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I always thought that the cosmic inflation happened after the big bang (10-36 sec) and lasted for a very tiny fraction of time). Now, I recently came across a couple of articles (links below) claiming that Big Bang happened after the inflation stopped and when the reheating phase started. I'm a bit confused as I always imagined the Big Bang as "when everything started", so how can something be happened before that?

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  • $\begingroup$ It's interesting that, in his very first sentence, the author of the piece destroys one of the most linked-to Big Bang explanations on this site. I'm referring to the Physics SE answer in which the spurious claim is made that, "the Big Bang happened everywhere at once". Anyone who asks about the Big Bang theory on Physics SE and Astronomy SE is, without fail, directed to that deeply shady and weird claim. $\endgroup$ – White Prime Aug 21 '20 at 23:12
  • $\begingroup$ In his very first sentence, the author of the piece gets it wrong. If he had inserted "observable" before universe, it might have some merit. $\endgroup$ – ProfRob Aug 22 '20 at 11:26
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    $\begingroup$ By writing 10-36 seconds, I wonder if you might be reading $10^{-36}$ seconds as meaning 10-36 seconds, or somewhere between 10 and 36 seconds after the Big Bang. $10^{-36}$ seconds is 1/1000000000000000000000000000000000000 seconds. $\endgroup$ – David Hammen Aug 23 '20 at 8:11
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The big bang (a singularity 13.8 billion years ago that was the beginning of time) didn't happen at all. Inflationary cosmology consists of:

  • ΛCDM (aka "big bang") cosmology starting at a cosmological time of some small fraction of a second, preceded by

  • an inflationary epoch lasting for an unknown amount of time (but at least 50-60 e-folds), preceded by

  • there may be no way to know, because inflation erases any hints of what came before.

Times quoted as "after the big bang" are ΛCDM model times and don't count from the true beginning of time. There's a common misconception that if inflation ended $10^{-32}$ seconds "after the big bang", that it must have lasted for no longer than $10^{-32}$ seconds. In reality, it has to last longer than that in order to accomplish its goal of producing smooth starting conditions for the ΛCDM epoch, and in most (all?) specific models it lasts for far longer than the minimum time. There are eternal inflation models in which it lasts forever.

We don't know how old the universe is. We only know that the current era of (non-inflationary) expansion started about 13.8 billion years ago.

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