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Offline DoctorBeaver

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Very basic cosmology question
« on: 11/04/2005 13:41:18 »
There's something I've been confused about for years but as yet I've had no satisfactory answer to. Cosmologists say we can now see objects 12 billion lightyears away. Now, if that light has taken 12 billion years to reach us, the objects concerned must have been almost that far away (minus the expansion of the universe)when the light started its journey. But surely, 12 billion years ago the universe wasn't anywhere near large enough for any object to have been that far from any other object: so how come we see them now? Wouldn't the light have gone way beyond us by now? Can anyone help me understand this?


 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #1 on: 11/04/2005 19:33:20 »
 

Offline Ultima

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Re: Very basic cosmology question
« Reply #2 on: 11/04/2005 22:31:43 »
Isnt the expansion increasing? So wouldn't this at some point in the future cause a cut off point where light will never reach us from a certain distance away?

wOw the world spins?
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #3 on: 11/04/2005 23:13:44 »
There's an article in the March '05 issue of Scientific American that mentions this.

See "Misconceptions about the Big Bang" at:
http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147&sc=I100322

Skip to page 5 to read the part about the cosmic event horizon.
 

Offline realmswalker

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Re: Very basic cosmology question
« Reply #4 on: 12/04/2005 01:11:29 »
In a nut shell:
At the time of the big bang universe was small, the big bang was not 1 point with space all around it, the big bang took place at all points in space at once. This caused the space to expand. The light travelling across that space is more or less fighting against the tide of the exspanding universe, if it goes 3 feet the universe exspands 1 foot (idk what it is really just an example), a process like that keeps the speed of light the same, but the actuall movement of light through the universe slows down
I think thats waht it is...
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #5 on: 12/04/2005 12:12:58 »
I've read those links & i'm just as confused as ever. So, answer me this... if photons travel at the speed of light there must come (or have been) a time when some reach the edge of the universe. What happens then? Do they just go phut? Or do they cause the universe to start expanding at the speed of light with the photons forming the edge?
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #6 on: 12/04/2005 13:23:33 »
>> Re: photons... reach the edge of the universe.
As counterintuitive as it may seem, the universe has no edge, and it has no center.
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #7 on: 12/04/2005 18:00:40 »
quote:
Originally posted by DoctorBeaver

I've read those links & i'm just as confused as ever. So, answer me this... if photons travel at the speed of light there must come (or have been) a time when some reach the edge of the universe. What happens then? Do they just go phut? Or do they cause the universe to start expanding at the speed of light with the photons forming the edge?



Actually, you are right. There was a time when the photons reached the end of the universe, about 10^-30 seconds after the beginning, give or take a few "-xx".  We know this, because we see it in the cosmic background radiation. The CBR is in thermal equilibrium, so the photons must have been able to interact long enough to reach thermal equilibrium at that time.

Then the expanding universe entered its inflationary phase, expanding outwards by a negative pressure. It expanded by a factor of 10^50 or more. It cooled during this expansion, then transitioned back to the normal gravitational mode we now see. The universe then re-heated from the release of phase-transition energy, but now the "ends" of the universe were vary far apart, and moving away from each other at a high rate.

The universe was still opaque to radiation at this time, due to the high temperature. About 300,000 years of age, the universe cooled enough to allow electrons and protons to combine into neutral hydrogen. The cooled remains of this is what we see today as the CBR. Since any observer can only see light that was emitted after this time, we can see only that far. The whole universe can be much larger than the observable universe. There are always new things coming into view, as light arrives here from further away. However, we will probably not ever be able to see the total universe, since inflation expanded it so far beyond any light-cone's reach.
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #8 on: 12/04/2005 21:24:56 »
The cosmological model does not permit the universe to have an edge. In big bang cosmology, there is no center and no edge; every point in the universe equivalent to every other.

The big bang was not an explosion that occurred at some point in a preexisting space. It occupied the whole of space. It created space. There is no 'empty space' that the universe is expanding into.
 

Offline realmswalker

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Re: Very basic cosmology question
« Reply #9 on: 13/04/2005 03:36:41 »
well the universe must have an edge, but, if the universe was 4 dimensional(which it might be) it would appear infinitly massive to 3 dimensional observers.
It would be like a person who lived in a flat world trying to look up (sorta, its more difficult for a 3 dimensional creature to comprehend 4 dimensions since we have never experianced interaction with what ever added depth the 4th dimension is).
The theroy that there are mutiple "bubble" universes relys on the fact that the universe are 4 dimensional, so although they seem 3 dim. and infinite, they arent and have more universes next to each other in that 4 dimensional space...
i think...
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #10 on: 13/04/2005 13:20:25 »
DrPhil - I'm aware of the fact that the universe isn't expanding into anything as there's nothing for it to expand into. But when it's said that at a given time after the big bang the universe was whatever size, there must have been an edge or it would always have been of infinite size. That clearly cannot be the case.
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #11 on: 13/04/2005 14:34:05 »
There is no problem with an expanding infinite universe. Big bang cosmology works even with an infinite universe. The universe simply needs to have infinite size at its creation. The big-bang beginning of our universe occurred everywhere throughout all of infinite space. A singularity of infinite size and infinite density.

The real answer is ... we just don't know. And this may be the best answer we ever get.
« Last Edit: 13/04/2005 14:52:17 by DrPhil »
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #12 on: 13/04/2005 17:19:59 »
DrPhil, philosophically that argument does not hold water. Infinite implies it's as big as possible & as such it cannot expand. If it expands it cannot have been infinite to start with. It's like the equation infinty+1=infinity.
 

Offline daveshorts

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Re: Very basic cosmology question
« Reply #13 on: 13/04/2005 19:32:14 »
Aaaah a poor innocent who has never been cornered by a Mathematician at a party. I am not a mathematician, but I will do my best.
 Just because something is infinite doesn't mean that something else can't be bigger. Consider this:

How many multiples of two (N2)are there?
an infinite number obviously

How many integers are there (N1)?
a infinite number

There are twice as many numbers as multiples of 2 so

N2/N1 =2

so although both N1 and N2 are infinite N2>N1, kind of wierd isn't it.

believe me it can get a lot worse!

Now imagine a universe consisting of an infinite number of stars all 1 light year apart, now you can expand the distance between all the stars to 2 light years, now whether the whole thing is bigger or not is a bit philisopical, but to all intents and purposes the universe has expanded.
 

Offline daveshorts

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Re: Very basic cosmology question
« Reply #14 on: 13/04/2005 19:37:48 »
showing the fact that I am not a mathematitian, where I said:
quote:
There are twice as many numbers as multiples of 2 so

I meant:

There are twice as many integers as multiples of 2 so
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #15 on: 13/04/2005 20:50:51 »
DoctorBeaver,
So, maybe you would accept a finite, expanding, closed, universe model that appears infinite because it wraps around and closes in on itself. You (or a photon) can travel as far as you like and you will never find a boundary. You simply follow the curved space and come back to where you started.

That's a valid model. There are a whole bunch of theories about this topic, and we are only now just beginning to figure out ways to test them.

Nevertheless, the Big Bang did not occur at a single point in space. It is better thought of as the simultaneous appearance of space everywhere in the universe. If space is infinite now, it was born infinite. If it is closed and finite, then it was born with zero volume and grew from that. In either the open or closed universe, the only "edge" to space-time occurs at the very moment of the Big Bang itself.
« Last Edit: 13/04/2005 22:29:52 by DrPhil »
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #16 on: 15/04/2005 06:07:59 »
AS a matter of fact I have been cornered by a mathematician at a party! Fortunately I was too blathered to take much notice of what he was wittering about
DrPhil I accept Riemanian (excuse my spelling) geometry & can understand that given a certain geometry the universe curves back on itself (saddles & spheres, I believe, have something to do with it). However, I also remember something about a theoretical puzzle about an hotel with an infinite number of rooms and then another room is added. Does that ring a bell with you? I can't remember the exact details.
You'll have to pardon my ignorance on this subject, I am but a lowly doctor of psychology so hence not au fait with anything more complicated than 2+2.
 

Offline gsmollin

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Re: Very basic cosmology question
« Reply #17 on: 15/04/2005 16:20:00 »
This discussion is becomming metaphysical, since it talks about things we can't really talk sense about, physically.

How big is infinity is not exactly the same question as how big is the universe, and an answer of infinity is not going to satisfy anybody either.

There is good evidence that the universe is hugely larger than anything we can see. No matter where we would go in the observable universe, we would see just what we see right here, even 14.8 billion light years away, where we see the observable edge from where we stand now. This is the cosmological principle, and it appears to be a good principle.

What lies beyond our observable universe is just more observable universe. What lies beyond that? Experimentally, no body knows, since we cannot see it. Actually, it does not matter, since it cannot ever effect us. However, the metaphysicist, and the philosopher are not satisfied with that either, so they formulate theories, or religion, as the case may be.

Contracting our inability to even see what we have in front of us today, back to a beginning time and saying that the universe was an infinite singularity point is speaking in riddles. It underscores our lack of true understanding. We say things like "expanding space", without defining space, let alone how it can expand. And if it is expanding, what did it expand into? There is not a comprehensive answer, so theorists write papers about bits of the problem. That's okay, since there is nothing else to do. There simply is no pat answer for such a fundamental question.
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #18 on: 15/04/2005 18:24:29 »
I am not a mathematician or a philosopher. I'm just an old over-the-hill retired physicist, and am ill equipped to engage in a philosophical discussion of infinity.

This all started with your question about what happens when photons reach the edge of the universe. All I can say is that I am unaware of any model, except for maybe the Star Trek model, that hypothesizes some kind of impenetrable barrier at the end of the universe where old photons go phooot.
 

Offline Ultima

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Re: Very basic cosmology question
« Reply #19 on: 15/04/2005 21:29:11 »
No there is a super being that lives beyond that boundary that collects them, cos Spok went there :D

wOw the world spins?
 

Offline DoctorBeaver

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Re: Very basic cosmology question
« Reply #20 on: 15/04/2005 22:16:10 »
DrPhil, you seem to be a very knowledgable person whether or not you consider yourself over the hill.
I was being a bit facetious when I mentioned about them going phut. But can I bring another dimension (no pun intended) to this? If there is enough matter in the universe to cause it to be 1 giant black hole, wouldn't gravity gradually slow the photons down and eventually pull them back?
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #21 on: 15/04/2005 23:45:03 »
The answer to that question hinges on knowing the density of the Universe. To determine that I'd need to solve the problem of the "dark matter."  If I knew that, I wouldn't be here posting at The Naked Scientists Forum, I'd be on my way to Stockholm  to collect my Nobel Prize.
 

Offline DrPhil

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Re: Very basic cosmology question
« Reply #22 on: 16/04/2005 19:53:09 »
The big bang was not an explosion in space; it was more like an explosion of space. There are no fragments of a big bang bomb flying around. Matter was not ejected at varying speeds. The galaxies are not traveling through space away from us. Individual galaxies move around at random within clusters, but the clusters of galaxies are essentially at rest. Instead the space between the galaxies and us is expanding.

There is no center. It did not go off at a particular location and spread out from there into some imagined preexisting void. It occurred everywhere at once.

The redshift is not really a doppler redshift like you hear with a moving train whistle. It is a cosmic redshift which is a bit different than a doppler shift. As space expands, light waves get stretched. If the space doubles in size during the waves' journey, their wavelengths double and their energy is halved.

Unfortunately the terms "explosion", "receding" and "moving away" are used a bit too casually.  The distances to remote galaxies is increasing because space itself is expanding.

(because I'm lazy parts of this reply were copied from: http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=2&catID=2  ) :)
 

Offline doughnut

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Re: Very basic cosmology question
« Reply #23 on: 16/04/2005 21:04:21 »
Thanks for the reply - the article is excellent just what I was looking for! :)

But now I have a problem... so the theory is saying space is expanding between (say) galaxies, but it doesn't have within it anything to explain why the further away they are the faster they are receding, it just says that's the way it is, v=Hd right?

 

Offline rosy

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Re: Very basic cosmology question
« Reply #24 on: 16/04/2005 21:12:36 »
Well, I think my understanding of it can be explained in terms of lamp-posts.
If you think of a row of lamp-posts 10m apart and the space between any two lamp-posts is increasing at a rate of 1m/s then after 1s the nearest lamp-post is 11m away (1m further) but the 10th lampost down is 110m away (as each gap has increased by 1m) and so the gap between you and that lamp-post is increasing by 10m/s as opposed to 1m/s for the nearest one. And the same will apply if you look the other way down the street, or are in a carpark with lamp-posts in all directions... generalising lamp-posts to 3D may be a challenge, I'd best not try and push the analogy too far. But you get the idea (I hope).
I'm sure that misses all sorts of subtlties but it seems to provide at least a plausible answer to your problem.
 

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Re: Very basic cosmology question
« Reply #24 on: 16/04/2005 21:12:36 »

 

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