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Astronomy

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    $\begingroup$ Oh, and who says that? Earliest Universe we can tell anything would be perhaps having density comparable to Planck density - that's completely different situation than you say. $\endgroup$
    – Mithoron
    Commented Apr 20, 2023 at 18:50
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    $\begingroup$ I don't think this assumed. Indeed the usual assumption is that the universe was always infinite in extent (even if this causes philosophical problems. If the universe had a start (and there wasn't an infinite period of inflation, for example, which come with their own philosophical problems) then our current understanding of matter has difficulty with times at a scale less than one planck time We don't really have a model of reality that can deal with such time intervals, so we can't currently talk about what happened in the first planck time. $\endgroup$
    – James K
    Commented Apr 20, 2023 at 19:29
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    $\begingroup$ Don't try to treat protons as "balls", or the universe as a fixed spacetime in which a ball of protons exploded. $\endgroup$
    – James K
    Commented Apr 20, 2023 at 19:30
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    $\begingroup$ The Planck units are not units which are fundamental in some way. They're just order of magnitude constants that are roughly at levels which mark the range where we don't have good theories. They feel natural for some purposes - a little easier to manipulate some equations using these units. $\endgroup$
    – StephenG - Help Ukraine
    Commented Apr 20, 2023 at 23:14
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    $\begingroup$ There's some relevant info at Did the Big Bang happen at a point? & physics.stackexchange.com/a/185943/123208 $\endgroup$
    – PM 2Ring
    Commented Apr 21, 2023 at 3:39