Ask Gugel Anything About the Big Bang

[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.

[FortunaMaximus]

Black holes have infinite density. Have the potential effects of something with such density having the capability to interact with other universal forces been explored?

Space interacts with time.

Gravity interacts with matter.

Singularities interact with the fabric of the universe, perhaps there are emergent effects, i.e. entropy, CMB, etc.

One also considers that whatever the interactions and consequences, the potential for those consequences to emerge outside the light cone unbidden to our explorations as of yet should be possible.

The Universe itself is far too small to truly be a full expression of what can be theorized in mathematics.

As to the matter of the potential spatial limits of this Universe, if it's 47 billion years, I have no reason to dispute that. What I can consider is that this is a number that experiences growth, as does our Universe's light cone.

In an infinitely expanding field of probability, there needn't be a center, and certainly since within our Universe, effects echo themselves on micro and macro scales. Seashells by the sea.

There are other forces that have yet to be thought of and explored. What may be a point of infinite density in the Universe may well be a fully expressed equation elsewhere.

It at least has that potentiality.

[2OuterJitsu]

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Could you expand on the quoted?

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Certainly.

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So as it stands:

A) More spatial dimensions have predicted things we can’t find in the CMB.
B) Current data on the CMB, precludes the possibility of more spatial dimensions.

A is correct and B is not correct?

Sorry, but sometimes scientists don’t make it clear that denying something isn’t the same as affirming its opposite, especially when trying to explain things in layman’s terms.

By the way, what’s your background?

[Arp220]

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Could you expand on the quoted?

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Certainly.

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So as it stands:

A) More spatial dimensions have predicted things we can’t find in the CMB.
B) Current data on the CMB, precludes the possibility of more spatial dimensions.

A is correct and B is not correct?

Sorry, but sometimes scientists don’t make it clear that denying something isn’t the same as affirming its opposite, especially when trying to explain things in layman’s terms.

By the way, what’s your background?

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(A) is true, but only for the very simplest possible cases - for example one extra spatial dimension that is not 'compactified' (theres a nice description of what a compactified dimension is here:

http://en.wikipedia.org/wiki/Compact..._(mathematics)

However, (B), as you say, is not true. There are many classes of braneworld model that make predictions that are entirely consistent with current CMB data. Many of these models have only one extra spatial dimension, but do very odd things to it. Examples include the Randall-Sundrum models, and a class of model called DGP gravity. Wikipedia has a succinct little writeup of the randall-sundrum models here:

http://en.wikipedia.org/wiki/Randall-Sundrum

And youre entirely right when you say that denying something is not the same as affirming its opposite. A good rule of thumb in astronomy is that, unless someone EXPLICITLY affirms the opposite of something, then theyre simply saying that we have no evidence for that something.

as for background - astronomy is my job [img]/images/graemlins/wink.gif[/img]