# Do string theorists speculate about unfolding dimensions?

by oldman
Tags: dimensions, speculate, string, theorists, unfolding
 P: 622 I understand from popular books such as those by Brian Greene that among the stock in trade of string theorists are as-yet-unexplored dimensions that are squashed into tiny geometrical configurations called Calabi-Yao shapes. Do stringy folk ever consider a reverse process, which might be called, say, the unfolding of dimensions? Especially of ordinary ones like the two kinds that we are familiar with; time and space? If so, is it understood how the unfolding of ordinary dimensions would manifest itself to us? What would it do to metric coefficients, for example? This is a probably a well-trodden path in string theory that I am sadly ignorant about. I'm asking to be guided to a starting point in these forums, or elsewhere on the web, suitable for someone who is quite uneducated about string theory, but is not entirely mathematically illiterate.
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P: 22,349
 Quote by oldman Do stringy folk ever consider a reverse process, which might be called, say, the unfolding of dimensions?

quite a bit of concern about this. they have dreamed up ways of wrapping imaginary bandages around the rolled dimensions to prevent them from unrolling.

One hears of "branes" being used to "stabilize" the vacuum, or "fluxes" wrapped around.

It can get quite complicated. I remember reading a paper back in 2003 by someone who was worried that the rolled-up dimensions might spontaneously unroll.

I believe that the theorists now consider the problem to be all solved and taken care of. The "moduli" are stablilized by various devices.
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Probably one of the well-informed string advocates at this forum will reply to your question with more authority and detail, but I had an interesting aspect of this business which I wanted to mention, namely INFLATION!

As you know there are estimated some 10^500 different vacua because there are so many ways to roll up the extra dimensions (and keep them from unrolling).
Well, suppose you choose one particular way of rolling up and you have one particular vacuum, and now you want it to INFLATE.

Apparently this runs into nasty problems. There was an article about it this year (Max Tegmark was a co-author, also Shamit Kachru a prominent Stanford string thinker).
Paul Steinhardt commented that the article confirmed what he had found independently and that he thought the string approach and inflation were INCOMPATIBLE and that one would have to give up one or the other.

Steinhardt has done a fair amount of string research himself, and has been a leader in developing inflation ideas. But he came to the conclusion some years back that string needed some way to obviate inflation. So he and Turok dreamed up the Ekpyrotic, or Cyclic, or CLASHING BRANES scenario. For people like Steinhardt who think string and inflation are incompatible, this is a way to preserve string by getting rid of the needs for inflation.

So, oldman, this is a curious aspect of the question you raised, which might not ordinarily be mentioned. The problem of maintaining the way you have the dimensions rolled up has even led to repercussions in cosmology!

You might even say that it lies at the root of Clashing Branes cosmology! Hope you find this as interesting as I do.
Mentor
P: 8,262
 Quote by marcus You might even say that it lies at the root of Clashing Branes cosmology! Hope you find this as interesting as I do.
I agree with you marcus; I've only heard a bit about braneworld cosmology, but what I have heard is rather interesting. One of the guys in my office is working on such models and gave a seminar last week. I think the jist of it was that inflation occured when our D3brane was attracted down a throat in the extra dimensional space by an anti-D3 brane. The distance between the two branes then gave the inflaton potential (I think). Very interesting seminar, but as I'm sure you can tell, a little above me at the moment!

P: 622

## Do string theorists speculate about unfolding dimensions?

Thanks for telling me about the incompatibility of inflation and strings, Marcus. I've heard of Clashing Branes, but hadn't realised that this had implications for cosmology. I take your post to imply that keeping dimensions rolled up has been of more interest to string theorists than what happens if they unroll.

Or can inflation be thought of as a breaking loose from "bandages" and sudden unrolling of compactified dimensions? I find this thought interesting because if so, it must involve the unrolling of our familiar dimensions of time and space, the consequences of which was what I was puzzling about in my OP.

I hope that as you say:
 Quote by marcus .... one of the well-informed string advocates at this forum will reply to your question with ... authority and detail...... Hope you find this .. interesting .
Indeed I do. Thanks.
P: 622
 Quote by cristo I've only heard a bit about braneworld cosmology, but what I have heard is rather interesting........ a little above me at the moment!
Snap! Cristo. Only in my case the subject is a miles above me, and this condition is likely to persist. But I hope to find bits that I can understand about the effects of dimensional unrolling.