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I saw the excellent answer here: Did the Big Bang happen at a point? but I have a hard time imagining the initial state. If the distances between all points in the universe were zero at the Big Bang, how is it not a single point in space?

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  • $\begingroup$ The distance between neighboring points is always zero in a continuum. You are basically just mistaking physics for mathematical minutia that require topology to sort out properly. In reality there was always a finite density, we just don't know what it was. $\endgroup$ Jun 6, 2023 at 21:35
  • $\begingroup$ Thanks for downvoting my question, that's helpful $\endgroup$
    – Goose
    Jun 6, 2023 at 22:16
  • $\begingroup$ Don Lincoln has a number of videos on the Big Bang. This is a good one. What really happened at the Big Bang? $\endgroup$
    – mmesser314
    Jun 7, 2023 at 1:05
  • $\begingroup$ The James Web Space Telescope's very recent discovery of galaxies much older and larger than expected (during a period that had been presumed to be the "Dark Ages", preceding the separation of photons from an extremely hot plasma) appears to be causing a very rapid decline in the acceptance of the "Big Bang" concept as representing a beginning of time, and an increase in the acceptance of past eternality as a concept that may be more plausible and less superstitious. Models supporting that view have been posited by Aguirre & Gratton, Poplawski, Steinhardt & Turok, & others. $\endgroup$
    – Edouard
    Jun 7, 2023 at 15:13
  • $\begingroup$ I should add that some of those views support local "Big Bangs" in a "multiverse", but at least one (Nobel winner Roger Penrose's past- & future-eternal "Conformal Cyclic Cosmology") doesn't. $\endgroup$
    – Edouard
    Jun 7, 2023 at 15:36

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We don't actually know what happened right at time 0 -- our present theories give a singularity there, an undefined point like dividing by 0. So it's not accurate to say it's a point, it's just something undefined. There is hope that a quantum theory of gravity might allow us to describe the beginning of the universe better, for example by providing a maximum density or other condition. But for any time $\epsilon > 0$ we can find in classical models a finite density of the universe, and space at that time is fully 3 dimensional. (It should be noted that the particular choice of time coordinates is somewhat arbitrary -- time is relative after all -- but the usual choice in cosmology is known as cosmic time in which the average density of the universe is uniform.)

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  • $\begingroup$ That makes sense. I just want to understand what a pure "big bang" model means. If we do assume a big bang initial state with infinite density - I understand that it still doesn't mean a point in space. So I'm trying to understand what it means, given that distances between all objects are zero. If I imagine a blob of matter with high density, different points on it have finite distances from each other. And if I imagine these distances go to zero, then it becomes a point in space... $\endgroup$
    – Goose
    Jun 7, 2023 at 1:31
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We just mean that there are points of the universe outside of our observable universe, and those can be slightly outside of the position of the Big Bang that we all started from.

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