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I have often read that our universe was once small enough to be subjected to quantum mechanical effects, potentially altering how our universe turned out. This is a large theme in Laura Mersini-Houghton's Before the Big Bang. Michio Kaku also mentions this in his book Parallel Worlds.

What exactly are they referring to? The initial singularity? The observable universe shortly after the big bang? The inflaton particle that inflated space giving rise to our universe?

It is my understanding that the universe might have always been spatially infinite after emerging from the initial singularity so I'm not quite understanding what aspect of the universe was subject to QM.

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  • $\begingroup$ Don Lincoln from FermiLab has a series of videos. A few are on the Big Bang. This one may help. What really happened at the Big Bang? $\endgroup$
    – mmesser314
    May 10, 2023 at 22:59
  • $\begingroup$ "It is my understanding that the universe might have always been spatially infinite" not in the mainstream Big Bang model.en.wikipedia.org/wiki/Expansion_of_the_universe $\endgroup$
    – anna v
    May 11, 2023 at 4:24
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    $\begingroup$ It seems odd to say that the universe was "once" subject to QM, since QM is required to describe the universe's behavior right now. $\endgroup$
    – g s
    May 11, 2023 at 15:07
  • $\begingroup$ Two quotes from Parallel Worlds -"The universe was once smaller than an atom. When we apply the quantum theory to the universe, we are then forced to admit the possibility that the universe exists simultaneously in many states." The second quote is "However, the universe, when it was very young was smaller than a subatomic particle. Therefore, perhaps the universe itself has a wave function. Since the electron can exist in many states at the same time, and since the universe was smaller than an electron, perhaps the universe also existed in many states, described by a super wave function. $\endgroup$
    – cosmicpawn
    May 11, 2023 at 16:26
  • $\begingroup$ @cosmicpawn Per FLRW, if the universe is closed, it expands from a zero size (Big Bang singularity) to some maximal size and then contracts back to a zero size (Big Crunch singularity). If the universe is flat or open, it starts from the infinite size of the infinite density and infinite mass (a complete nonsense) and then expands forever. When the measurements showed that the universe was likely flat, all Wiki pages were rewritten overnight to remove any reference to the small size, but you can still see it in other sources. FLRW is a failed cosmology, so none of this matters anymore. $\endgroup$
    – safesphere
    May 14, 2023 at 1:12

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Generally when it's stated that the universe was very small shortly after the big bang it refers to the size of the observable universe (option 2). This is the region from which light has had sufficient time to reach us on Earth at present. An overall infinite universe is not inconsistent with the fact the observable universe was much smaller in the past.

The cosmic scale factor describes the relative expansion of the universe. The initial singularity in the theory refers to where this was zero and any region of the universe was infinitely smaller than it is at present. This is considered an indication that cosmological theory breaks down due to failure to describe quantum gravity at the very earliest times $\sim 10^{-43}$ s.

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  • $\begingroup$ I posted a few of the quotes my question refers to above. In your answer you state that the observable universe is what they are saying was once that small. I want to make sure I'm understanding correctly. You're saying that the observable universe and all of the matter/energy it contains was once in an area so small that it could possibly be described with a wave function? $\endgroup$
    – cosmicpawn
    May 11, 2023 at 16:31
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    $\begingroup$ @cosmicpawn I think the quoted claim is sufficiently imprecise that it's best to just ignore it entirely. What does it mean to have a "wavefunction" if you don't have a model that works properly at that energy density? Which observable universe, observable in what frame, when? If universes can have wavefunctions, and we don't have to know the function anyway, why impose an arbitrary size limit or temperature floor - nothing in QM ever stops us in principle from adding another particle. Superposed states for which observer? Whose past are we talking about, anyway? $\endgroup$
    – g s
    May 11, 2023 at 19:02
  • $\begingroup$ @cosmicpawn The observable universe has a well-defined meaning (see en.wikipedia.org/wiki/Observable_universe). When talking about the apparent big bang singularity, my point was that this is probably due to the neglecting quantum gravity (see simple.wikipedia.org/wiki/Planck_epoch). $\endgroup$
    – FTT
    May 11, 2023 at 22:18
  • $\begingroup$ @cosmicpawn While the observable universe stared small, note that what the observable universe is depends on the observer. If you are the observer, then the observable universe for you left eye is different than the one for your right eye and they did not start from the same small area, bur from 2 different ones. Also note that the so-called “observable universe” is a concept of the failed FLRW cosmology, but even in this cosmology all universe is observable, if there is no “dark energy”. And since “dark energy” has been made up to save the failing FLRW, in reality, all universe is observable. $\endgroup$
    – safesphere
    May 14, 2023 at 1:39

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