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As per my understanding of the big bang theory, it stipulates that the entire universe was contained in this single point of "pure energy" and that the big bang happened when the universe rapidly expanded out. This, again as per my understanding, is derived from the observation that the universe seems to be expanding right now (Red-shift etc.) and so if we were to go back in time, the universe would be smaller and at some point back in the past, the universe would've been contained in one single point.
However, my question is that if the universe turns out to be infinite then how can something infinite be smaller or contained in a single point? If you go back in time, the universe should be the same size as it is now (infinite) because it has always been infinite. An expanding universe would not be infinite but be finite, you would be able to traverse the entire diameter of it. A universe that expands out of something has a boundary, and so by definition should not be infinite. So, if the universe turns out to be infinite, will we have to look for an alternative to the big bang theory? I hope my question based on a meagre understanding of physics is clear.