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When the size of the observable universe was small as a proton what was the physical content of it?

Was there a limit of particles in such a tiny volume with enormous high density and energy?

Can we find more than one proton inside such a volume, can we assume that because an electron, a quark, or a photon are point particles there was no maximum limit for the number of say electrons instide a volume the size of a proton?

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    $\begingroup$ The universe may be infinite in size, and if so, it was always infinite after the start of the Big Bang. See physics.stackexchange.com/q/346750/123208 However, the region which became what is now our observable universe was certainly quite small a tiny fraction of a second after the Big Bang started, when the universe was too hot for our familiar matter (and antimatter) particles to exist. $\endgroup$
    – PM 2Ring
    Commented May 1, 2023 at 13:50
  • $\begingroup$ So only photons existed back then? Absolutely no matter particle when the size of the observable universe was as small as the size of the proton? $\endgroup$
    – VVM
    Commented May 1, 2023 at 15:20
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    $\begingroup$ During the Grand unification epoch it was too hot even for photons. See en.wikipedia.org/wiki/… & the following sections. $\endgroup$
    – PM 2Ring
    Commented May 1, 2023 at 15:29
  • $\begingroup$ What I particularly like about this question is the fact that it brings out the difference between two of Einstein's relativistic theories, General Relativity and Einstein-Cartan Theory. The latter, accepted by Einstein in a 1929 telephone conversation with the mathematician Cartan, is described at en.wikipedia.org/wiki/Einstein%E2%80%93Cartan_theory, and prescribes a finite spatial extent (several orders of magnitude greater than the Planck length) for fermions. In focusing on photons and leaving out a GR tag, the OP seems to be aware of ECT, but PSE currently lacks a tag for it. $\endgroup$
    – Edouard
    Commented May 1, 2023 at 16:44
  • $\begingroup$ Eternality to the past seems to be implied by ECT, and was accepted as a possibility by Borde, Guth, and Vilenkin, in the last footnote to their last (2003's) revision to their Borde-Guth-Vilenkin Theorem. $\endgroup$
    – Edouard
    Commented May 1, 2023 at 16:52

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According to current cosmology, there were no particles at that time, and it’s not even clear that the universe ever was that small. We don’t know much about the physics that prevailed during the inflationary period, and at the end of that period when it becomes meaningful to talk about particles, the universe was already far larger than a single proton, perhaps around the size of a grapefruit, though that estimate is extremely speculative.

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  • $\begingroup$ I could ask the same question about the time when the size was big as a grapefruit, was there any max limit on the number of different types of particles? $\endgroup$
    – VVM
    Commented May 1, 2023 at 11:56

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