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This question already has an answer here:

If the spacetime of the universe is infinite now, then it must have been infinite at every point in its history, right? How then can Big Bang be an expansion of space and time? Can somebody show the compatibility of the idea of a singularity and the possible infinity of spacetime extent.

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marked as duplicate by John Rennie cosmology Feb 7 '18 at 14:09

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Infinities in physics tend to demonstrate a failing of the theory. For example the infinities that plague QFTs.

One of the last bastions of the infinite in physics was in cosmology. Einstein introduced the cosmological constant to model an infinite (in the past) static universe. If one takes finitude seriously then one could have already said that the universe had a finite extent in the past (and hence a beginning), and a finite extent in space. And if it had a beginning then its plausible to say that it must have grown or expanded. For its hard to imagine the universe, as large as it is now, appearing all at once; but easier if it appeared at some point.

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