The Big Bang is the name of a scientific theory that explains how the Universe started, and then made the groups of stars (called galaxies) we see today.
In the Big Bang theory, the universe begins as very hot, small and dense, with no stars, atoms, form, or structure (called a "singularity"). Then about 14 billion years ago. the space in the universe expanded very very quickly (like a big bang), and later atoms formed, and then the stars and their galaxies. The universe is still expanding today, and getting bigger, but colder.
As a whole, space is growing and the temperature is falling as time passes. Cosmology is the name given to how the universe began and how it has developed. Scientists that study cosmology agree the Big Bang theory matches what they have seen so far.
Fred Hoyle called the theory the "Big Bang" on his radio show. He did not believe the Big Bang was correct. Scientists who did not agree with him thought the name was funny and used it. Since then, Fred Hoyle's reasons for not liking the theory have been shown to be wrong.
Scientists base the Big Bang theory on many different observations. The most important is the redshift of very far away galaxies. Redshift is when the light from an object moving away from the earth looks like it has lost energy. Objects moving towards the earth look like their light has gained energy. This is because of the Doppler effect. The more redshift there is, the faster the object is moving away. By measuring the redshift we can work out how fast the object is moving. Since everything is moving away from everything else at a carefully measured rate, scientists calculate that everything was in the same place 13.7 billion years ago. Because most things become colder when they become bigger, the universe must have been very hot when it started.
Other observations that support the Big Bang theory are the amounts of chemical elements in the universe. Amounts of hydrogen, helium, and lithium seem to agree with the theory of the Big Bang. Scientists also have found "cosmic microwave background radiation". This radiation is radio waves that are everywhere in the universe. It is now very weak and cold, but a long time ago it was very strong and very hot.
The Big Bang might also have been the beginning of time. If the Big Bang was the beginning of time then there was no universe before the Big Bang. Other ideas that also have a Big Bang do not have a beginning of time at 13.7 billion years ago. Instead, these theories say that the beginning of the universe as we currently know it began at that time. Before then the universe may have been very different.
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