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My point of view is that first there was the Big Bang singularity, and then the period of inflation which resulted in the observable Universe becoming many times bigger. But was the Universe expanding between the Big Bang singularity and the beginning of inflation?

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  • $\begingroup$ Are they even certain the Big Bang was a singularity? I think they're leaning towards an infinite universe now, enormous expansion, but not necessarily from a singularity. The Hubble expansion implied a singularity so it was believed for a while, but the infinite universe suggests not a singularity . . . er, I think. Not 100% sure though. :-) $\endgroup$ – userLTK May 21 '15 at 0:26
  • $\begingroup$ The current theory of inflation has the universe at a radius of 15 Mpc after $10^{-34}$ sec. Does it really matter if the universe was slowly expanding after creation and before the inflation? How long could the period possible be if the inflation took most of the time? $\endgroup$ – LDC3 Jul 19 '15 at 21:59
  • $\begingroup$ @LDC3 In what sense did the universe have a 15 MPc radius? The "visible universe" couldn't have been larger than light travel time distance, right? $\endgroup$ – LocalFluff Jul 20 '15 at 5:32
  • $\begingroup$ @LocalFluff It's true that the "visible universe" is no larger than light travel time distance, but the universe can easily be larger than that. See pages.uoregon.edu/jimbrau/BrauImNew/Chap27/7th/… $\endgroup$ – LDC3 Jul 20 '15 at 6:02
  • $\begingroup$ @LDC3 Unless universe is infinite, and there's no indication of it being curved. Space can expand anyway, but universe would never have a size. $\endgroup$ – LocalFluff Jul 20 '15 at 6:59
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This is likely unanswerable. Inflation was in part a resolution to a fine tuning problem: without it, it seemed we needed the early universe in a very specific and precisely balanced state to get to where it is today, and there was no solid scientific way to explain why things were so perfectly arranged (other than to simply assert that they were).

With inflation, the state of the universe before the inflationary epoch is fairly irrelevant. Mostly all it needs is that the region that inflates into what we will know as the observable universe had enough time to achieve thermal equilibrium (which we need to explain why the universe looks pretty much the same in all directions, exactly as if things well outside of light speed communication had nonetheless achieved thermal equilibrium at some point). Curvature and such get "smeared out" by the inflation to give us the mostly flat and homogeneous universe we see today.

On a pedantic note, some researchers consider inflation and the big bang to be the same thing. At least in the sense that talking about "before inflation" is scientifically meaningless, so if the Big Bang is the (scientific) beginning then we might as well take it to be inflation.

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Inflation has started with the bigbang. In fact for anything to take place you need an expansion right away. If you had a static universe just after the bigbang, it couldn't have formed subatomic particles or atoms, without any increase in size. Inflation was always happening and will always happen. The process is behaving (in terms of duration) like radioactive material, that means that inflation slowy decays wth a halflife of about 10^-32 seconds. I hope this answer suffices.

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