If our universe didn't exist before the big bang, the big bang can not have taken place in our universe, but in some external space. How does current physics explain this?
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3$\begingroup$ Effectively a duplicate of Did spacetime start at the big bang and multiple other similar questions $\endgroup$– Kyle KanosCommented Oct 12, 2015 at 18:40
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$\begingroup$ @HDE226868 No current theory predicts that the universe did not exist before the big bang. See the multiple other similar questions. $\endgroup$– garypCommented Oct 12, 2015 at 18:44
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$\begingroup$ @HDE 226868: I find this philosophically unsatisfying, because it would then not be very far from "the universe appeared out of thin air". $\endgroup$– Alexander TorstlingCommented Oct 12, 2015 at 18:48
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$\begingroup$ @garyp I shouldn't have made such a strong statement; I should have said that the events that happened are not well known. Alexander Torstling - both the ideas of a finitely old and an infinitely old universe are a big unsatisfying, but physics has never stopped for philosophy. $\endgroup$– HDE 226868Commented Oct 12, 2015 at 18:54
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3$\begingroup$ Possible duplicate of Did spacetime start with the Big bang? $\endgroup$– CR DrostCommented Oct 12, 2015 at 19:51
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2 Answers
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The Big Bang model is based on General Relativity, a classical theory. Our observations tell us that all points of our observable universe are receding from each other, all three space dimensions are expanding, like a raisin bread expanding in the oven .
The theory of General Relativity posits a function that fits the observed data, which when extrapolated to an origin of (0,0,0,0) of the spacetime dimensions, gives all the present matter and energy a point location. This means that all points in our present universe were at the theoretically (0,0,0,0) point, the beginning.
If our universe didn't exist before the big bang, the big bang can not have taken place in our universe, but in some external space.
One can approach the beginning of the BB mathematically, and there space time is well defined up to the actual beginning. There is no need for an external space to be invoked, up to the (0,0,0,0) point.
There are various speculations of what existed before the BB, like multiple universes.
It seems reasonable that at the very small dimensions very close to (0,0,0,0) quantum mechanics has to come in. At the moment quantization of gravity and the unification of forces is at the research stage. One can speculate that due to the Heisenberg Uncertainty Principle there may exist probabilities for other universes. There is no way to validate these hypothesis.
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You don't need a larger space. Here is a simple example.
Imagine a 4d spacetime $\{(a,d,c,d):a,d,c,d\in \mathbb R\}$ and let time be the distance from the origin, then $t=\sqrt{a^2+b^2+c^2+d^2}$ and for every time $t$ there is a whole surface in 4d that corresponds to the universe at that time $t.$ And now if you ask where it expands, the answer is it expands into the future.
And if you ask about before the big bang, that's like asking what is deeper inside the earth than the center.
To do General Relativity with such a model you just need to define a metric on the 4d spacetime and a stress energy tensor and make the Einstein equation be satisfied everywhere that things aren't infinite.
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$\begingroup$ I hadn't heard "deeper than the centre of the Earth" before. Well done! $\endgroup$ Commented Feb 4, 2016 at 9:23