Answer to Question #173353 in Astronomy | Astrophysics for Elaine

Question

What is big bang

Answer 1

The Big Bang is the earliest stage of the Universe that we know now. It was a time when something (super dense and super hot) began to expand. Science knows that the Big Bang is real because there are various proves of it. For example, if you toss a stone into the water, there will be concentric circles - waves from the moment when the stone reached the surface of the water. Big Bang has an analogy of such waves - thermal emission at T=2.7K. There are also gravitational waves from the Big Bang.

So, Big Bang is a moment when the Universe that is known to us, started to exists. 'What was before the Big Bang' or 'what caused something hot and dense to explode' are the questions that don't have an answer within the competence of modern science.