[Arp220]

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Many people see this example and conclude that the expansion is simple - we are three dimensional, but the universe is expanding 'into' a fourth spatial dimension. This is however not accurate. If this was the case we would see some very clear indicators from particle physics, but we do not.

...We have evidence that the expansion is not due to a simple increase in the number of spatial dimensions.


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Could you expand on the quoted? I keep hearing/reading this, but never much more. Explain the both the indicators that would suggest expansion into another spatial dimension, and the evidence that the universe isn’t expanding into another spatial dimension.

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Certainly.

This is really not a single question though!I'll try to answer it in a number of ways:

Ignoring extra dimensions for a minute - lets look at the question of the expansion of the universe. There are two posible cases to consider; one where the universe is infinite, and one where its finite.

If the universe is infinite, then by definition it's not expanding 'into' anything.

If the universe is finite though, then it certainly could be expanding 'into' something. The trouble is, we have virtually no way of telling what that 'something' might be, as it would lie outside the boundary of the observable universe. Hence, for all practical purposes, its not a particularly scientific question.

Now, when it comes to extra dimensions, we can do a little better. There are two ways to look for extra spatial dimensions - particle accelerators and by studying the cosmic microwave background. I'm not really up to speed on the poarticle physics side of things, so I'll concentrate on the CMB.

The CMB is very very smooth and homogenous, but does exhibit very small scale anisotropies. These little irregularities have been imaged in great detail by several experiments, most recently a satellite called WMAP.

Because the CMB was formed so soon after the big bang, the exact nature of these anisotropies is governed by the nature of the expansion of the universe at very early times. One class of model for describing this expansion is called 'braneworld' models, which posit that 'our' universe, consisting of three spatial and one temporal dimension, is actually 'embedded' in a higher dimensional spacetime.

There are many MANY variants of braneworld models. And depending on their nature (mostly the number and nature of extra dimensions), they make specific predictions for the pattern of anisotropies in the CMB. So, we can look at the CMB and use it to rule out certain classes of braneworld model.

The simplest possible braneworld models are ruled out by what we currently know about the CMB. Unfortunately, the newer generation of braneworld model are sufficiently complex that making concrete predictions from them is hard, and current data cannot effectively discriminate between them. Which is why a satellite called Planck is being launched next year, to get better quality CMB data.