas