# Circumnavigating the Universe and 3D 'Branes

1. Dec 7, 2011

### gizander

I read (or heard or saw) somewhere long ago that if you traveled as far as you could go in the Universe you'd return to the exact same point at which you started. If our U is indeed a 'brane, and the previous statement is true, doesn't that whole circuit of travel ending up back at "point A" describe the type of journey one would make traversing the length of a Mobius strip? Thus, are 'branes really 2 dimensions of the 11 theorized or are they 3 dimensional Mobius strips where, if a 'brane were a 2D square with corners A, B, C and D, clockwise from top left, and corners A & C and B & D were joined (and thus the sides AB and CD joined)?
BTW, this also begs the question, "What about travel ACROSS the strip?" Does one "fall off the edge or walk onto a parallel strip? Or is travel (read: Motion) actually only possible in only one dimension in both the positive and negative directions (i.e. going "forwards" and "backwards" on the strip)?

2. Dec 7, 2011

### phinds

There's nothing to prove whether the universe is finite or infinite and if it's finite whether it's bounded or unbounded (your example is finite, unbounded). It's all speculation.

3. Dec 8, 2011

### PatrickPowers

I read that too, but that idea is obsolete. That's nice, because I never believed it :-).

Observation of dark matter and accelerating expansion of the universe have changed everything. The "standard model" is now of a flat, infinite universe. Second choice is a near-infinite universe.

4. Dec 8, 2011

### phinds

No, that is not correct. As I said above, there is no proof one way or the other.

Nonsense. There is no such thing as "NEAR infinite". Something is either infinite or it is not. Very, very big is not any closer to infinite than very very small.

You are correct that the consensus is that the U is very nearly flat and MAY be infinite, but MAY be is not "IS" as you have stated.

5. Dec 8, 2011

### marcus

That's a figurative way of describing a universe that is spatially a 3D hypersphere---analogous to a 2D spherical shell. I don't think cosmologists ever suggested that circumnavigation was a practical possibility. You'd have to imagine traveling much faster than light. But it is good "thought experiment"way of describing the experience of living in the 3D analog of a balloon surface world.
That's one possibility which is taken very seriously by cosmologists---the positive curvature near flatness case.
Actually the idea is not obsolete.
The idea is far from obsolete and it is certainly compatible with dark matter and accelerating expansion! They fit nicely into the finite universe picture. The 5-year
WMAP report, Komatsu et al, calculated parameters for both the finite spatially closed standard model and the infinite. The issue of which fits best has not yet been decided.

I agree with Phinds about that. The issue remains to be resolved, hopefully by more precise measurement of the overall curvature. I am not sure that it can be resolved in the foreseeable future, but one can hope.
Again I agree with Phinds. "Nearly flat" does not mean "nearly infinite" (that has no meaning). Nearly flat means that the largescale curvature measured so far is very close to zero (the 95% confidence interval includes zero).
This could mean either spatially finite or infinite.
If there is a consensus it would, I guess, be that both cases are possible. Neither has been ruled out yet.

Last edited: Dec 8, 2011
6. Dec 8, 2011

### Schneider55

I just saw a documentary entitled:

BBC Horizon: WHat Happened Before the Big Bang?
http://captured-beats.blogspot.com/2011/12/what-happened-before-big-bang.html)

The documentary mentions a theory about 'Branes'. What exactly is a 'Brane'? As you can tell I am not a physicist, but I would like a simple of an answer as possible, I'm too old to study Physics :(...

7. Dec 10, 2011

### gizander

Schneider,

A great documentary to watch about Membranes ('Branes) and all things String and M Theory is "The Elegant Universe" It's a Nova production so it should be available for viewing or download from their site or try PBS.org
It explains a lot in terms non-physicists like us can understand. It's a 3 part film so be sure to watch all three as 'branes are discussed in the 3rd part if memory serves...
Pretty mind-blowing stuff (and that's without the math ;)--it's one of the things I saw/read that got me interested in Theoretical Physics and Quantum Mechanics. (I'm now taking courses using the OCW program at MIT)...And, as the saying goes, "No one's ever survived long enough to be too old to learn Physics." ;)
We think, therefore we are; thus, when we stop thinking we are no more...
Cheers!