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Its quite stunning to assume that not only space-time is expanding but also the rate of expansion is greather that speed of light.

But after the initial surprise, I ve been wondering if that fenomena would not be completely fundamental and inherent to the existence of the universe. I mean, would it be possible for a Universe to exist with expansion rate lower than speed of light? Or even more, would it be possible to exist a universe without expansion?

Let me explain, maybe you can help me with a better explanation or correct the unaccuracies in my arguments.

Imagine a photon originated early in the formation of the universe. Lets say that a certain point of time of the universe, when this photon was originated, it started traveling through space time at speed of light. Even if universe would be expanding but a lower rate than speed of light, this traveling photon at c will always reach, at some point in future space time, the, lets say,’ edges of the universe’, the limits because it would had not enough time for Universe to grow to let ‘more space time’ to the photon to travel. What would happen in this case? What would be the behavior of a photon at the ‘edge of the universe’. It looks like a kind of impossible or difficult to explain situation. So an expanding universe a rate >c its just the only viable possibility for universe to exist.

Qmechanic
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Jordi Martí
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    What edge? Please see https://physics.stackexchange.com/q/136860/123208 – PM 2Ring May 24 '21 at 07:56
  • Also, the expansion speed isn't a single value, it depends on the distance between the two points. It's roughly 70 km/s per megaparsec. Eg, 2 galaxies that are 50 Mpc apart are expanding away from each other at 350 km/s. – PM 2Ring May 24 '21 at 08:01
  • So let say, talking about the shape of universe. If its closed , with expansion rate lower than c, we would see in the sky infinite versions of this photon? – Jordi Martí May 24 '21 at 08:22
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    Well, you can only see a given photon once, since seeing it absorbs it. But sure, if a closed universe were small enough then you'd get multiple images of a galaxy at different apparent distances. – PM 2Ring May 24 '21 at 08:28
  • Thats the point in my question. ‘Small enough’ would not care as long as expansion rate is <c. With expansion <c , there would be always ‘time’ for the photon to close the loop. – Jordi Martí May 24 '21 at 08:36
  • Expansion is not relative motion: It's something getting larger. The universe isn't something: It's everything. Everything doesn't actually get larger, or isotropy would be total, and everything would look like everything else. That your question brought that out is why I rated it as a good one. – Edouard May 24 '21 at 17:43
  • I'm saying "universe" in the sense used before inflationary models were hypothesized, but I figure it's what you mean because you didn't say "local universe", "pocket universe", "bubble universe", or "iteration": In the context of those models, the pre-1980's universe would be a "multiverse". – Edouard May 24 '21 at 18:26
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    Expansion in the sense of new "space time" creation. is there any calculation of how much is this expansion at infinitesimal or atom level? – Jordi Martí May 24 '21 at 21:20
  • what would be the " cut expansion rate " from a inflationary universe where two starting points separated 1 m that would been disconected causally from each one? – Jordi Martí May 24 '21 at 21:36
  • maybe it has more sense to determine at which point in time in the age of the universe these two points became disconnected – Jordi Martí May 24 '21 at 21:40
  • The atom level's described here & there as a "cosmic substratum" of helium & hydrogen atoms. Neither the spacetime nor its contents would necessarily be "new", as mechanisms for expansion (some of them, such as those described by Nikodem Poplawski in many 2010-2021 papers whose preprints are at arxiv.org/a/poplawski_n_1.html, using torsion in an updated version of relativity known as Einstein-Cartan Theory that takes particulate spin into account, and others, such as the cyclic cosmology devised by 2020 Nobel winner R. Penrose) are used in past- & future-eternal models. – Edouard 9 mins ago – Edouard May 24 '21 at 22:20

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