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It is my understanding that the universe is expanding and that matter takes up space. While the universe only contains small percent of matter, wouldn't expansion indicate that the universe is encroaching/growing into other space(other universes, dark matter, dark energy, something)?

What is the universe expanding into (or don't we know)? Or is there some other way of understanding the expanding universe?

Related question about universe itself, not outside of it (unless I misread/misunderstood it)

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  • $\begingroup$ Nobel Prize winner John Mather has mentioned that "the universe is expanding into itself": To me, that suggests that the scales of space and time must be decreasing, but the only cosmological model I've seen which even seems to imply that, is Nikodem Poplawski's "cosmology with torsion". It's described in numerous preprints (many of them later published), written between 2010 and 2021, which can be found by his name on Cornell University's << Arxiv >> website. The papers actually describe causally-separated "local universes", whose shape he's analogized to "the skin of a basketball". $\endgroup$
    – Edouard
    Commented Sep 14, 2022 at 3:20
  • $\begingroup$ If Poplawski's right, the acceleration of spatial expansion (observed in the SN1a studies of the late 1990's) would seem to be a reduction in the rate of spatial contraction, although that's "original work" that did not originate with Poplawski.... $\endgroup$
    – Edouard
    Commented Sep 14, 2022 at 3:31

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I find it helpful, when considering a universe that may be infinite, to think of everything within space getting smaller, rather than space itself getting "bigger". Rulers, planets, stars, galaxies. That allows visualisation of an expanding universe without having to worry about the "edges". Not very scientific as it assumes space itself is empty, but it eases my brain!

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    $\begingroup$ several articles I've seen seem to indicate the universe has a size one source. While the edges are probably not clearly defined, what was in the 'space' before the universe reached that point? $\endgroup$
    – depperm
    Commented Jul 15, 2021 at 19:35
  • $\begingroup$ @depperm--In Poplawski's model, as analyzed in a paper titled "Big Bounce or Double Bang?" by David Lindford, the answer is "something", because the multiverse containing the local universes is past- and future-eternal. $\endgroup$
    – Edouard
    Commented Sep 14, 2022 at 3:35
  • $\begingroup$ The "something" would be "time", for beings existing only in space, or "space", for beings existing only in time: Since we exist in both, it would account for the truism that "space and time exchange roles at the event horizon of a black hole". (Poplawski's model is a version of the "black hole genesis" of local universes that was first formulated by the physicist Lee Smolin: It provides a mechanism for the operation of black hole genesis.) $\endgroup$
    – Edouard
    Commented Sep 14, 2022 at 3:55
  • $\begingroup$ Sorry, but I had one of the author's names wrong: It's Daniel Linford, & his paper's preprint's also found by his name on Arxiv. $\endgroup$
    – Edouard
    Commented Sep 14, 2022 at 4:13

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