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What triggered the Big Bang?

What triggered the Big Bang?
Roger Inkpen from Portsmouth (Age 45-54)

How and why did the Big Bang start?
James Kelly from Nottinghamshire (Age: 5-14)

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One Response

  1. Hi there!

    The short answer is this: we don’t know!

    Here’s a slightly longer-answer, with a bit of speculation at the end:

    The question of what happened at the big bang, or before the big bang, or even right after the big bang is difficult because it’s the intersection of two theories that don’t seem to play well together: general relativity and quantum theory. General relativity describes the universe well on large scales, where it is able to tell us about the geometry (shape) of the universe. It’s been tested many times over the past century; and while there are still plenty of questions to answer about cosmology, we’re quite happy with GR as a framework for the large-scale geometry of spacetime. Cosmological observations within the framework of general relativity describe a point in the distant past call the big bang. In a nutshell, because the universe is expanding, we can `play the tape backwards’ and expect to see a time when the universe was infinitesimally tiny.

    However, we don’t expect general relativity to still be valid at such very tiny scales. At such scales, quantum theory takes over as our well-tested theory describing nature at that scale. Quantum theory is very different from general relativity and itself makes very strange predictions.

    At the big bang (and also near the centre of black holes), we need to have a description of nature that combines the geometry of relativity and the small-scale predictions of quantum theory. Unfortunately we don’t quite have such a theory, though there are many theoretical physicists who think about this problem.

    Let me offer two very speculative *possible* answers. These are hypotheses put out by some of our leading cosmologists that are interesting ideas, but are still very, very speculative:

    (1) It is possible that our universe emerged out of a black-hole like singularity of another universe. This is related to Stanford cosmologist Andrei Linde’s multiverse scenario.

    (2) It is possible that the universe is cyclic, after a big bang, it expands, then contracts, and then `crunches’, only to bang once again. This is a model proposed by Princeton cosmologist Paul Steinhardt and Cambridge cosmologist Neil Turok.

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