Ask Gugel Anything About the Big Bang

[Ohgod]

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Okay, new question about whether there is a center point to the universe. I understand the idea that space itself expands. The universe itself may be endless, but what about the matter originating in the BB? None if it is more than 14 billion or 47 billion apart.

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I dont't think this is 'knowable' - nothing more than 47 BLY from us can have emitted light that has reached us, so it cannot affect us or be observed by us in any way. Thus, we are at the centre of our own obsevable universe. Similarly, another galaxy that we can see is at the centre of its own observable universe, which overlaps ours but is not identical to it. So there is nothing particularly special about the 'centre of the observable universe'.

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Or I guess that would be actually 94 BLY apart, if it's 47 in each direction. So within the infinite, shapeless universe, isn't there an expanding globe of stuff -- our observable universe -- that has a center point? If the universe is 94 billion LY wide, can't 47 BLY in from the frontier be considered a center point? And where in the sky would it be?

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Here. But that's not really surprising, and would also be true anywhere else.

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I agree it is slightly subtle - on the other hand, it IS basic information for someone setting themselves up as any kind of 'authority' on matters cosmological

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He understands that. The critics are just pouncing on something ambiguous, inadequately explained prose. It would be much more adult to say "Don't you mean 47 BLY?" rather than "gotcha, you wrong, wrong, wrong, bow before my superior intellect for all the world to see."

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On the other hand, it's not a minor matter. It is an easy slip to make, but it could also indicate that the OP has some fundamental misconceptions. Further discussion should help to indicate which. Similarly, if I have made any mistakes (since I am definitely not a specialist in the field), I welcome corrections.

[Arp220]

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Okay, new question about whether there is a center point to the universe. I understand the idea that space itself expands. The universe itself may be endless, but what about the matter originating in the BB? None if it is more than 14 billion or 47 billion apart. Or I guess that would be actually 94 BLY apart, if it's 47 in each direction. So within the infinite, shapeless universe, isn't there an expanding globe of stuff -- our observable universe -- that has a center point? If the universe is 94 billion LY wide, can't 47 BLY in from the frontier be considered a center point? And where in the sky would it be?


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I agree it is slightly subtle - on the other hand, it IS basic information for someone setting themselves up as any kind of 'authority' on matters cosmological

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He understands that. The critics are just pouncing on something ambiguous, inadequately explained prose. It would be much more adult to say "Don't you mean 47 BLY?" rather than "gotcha, you wrong, wrong, wrong, bow before my superior intellect for all the world to see."

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Ok:

The 'comoving radial distance' is about 47 billion light years. Now, from this we can infer that there exist (say) two galaxies that are 94Glyr apart, but those two galaxies cannot see each other. We can see both of them though.

This alludes to this 'center of the universe' stuff. Its often stated that the universe has no center, but this is not exactly true. For all practical purposes, *we* are at the center of the universe, because we see everything expanding away from us in all directions. Now, its also true that, if we were to go live in another galaxy far far away, we would also see everything expanding away from us. In other words, wherever we are, we are at the center of the universe!

This sounds a touch egotistical, but in fact what its saying is that we are at the center of the *observable* universe. Which makes much more sense.

As to whether the universe has an absolute center - well, its rather a moot question, because, unless we work out a way to look beyond the particle horizon, its almost impossible to answer. We have no observed evidence for an absolute 'center', therefore we can say there is no center. Of course, if evidence comes along to contradict this then things will change.

There is however an observable absolute 'rest-frame' for the universe. If we look at the cosmic microwave background, we see that it is marginally hotter in one direction. This is called the 'dipole anisotropy', and is caused by our movement relative to the CMB.

As for Gugel - I applaud his efforts, but the fact remains that he said something demonstrably incorrect. It wasnt in the least bit ambiguous.

[Bill Haywood]

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we are at the center of the *observable* universe.

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Okay, keep helping me through this conceptual block I'm having.

Posit an observer way out there, say 200 BLY from our universe neighborhood. And let's ignore the complication that it takes awhile for our light to reach the observer.

The observer looks, and sees odd happenings over there -- an expanding bunch of hot specks.

From a god's eye point of view, isn't our observable universe an expanding globe? What am I missing?

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As for Gugel - I applaud his efforts, but the fact remains that he said something demonstrably incorrect. It wasnt in the least bit ambiguous.

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Okay fine, but his critics shouldn't be diccks about correcting him.

[Arp220]

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we are at the center of the *observable* universe.

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Okay, keep helping me through this conceptual block I'm having.

Posit an observer way out there, say 200 BLY from our universe neighborhood. And let's ignore the complication that it takes awhile for our light to reach the observer.

The observer looks, and sees odd happenings over there -- an expanding bunch of hot specks.

From a god's eye point of view, isn't our observable universe an expanding globe? What am I missing?


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Youre not necessarily missing anything [img]/images/graemlins/wink.gif[/img]

We, unfortunately, do not have a 'gods eye' point of view. If we did we might see many things.

Heres an example:

One way to visualize the expansion of the universe is to take a balloon, paint lots of tiny specks on the balloon, and then gradually blow it up. If you, as 'god', look at the balloon, you see all the points expanding away from each other. You see a 3 dimensional object (the balloon) expanding into a 3 dimensional space (the room youre in).

Ok, now lets change our point of view. The surface of the balloon, with all the galaxies on it, is effectively a two dimensional object (lets ignore the thickness of the balloon for now) that is 'curved' in a three dimensional space. So, now imagine you are a TWO dimensional resident of one of those little dots on the balloons surface. You look at all the other dots, expanding away from you in all directions. From your point of view, you are at the center of the observable universe, but you cannot identify an 'absolute' point from which everything is expanding away. Because, if you went and lived in another dot, you'd see everything expanding away from that dot.

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.

So, the ultimate answer to your question is that nobody really understands the fundamentals behind the expansion of the universe. We see that it is expanding. We see that it is not expanding into a bigger volume. We have evidence that the expansion is not due to a simple increase in the number of spatial dimensions.

So we're still trying to find out what exactly is going on.

[2OuterJitsu]

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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.

[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]