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I recall witnessing a discussion with Dr. Brian Cox from the University of Manchester discussing measurements that have concluded a flat universe. He brought up the suggestion that this is perhaps because the observable universe is infinitesimal relative to the actual size of the universe, while the entire universe may contain nonzero curvature. Later in the discussion, he mentions we can also view the early universe. As I understand, this can be done by looking sufficiently far away that the image we see is nearly as old as the universe itself (younger by hundreds of thousands of years).

I preface to ask this: How is it possible to look at the early universe in a universe that exceeds the observable universe? My assumption is if the universe is spatially infinite, or much larger than the observable universe, then we would not be able to view the early universe. Said another way, I assume if we can see the infant universe, then we've "looked beyond" all space in the universe, and the entirety of the universe is observable. In this case, the ability to view the infant universe is incompatible with the suggestion that the limits of the universe exceed our observability. Why is this wrong?

I am uneducated in physics beyond engineering physics courses in electromagnetics. I hold no convictions in this area. Thank you for your help.

Qmechanic
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Mitchell Easley
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  • You may find this article about cosmological expansion helpful: https://people.smp.uq.edu.au/TamaraDavis/papers/SciAm_BigBang.pdf – PM 2Ring May 05 '20 at 02:10
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    Your conclusion is incorrect. If the universe is flat and starts as infinite, our observable universe starts from a single point in this initial infinite state. So for us it makes no difference if the universe starts as infinite or as infinitely small, because our observable universe starts as infinitely small either way. We have no way of knowing if the rest of the infinite universe even exists. See this: https://physics.stackexchange.com/questions/456071/ - All this is specific to the Friedman metric and may be totally different in reality (e.g. the entire universe may be observable). – safesphere May 05 '20 at 15:43
  • I think one difficulty you're having lies in the definition of "flat", which is usually taken as meaning "having non-negligible curvature" (in other words, having gravitation that might, under certain circumstances, be inherently perceptible to us), and isn't considered to be an absolute flatness. (Inflationary cosmologies, whose space is expanding at an asymptotically-exponential rate, are considered to resolve the "problem" of why space appears flat, but only result in a "flatness" that's several decimal points beyond 0.99.) If space was totally flat, the moon wouldn't be ball-shaped. – Edouard May 12 '20 at 21:38
  • There are metrics that allow for a universe that's completely empty, but even that might only mean "having matter in such a tiny proportion of their volume that, even using all the mass / energy available in our Observable Region, it would not be perceptible by us", although they'd leave "us" being ethereal. – Edouard May 12 '20 at 21:51

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