There are arguers that say the Universe is streching about every its point but also tell it is infinite. How it can possibly be?Maybe we should reinterprete what infinity means.
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$\begingroup$ astronomy.stackexchange.com/q/12576 $\endgroup$– leonbloyCommented Sep 8, 2020 at 11:34
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$\begingroup$ Are you aware of the $k=0$ Friedmann metric? If so, please clarify what you don’t understand. $\endgroup$– G. SmithCommented Sep 8, 2020 at 16:58
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2 Answers
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Imagine a grid. As a fishing net. Imagine it is infinitely large; it spans to infinity in all directions.
Now stretch this fishing net, so all net cells become larger. It is still infinitely large, because the concept of infinity is abstract and not a mathematical point that you can pass. But now all the "content" within this infinitely large grid has grown and expanded.
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The expansion regards the mutual distance of far physical objects, more precisely galaxies or clusters of galaxies.
Universe expansion of modern cosmological models, when the space sections of the spacetime are open, just means that the relative distance of far bodies evolving along preferred worldlines increases in time, isotropically and uniformly in space.
It does not matter if the space where all that happens is infinite.
When dealing with models with closed (compact) spatial sections, the above presented expansion also implies an expansion of the total volume of the space, but this is just a consequence.
answered Sep 8, 2020 at 11:52