Different people use the term.
Big bang theory earth science definition.
It is the idea that the universe began as just a single point then expanded and stretched to grow as large as it is right now and it could still be stretching.
In 1912 vesto slipher measured the first doppler shift of a spiral nebula spiral nebula is the obsolete term for spiral galaxies and soon discovered that almost all such nebulae were receding from earth.
The big bang theory developed from observations of the structure of the universe and from theoretical considerations.
The big bang theory tells us that the entire universe was once condensed in the form of a primeval atom or a dense mass.
The big bang theory is the leading explanation about how the universe began.
Then between 10 billion to 20 billion years ago the exact timing is not.
The big bang theory represents cosmologists best attempts to reconstruct the 14 billion year story of the universe based on the sliver of existence visible today.