Size of the Universe

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Size of the Universe

Postby philip pugh » Mon Jul 05, 2010 1:46 am

How can we be sure that the universe can be bigger than 13.7 billion light years? Also, do the meanings of radius and diameter apply when we're probably talking in at least 4 dimensions, if not 11.
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RE: Size of the Universe

Postby brianb » Mon Jul 05, 2010 10:56 am

[quote]How can we be sure that the universe can be bigger than 13.7 billion light years?[/quote]
This depends on your model of the universe. If you believe in a creator, the creator is necessarily larger than the creation. If you believe that the universe is the reault of a random qaunatum fluctuation, you're forced by theory (backed up by cosmological observation) to accept in "inflation" - a period during the very early universe when the electromagnetic & weak nuclear forces seperated out, causing the universe to double in size every Planck interval (~10^-43 sec) until it was already much bigger than expansion limited boundary of the universe that we see.

The theory of the early universe needs modifying depending on the flavour of quantum gravity you favour, but since gravity and electroweak forces must have seperated out well before the electroweak symmetry break driven inflation, it can hardly have much effect on the scale of the inflation.

[quote]Also, do the meanings of radius and diameter apply when we're probably talking in at least 4 dimensions, if not 11. [/quote]
The dimensionality doesn't matter, curvature & distortion does. There's no such thing as absolute size, or time. This is the true meaning of the Heisenberg uncertainty principle ... you can't grasp the quantum nature of the universe so long as you think in classical terms like distance.

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RE: Size of the Universe

Postby philip pugh » Mon Jul 05, 2010 12:53 pm

Thanks, Brian. I'm beginning to think that the universe is a more complicated place than we imagine. This could be a long story but is there not the minutest possibility that inflation didn't stop at all and is still going on? If not, what could have stopped it?
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RE: Size of the Universe

Postby brianb » Mon Jul 05, 2010 4:46 pm

[quote]inflation didn't stop at all and is still going on? If not, what could have stopped it?
[/quote]
Ummm, dark energy is different to cosmic inflation associated with the electroweak decoupling.

The decoupling is like a phase change - a poor analogy is water boiling. At some stage you have just liquid water, you then have an event where there is both water and water vapour, finally you have no liquid water left, but a much larger volume of water vapour (or "steam" if you prefer). Once the water is all vapourized, you can have continued expansion but you don't get the sudden large increase in volume that occurs when the water boils (at constant pressure).

So, once the decoupling is complete and the weak nuclear & electromagnetic forces have differentiated from each other, there is nothing left to drive inflation. The decoupling would have occurred at ~10^-35 sec assuming our understanding of particle physics is anything approaching accurate - and we now have experimental evidence from both the very small high energy scale (particle accelerators) and very large scale (cosmology) that it is; the distribution of galaxies in ancient clusters is consistent with the observed mass/energy of the W & Z particles and the distribution of clusters of galaxies is consistent with the mass/energy of neutrinos, both of which would have begun to "crystallize" out of the continuum very shortly after the inflationary epoch and in relative quantities which depend on the effects of inflation.

Dark energy is a different thing altogether - the evidence for its existence is almost by definition weaker the nearer to the big bang you get, and there may be some flaw in the model of supermassive stars formed in the very early universe which accounts for the apparent change in "Hubble's Constant" i.e. acceleration expansion of the universe.
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RE: Size of the Universe

Postby spartacus » Mon Jul 05, 2010 7:33 pm

Good of you to clear that up for us Brian ! [&:]
Phil, you posted your question in the wrong place. The size of the Universen is fundemental to understanding everything though I'm quite sure there's a huge error in 13.7 billion ly. Your question should have been posted in the "Proper Big Boys Questions" section [:D]

Someone made a very enlightening comment on the accelerating expansion of the Universe in that we could still be in the early years of the Universe' "ontogeny" ! We assume the Universe is old from our own perspective but from that of a universe, it could be just a pup !

There's a word for this sort of fascinating stuff but it's rude, PM me ! lol [:D]

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RE: Size of the Universe

Postby philip pugh » Tue Jul 06, 2010 12:33 am

Thanks, Brian. I now get some idea of why we had inflation and no longer do. I never really "got it" before. Keef, thanks for your answers, too. I thought mine was quite a silly question because the way I saw it, I could see that the minimum size of the universe was derived from its age and the speed of light. I also knew about the cosmic background radiation that occurred about 380 000 years after the Big Bang.

So we can't see anything that happened in the first 380 000 years of the universe's history and there's a whole lot of universe created before then that could be receding from us faster than the speed of light.

So all I need to work out now is:

1. What happened in the first 380 000 years

2. How big the universe really is (some say 40 billion light years)

3. What is dark energy

4. What is dark matter
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RE: Size of the Universe

Postby lancashire astroguy » Sat Jul 10, 2010 9:13 pm


[quote]ORIGINAL: Spartacus

The size of the Universen is fundemental to understanding everything though I'm quite sure there's a huge error in 13.7 billion ly.

[/quote]

The measurement of 13.7 billion years for the age of the Universe from Cosmic Microwave Background measurements is accurate to a few percent I believe (a few hundred million years). This doesn't mean that the diameter of the visible Universe is 2 times 13.7 = 27.4 billion light years. In fact, Inflation theory suggests that space expanded faster than the speed of light during the big bang, such that the actual diameter of the visible Universe is more like 90 billion light years.

I think that there is always confusion between the visible Universe and the entire Universe. When we look backwards in time to objects more and more distant we come to an event horizon beyond which the light from any object could not have reached us in the time since the Big Bang. The visible Universe (which is sometimes just referred to as "The Universe" and is probably what Phillip is referring to in this thread) is thus the "bubble" of Universe which lies within this event horizon. Move to a different location in the Universe and you will have a different event horizon.

There is no suggestion, therefore, that our event horizon marks the "boundary" (whatever that would mean) of the Universe in any sense. A Universe which was closed (had enough mass to overcome cosmological expansion) might have a finite size, but all measurement seem to indicate an open Universe which will expand forever (if not accelerate in its expansion). There is no theoretical reason why an open Universe could not have an infinite total size, and indeed I think this is most likely answer.

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RE: Size of the Universe

Postby tim allan » Wed Jul 14, 2010 4:36 pm


[quote]ORIGINAL: Philip Pugh

Thanks, Brian. I'm beginning to think that the universe is a more complicated place than we imagine.[/quote]

Well as Sir Arthur Eddington said "Not only is the universe stranger than we imagine, it is stranger than we can imagine".
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RE: Size of the Universe

Postby lancashire astroguy » Thu Jul 15, 2010 10:06 am

To quote one of my favorite books of all time, the Hitchhikers Guide to the Galaxy:

[color=#00FF00][i][b]"Space is big. Really big. You just won't believe how vastly hugely mindbogglingly big it is. I mean you may think it's a long way down the road to the chemist, but that's just peanuts to space"[/b][/i][/color]
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RE: Size of the Universe

Postby afmanchester » Thu Jul 15, 2010 11:46 am

This song actually quite accurately explains how big the universe is:

http://www.youtube.com/watch?v=dF2HG1PVZok&feature=player_embedded#!
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