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A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me.

What I can't grasp yet is how a quantum fluctuation can even start without an existing universe. Aren't all physical laws created with the universe? I understand that there is no notion of "before" with respect to time, however the big bang is theorised to have occurred, but for that to occur there must have first existed something right?

I wonder also, if there was a more nothingness instead of vacuum before the universe existed and how a quantum fluctuation could have started really from ex nihilo instead of a vacuum.

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touche my friend, +1 – lurscher Dec 2 '11 at 3:21
""A theory among scientists"" First read about what a theory is! And second I can't remember that some serious scientist did not mention that any thoughts/questions on before BB are free of sense? – Georg Dec 2 '11 at 11:15
I know what a theory is and I agree it's nonsense to ask what is north of the north pole. However, my question is how quantum fluctuations can happen altough they shouldn't even exist yet. Is that a question free of sense? Please explain that to me. – Sven Dec 2 '11 at 11:50
Sidney Coleman is known as the source of the idea of quantum fluctuations as a framework for getting the universe out of nothing, but he meant it as a guide or raw principle in the same spirit that Mach principle was a informal guide. Obviously as a guide, doesn't Have to make sense. – lurscher Dec 2 '11 at 18:16
A guide, an idea, but not a theory! BTW, most thoughts on something before BB are therories in which the rebounce is not a real BB in the original sense. – Georg Dec 2 '11 at 19:10

Indeed, there exist some pre-big-bang theories that arose from string theory. A notable name in this area is Gabriele Veneziano. You can find some information here. This article is somehow technical but should convey the right flavor of these ideas. As you can read from this, there is a two-dimensional space where strings live and a target space, the one we live on, that should start, with inflation. Quantum fluctuations that start the big-bang are happening on the two-dimensional space. These theories have some problems, e.g. the graceful exit problem from the inflationary phase, but are an active field of study.

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It is possible the number of degrees of freedom of the universe goes up as the universe expands. The larger the volume, the more degrees of freedom there are. A distinction most scientists don't make is between determinism and reversibility. Determinism states that if the universe is in state A at time t, in principle it will be in a uniquely determined state at time t' provided that t' comes later than t. Reversibility states that this will also be so if t' comes before t. Our universe is not classical by any means, but the density matrix describing our universe evolves deterministically. If the number of degrees of freedom is going up, this evolution might not be reversible. The Lindblad master equation for density matrices can handle this sort of thing. If the universe started off from a big bang of zero volume and no degrees of freedom evolving under Lindblad dynamics, there is no reversibility. It is entirely possible that there might not actually be any state at time t' which can evolve into state A at a later time t. Hence, one cannot infer the existence of the past of a state even though one can always infer the existence of a future state.

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I'm sorry, but I didn't understand a word.. – Sven Dec 2 '11 at 14:33
+1: This makes sense, but is very, very speculative. It is a possible way to understand what the cosmological horizon means in an expanding dS space. I don't understand the downvotes, this isn't nonsense. The "Lindblad master equation" is just the most general linear operator on the density matrix, while reversibility loss would be intrinsic to compact cosmological horizons. I have often thought such things, but I have never had the balls to say them out loud. – Ron Maimon Dec 3 '11 at 7:41
+1. This answer is ok. – centralcharge Jun 30 '13 at 6:29

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