When did the big bang happen?

Martin asked:

If the Big Bang was a reality, where exactly did it happen?

We are led to believe that the Big Bang took place between 10 and 20 billion years ago a humongous explosion at a point in the universe, resulting in the expanding state that we recognize today. For this to be true there must be an ever-increasing ‘dead zone’ centred around the point where the Big Bang took place, void of everything as all the exploding arisings race away from the original point so this would mean the universe is hollow and have we found the dead zone?

Answer by Craig Skinner

Your error is to think of the BB as starting at a point in the universe, proceeding like an explosion with things flying apart through space producing, as you say, a central hollow. The ‘point’ at which the BB began was the whole universe, and it is this which has expanded, not things exploding outward but space itself expanding. So, wherever you stand in the universe, the other galaxy clusters all recede from you giving you the impression you are at the centre. Everything recedes from everything else, there is no centre, hollow or otherwise. The usual 2-D model to aid understanding is a balloon on which you paint dots, each representing a galaxy cluster. Blow up the balloon and as it stretches the dots all move away from each other. Note that I say galaxy clusters recede from each other. Within galaxies and clusters, gravity counteracts spatial expansion, so that, since you started reading this, the distance between you and the door hasn’t expanded, or the distance to the sun or to Andromeda. Where you are right is that continued expansion does mean galaxy clusters all get farther and farther apart from one another, eventually beyond each other’s horizons so that we could never see them, or them us.

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