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My question is undoubtedly naive, due to lack of background, but any answers, or simply comments with short explanations of my incorrect assumptions, would be appreciated.

I must point out this question: Why does space expansion not cause matter expansion , in case of duplication concerns. I do understand the points involved in the answers of this "normal expansion" question, but my question involves the inflationary period only, not the current observed expansion, as I feel it can be easily argued that the proposed possible inflation of the very early universe was a completely different phenomenon.

My question is: If the normal 3 spatial dimensions that define the current volume of the universe were given a "kick start" in their initial expansion by the inflation process, why were the other proposed 6 dimensions of Calabi-Yau spaces not also affected?

I cannot stress enough my lack of knowledge of this area of physics, but despite that my curiosity leads me to speculation on two possible answers and I wonder do they have any validity:

  • The extra dimensions only involve particles, and particles did not form until after the inflationary period ended, but this would imply that particles are involved in creating spatial dimensions, so this is probably totally incorrect.

  • The extra dimensions did expand, but were so small initially that even inflation failed to expand them to beyond their present proposed minute proportions.

The final, most likely "answer" is simply that I don't yet have enough understanding of the topic to make any sensible suggestions, but any answer (or comments including a related relevant link) that involves even a small increase in my knowledge of this topic would be appreciated.

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As far as I know, this is an open question. The technical name is the problem of "moduli stabilization" in string theory, because there some fields (called moduli) the values of which determine the size of the dimensions. The problem is that string models are consistent for a big range of these values, so indeed, it could have been that all directions are small, or all big or any crazy combination. There have been many attempts to stabilize the moduli in different string constructions, ie to find a way to predict the size of all dimensions in nature without putting them in by hand, but I think it is fair to say that none of the approaches are fully satisfactory.

One possible answer to this question, (which appears quite often when we simply don't know!), is the anthropic principle. So there might be other universes where more or fewer dimensions were decompactified (either at inflation or at different stage of the evolution) but did not lead to worlds in which life could develop. This is definitely a logical possibility, but maybe someday we will come up with a better explanation that gives further intuition on how nature works.

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  • $\begingroup$ Thanks very much for taking the time to answer. My knowledge of the field is very shallow, comparable to trying to explain to a dog how his food comes in tins, but your answer is a lot more understandable to me than I anticipated. Regards $\endgroup$ – user81619 Nov 3 '15 at 11:00

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