If we assume the universe is a closed 3-sphere, how big was the universe after inflation, compared to nowadays? Was, relatively, most space already there? If we envision the universe as a 2-sphere, a balloon, how big was the balloon back then compared to its present size, assuming the universe continues at least 500 times the radius of the observable universe behind the horizon?

  • 2
    $\begingroup$ Note that the big bang did not happen at a point. It makes sense to ask what the size of the observable universe was after inflation ended, but not what the size of the universe was. $\endgroup$ Commented Jan 25 at 12:22
  • $\begingroup$ The estimates come from the idea that inflation is responsible for the flatness of the universe. There are IMHO credible scaling arguments for the notion that the universe has to be globally flat no matter what, so the "flatness problem" may not even exist to begin with. I can't comment on the other reasons for the model (like magnetic monopoles), but personally I would not rule out the possibility that inflation is the wrong explanation for the observations. It might be more of a Bohr atom model kind of intermediate step in our understanding of the early current era of the universe. $\endgroup$ Commented Jan 25 at 14:29
  • $\begingroup$ Was, relatively, most space already there? What does that mean? $\endgroup$
    – Ghoster
    Commented Jan 25 at 19:54
  • $\begingroup$ @Ghoster What I mean is if the size of the balloon (if we assume the universe to be a 3-sphere and look at it in 2D equivalent to be able to imagine), after inflation, was relatively not so different from the size it has nowadays. $\endgroup$
    – Il Guercio
    Commented Jan 26 at 0:13
  • $\begingroup$ It was not anywhere near the size today. It was on the order of meters then, not on the order of billions of light years like now. The universe has expanded by a linear factor of about 1,000,000,000,000,000,000,000,000,000 since inflation ended. $\endgroup$
    – Ghoster
    Commented Jan 26 at 0:24