• Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.


PO is now at the Physics Department of Bielefeld University!

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback


(propose a free ad)

Site Statistics

205 submissions , 163 unreviewed
5,054 questions , 2,207 unanswered
5,345 answers , 22,721 comments
1,470 users with positive rep
818 active unimported users
More ...

  What does string theory say about the metric expansion?

+ 5 like - 0 dislike

Specifically, what happens to those small intertwined hidden dimensions? Do those expand too?

This post imported from StackExchange Physics at 2014-03-07 16:38 (UCT), posted by SE-user laggingreflex
asked Jul 29, 2013 in Theoretical Physics by laggingreflex (50 points) [ no revision ]
Expansion is only observable for very high distance scales. For instance, you could not observe expansion in our galaxy,the Milky Way, and even in the local group, because of (local) gravity. For instance, Andromeda is actually falling towards us. Moreover, I suppose that we need a specific cosmological string model.

This post imported from StackExchange Physics at 2014-03-07 16:38 (UCT), posted by SE-user Trimok
@Trimok: "Expansion is only observable for very high distance scales." Not true. For example, an electromagnetic wave-packet that's localized to within a very small region will suffer a cosmological redshift, and one valid way of interpreting this redshift is that it's a cosmological expansion of the space occupied by the wave. A more correct statement is that cosmological expansion has an unmeasurably small effect on strongly bound systems, if the equation of state doesn't have unusual properties: physics.stackexchange.com/q/70047

This post imported from StackExchange Physics at 2014-03-07 16:38 (UCT), posted by SE-user Ben Crowell

3 Answers

+ 3 like - 0 dislike

The metric expansion can for example be described by the Robertson-Walker metric given by

$$ \mbox ds^2 = -\mbox dt^2 + a^2(t)\mbox d\Sigma^2 $$

where $a$ is the scaling factor and $\mbox d\Sigma^2$ is spatial part of the metric. If the scaling factor obeys the equation

$$ \dot{a} = a H $$

where dot means time derivative and H is the Hubble constant, the universe not just linearily expanding but the expansion is accelerated (or exponantial) which is called inflation.

From the observations we have up to now (?) it is known that the universe is rather flat, homogeneous, and isotropic. The isotropic means that the universe looks the same in all direction, which means in addition that all spacial directions have to be treated equally.

In string theory, there are small extra dimensions and it is therefore natural in string cosmology to correspondingly relax the condition of isotropy and treat the (compactified) extra dimensions different from the large macroscopic dimensions.

One possibility to implement this anisotropy due to not all spatial dimensions behaving the same is by assuming that the universe can be described by the Kasner metric

$$ \mbox ds^2 = -\mbox dt^2 + \sum_{j=1}^D t^{2p_j}(\mbox dx^j) $$

By this metric, it is possible to not only describe the cosmological expansion of some spatial directions (the ones with positive Kasner exponents $p_j$, but this metric allows for some dimensions to contract too, those have negative $p_j$. Carrying over to what is asked in the question, the expanding dimensions could for example correspond to the dimensions with positive Kasner exponents and the compactified extra dimensions could be described by a negative Kasner exponent. The Kasner exponents must satisfy the Kasner conditions

$$ \sum_{j=1}^{D-1} p_j = 1 $$


$$ \sum_{j=1}^{D-1} (p_j)^2 = 1 $$

which can be shown to boil down to the fact that not all exponents can have the same sign and therefore there have to be contracting and expanding directions present at the same time.

Also in string theory, the extent of the size of the (extra) dimensions is described by dilaton fields ;-) $\phi$ and they are related to the Kasner exponents in Models making use of the Kasner metric for example by

$$ \phi = -\left(1- \sum\limits_{j=1}^D p_j \right)\ln t $$

This introduces some kind of duality between expanding and contracting dimensions, such that something like a cyclic pre big bang cosmology can be described in this model in the following way:

  1. The universe starts from a large, flat, cold state

  2. It contracts until it reaches the small self dual radius where it is higly curved and hot (this is the big bang)

  3. Due to the mentioned duality the universe is in this state
    equivalent to an expanding universe, which corresponds to the
    universe we live in.

In such cosmological models, the contracting dimensions can cause the expanding dimensions to accelerate, but I did not fully understand this inflation mechanism. I am not sure if maybe the potential energy needed to keep the contracting dimensions small could take to role of an appropriate inflaton potential or something along these lines.

Other string cosmilogy models make use of brane worlds which are based on Randall-Sundrum models

enter image description here

which describe our universe as a 3-brane connected by an extra dimension to an analog 3-brane called the hidden sector. In their "stationary form" where the branes dont move, these models give among other things an approach to solve the hieararchy problem in particular when warped dimensions are allowed for as for example nicely explained here. In cosmological applications, the branes are allowd to move along the extra dimension, such that cyclic ekpyrotic models are obtained where the universe always existed and the big bang is not the origin of time:

  1. First, the two branes are located at the boundaries of the extra dimension (s). This state corresponds to an empty cold universe
  2. The branes are attracted towards each other and collide in a big bang, matter and radiation are created from the collision energy
  3. The branes move away from each other and the universe cools down

This cycle of events can take place repeatedly. In these cosmological models, the isotropy can be explained by the fact that the two (parallel) branes collide roughly like two plates evetywhere at the same time, the initial state is empty and therefore flat, and brane ripples (oscillations of the branes in the extra direction) provide seeds for the large structures of the universe.

This post imported from StackExchange Physics at 2014-03-07 16:38 (UCT), posted by SE-user Dilaton
answered Aug 4, 2013 by Dilaton (6,240 points) [ no revision ]
+ 1 like - 0 dislike


At least as far as I know. See, for example, Why and How Energy is not Conserved in Cosmology (TRF) by Lubos Motl. To quote,

I deliberately wrote "K^3" in order to appease the people who don't realize that there are extra dimensions of space or those who do realize that there are extra dimensions but who also know that only 3 of them have been expanding in the recent 13.7 billion years. ;-)

In fact, if I'm not wrong, this result, in string cosmology, is sometimes used as a reason why these extra dimensions are so small.

enter image description here

Notice that the $y$ dimension doesn't expand but the $x$ direction does. Thus, after a long time, the reigon looks 1-dimensional, but initially, it lookks 2-dimensional .

enter image description here


Sorry I couldn't write a more detailed answer, I don't know much (or anything) about string cosmology Hope someone else can write a more detailed answer.

answered Jul 30, 2013 by dimension10 (1,985 points) [ revision history ]
edited Aug 30, 2014 by dimension10
+ 0 like - 0 dislike

The extra dimensions in string theory have always been assumed to be curled up on a scale equal to the Planck length, simply because there is no other natural scale for them. If they had been curled up on that scale at the time when the temperature was the Planck temperature, and then expanded since then at the same rate as the ordinary 3+1 dimensions, then they'd be big enough now to be easily observable.

Furthermore, I assume that the masses of fundamental particles in string theory is closely related, via the de Broglie relation, to the scale of the extra dimensions. If there were an increase in this scale over time, then we should observe, for example, that the ratio of the classical electron radius to the Bohr radius was increasing over time. Since this ratio is known to about one part in $10^9$, I would imagine that we would have detected a change in it when the universe expanded by one part in $10^9$. That would have only taken about 10 years.

This post imported from StackExchange Physics at 2014-03-07 16:38 (UCT), posted by SE-user Ben Crowell
answered Aug 4, 2013 by Ben Crowell (1,070 points) [ no revision ]

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
Please complete the anti-spam verification

user contributions licensed under cc by-sa 3.0 with attribution required

Your rights