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I usually hear physicists say that the Big Bang was probably caused by vacuum fluctuations. However, as I understand from answers here (this one for example), what we refer to by vacuum fluctuations is the different values we get when making measurements due to the quantum nature of the system rather than fluctuations in time.

If my understanding is correct, my question then is:

What caused these fluctuations before the Big Bang when there was nothing to interact with and so no measurements were taken?

I have a degree in Physics so feel free to use equations/technical terms in your answer.

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If you understand the Big Bang as the literal beginning of the universe, then there weren't any quantum fluctuations before the Big Bang. There weren't any quantum systems before the Big Bang. There weren't any 'before Big Bang'.

And that leads to a different definition of Big Bang. Instead of saying that it was a literal begining of the Universe (which might or might not actually happen), we only understand it as the begining of the Universe as we see it. That is, not a starting point, but a transition from whatever was before to what is now. With this interpretation of the term "Big Bang", the quick initial expansion disn't happen after the Bib Bang, but it was a part of the process known as the Big Bang. If anyone says about anything before the Big Bang, it can only make sense if they understand Big Bang as the transition from something before.

And if we accept there was something before, this something might be the cause of the Big Bang. Whether it were quantum fluctuations or not, we cannot really tell, as none of our theories were tested in the conditions of Big Bang, so we don't know if they describe it accurately.

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  • $\begingroup$ Thank you! So if I understand right, if vacuum fluctuations are what caused the Big Bang then there should have been something before which caused these fluctuations. It is not something that happens spontaneously. Right? $\endgroup$
    – S.Adham
    Feb 13, 2020 at 17:55
  • $\begingroup$ @S.Adham In quantum physics there's also a possibily that some things just happen without a clear cause. Think about quantum tunneling. A particle may exesist in a bound state inside some potential well for some time, until at some random time it tunnels outside. There are some theories that the Universe may be something like that. It existed in some other state for undetermined amount of time until some quantum effect (maybe vacuum fluctuations, maybe something else) caused it to 'tunnel' to a different staten. It could happen, so it eventually happened, and that's it. $\endgroup$ Feb 13, 2020 at 21:53

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