# Dimensions of universe

Snip3r
In the distant past at bigbang the universe was very hot and dense. Can i say it was dimensionless then? If so did it come to this 3 dimension through 1 and 2?a little more speculation... is/will it go to higher dimensions?

Homework Helper
If I understand your question - the very early universe in the kind of model you are thinking of has 4 dimensions ... one of them separates out to become time so we get the current 3+1 set of today. One way to visualize how you can have more than zero dimensions in a singularity it is to consider that any flat projection of the surface of the Earth has singularities - but the surface of the Earth (and, indeed, the map) is 2D at the singularities.

But all this is before the plank epoch - and thus highly speculative. There are a range of models for this "period" and they don't all have a singularity(? someone correct me?) or, even, a beginning. See:
http://en.wikipedia.org/wiki/Chronology_of_the_universe
http://www.hawking.org.uk/the-beginning-of-time.html

Naty1
There are a range of models for this "period" and they don't all have a singularity(? someone correct me?) or, even, a beginning.

yes, the bang is itself speculative...As the Hawking reference says, all our equations fail at what appears to be a 'singularity'. A finite 'bang' has many variations, one from Paul Steinhardt and Neil Turok, discussed briefly here:

http://en.wikipedia.org/wiki/Cyclic_Universe

Their book on the subject for the general population is very interesting:
THE ENDLESS UNIVERSE

Eternal inflation is yet another hypothesis for a never ending set of universes...a multiverse. In such a situation with an infinite number of universes, many should be 'born' with varying numbers of dimensions...so for example, some may start without time, beunable to evolve, and so remain 'dead'...unevolved...others may live their lives in similarly short time spans.

i haven't come across cosmological models with higher dimensions, but I would not be surprised to see some. A number of de Sitter based spacetime models suggest that the universe evolves so as to increase entropy. And that seems to lead to a rather flat universe devoid of anything except the current 3+1 dimensions. cold,dark,empty.

Mark M
i haven't come across cosmological models with higher dimensions, but I would not be surprised to see some. A number of de Sitter based spacetime models suggest that the universe evolves so as to increase entropy. And that seems to lead to a rather flat universe devoid of anything except the current 3+1 dimensions. cold,dark,empty.

Well, the Ekpyrotic model of Steinhardt/Turok is a higher dimensional model, as it is based off of M-theory, which has 10 dimensions of space. In the superstring theories, which have 9 dimensions of space, 6 dimensions are compactified into an extremely small shape called a Calabi-Yau manifold. When M-theory emerged, one realization was the one extra dimension. Rather than conpactifying it, the 'Randall-Sundrum model' leaves one of the extra dimensions large, so that the universe has 4 large dimensions. In this fourth dimension are two large D3-Branes, one of which would be our universe. These branes would be separated by space in this fourth dimension, called the 'bulk'.

In Steinhardt and Turok's model, they consider a field $\phi$, called the radion field, which would attract the two branes together. After enough time, they would collide via the large fourth dimension. This is the key - if you took two two dimensional plates and slammed them together through the third dimension (by, say, dropping one onto the other), they would collide along every point on the two dimensions. Similarly, these two branes collide at every point in Their three dimensions. This produces the homogenous universe we observe, since they collide at every point.

Naty1
Well, the Ekpyrotic model of Steinhardt/Turok is a higher dimensional model,

sure is... I should have said 'traditional cosmological models'...and even so what I have 'come across' is rather irrelevant as there is a LOT of scientific stuff I haven't even seen.

In the distant past at bigbang the universe was very hot and dense.

That's the traditional view...for another that has been very recently discussed see here:

Spontaneous Inflation and the Origin
of the Arrow of Time
Sean M. Carroll and Jennifer Chen 2004

http://arxiv.org/pdf/hep-th/0410270v1.pdf

My quick summary:

According to everything we know about gravity, large curvatures are entropically disfavored, tending to ultimately smooth themselves out under ordinary evolution.
our proposal imagines that there do not exist any maximum-entropy equilibrium states, but
rather that the entropy can increase from any starting configuration

The Big Bang in our past is not a unique moment in the history of
the universe; it is simply one of the many times that inflation spontaneously began
with fractal distribution of pocket universes to the far past and far future. [Those which
expand forever are far more likely to lead to the origin of other universes.]

...if “empty space” is not a perfectly stable state, but rather is subject to instabilities that can produce universes like our own. A mechanism for such an instability may be provided by quantum fluctuations in a nearly-empty universe [associated with small positive cosmological constant] and an appropriate inflation field.

I am not personally overwhelmed by the force of the logic, but the scope of the discsussion [not a lot of math] and insights is fascinating.

Homework Helper
Um... I thought the question concerned the dimensionality of the singularity in the big-bang model folks, not the number of dimensions in various models of the Universe.

Do any of the models discussed have a progression where the Universe starts out in 0D then becomes 2D then 3... I suspect not.

Xyooj
i was thinking more in line with
our dimension, spirit dimension, etc... :)

we are able to understand our dimension better perhaps because we are created from the matter in our dimension. inter-dimensional beings are made with matter in those dimensions?

Homework Helper
i was thinking more in line with
our dimension, spirit dimension, etc... :)
Well you are not the OP :)
You would also need to define your terms more closely for those concepts to make any sense - how would you go about making measurements in the "spirit dimension" for example?

we are able to understand our dimension better perhaps because we are created from the matter in our dimension. inter-dimensional beings are made with matter in those dimensions?
You seem to be using the word "dimension" in a way that is not usual in physics ... beware of confusing technical with common terms.

twofish-quant
In the distant past at bigbang the universe was very hot and dense. Can i say it was dimensionless then?

No. After inflation and probably during, the universe is 3+1 dimensional.

If so did it come to this 3 dimension through 1 and 2?a little more speculation... is/will it go to higher dimensions?

A lot of the speculation involves assuming the reverse, that the universe in fact has a lot of hidden dimensions which you can't normally see. These hidden dimensions are extremely small, and it was near the beginning of the universe that the universe "inflated" along the dimensions that we normally see.

The idea is that the "hidden dimensions" have effects that we can see. For example gravity is particularly weak because some of the force goes down the hidden dimensions.

Note that is all speculation and guesswork and may not be true.