# Thread: Size of the observable universe?

1. ## Size of the observable universe?

I'd always thought that the size of the observable universe, its radius if you will, was equivalent to the age of the universe in years times the distance light travels in a year, i.e. about 14 billion light years. But I just read on Wikipedia that the radius is in fact closer to 46.5 billion lightyears!

Is this really the generally accepted definition of "observable universe"? How can a part of the universe which we can't possibly observe for another 32.5 billion years possibly be part of the observable universe?

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This answer won't help all that much, because I have a hard time understanding it myself, but the cosmos has more than one horizon as far as observation goes. There's a universal event horizon, the particle horizon, and the Hubble distance, I think.

Basically, we can see the light of objects that are farther away from us than 14 billion light years because the space between us is expanding, and the space near an object that far away/back in time is expanding incredibly quickly.

3. Originally Posted by DanishDynamite
I'd always thought that the size of the observable universe, its radius if you will, was equivalent to the age of the universe in years times the distance light travels in a year, i.e. about 14 billion light years. But I just read on Wikipedia that the radius is in fact closer to 46.5 billion lightyears!

Misconceptions
Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these are listed below.

• 13.7 billion light-years. The age of the universe is about 13.7 billion years. While it is commonly understood that nothing travels faster than light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.7 billion light-years. This reasoning might make sense if we lived in the flat spacetime of special relativity, but in the real universe, spacetime is highly curved at cosmological scales by virtue of the Hubble expansion (though space is roughly flat). Distances obtained as the speed of light times a cosmological time interval have no direct physical significance.
• 15.8 billion light-years. This is obtained in the same way as the 13.7 billion light-year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006
• 27 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.
• 78 billion light-years. This is a lower bound (not an estimate) for the size of the whole universe (not the observable universe). If the universe is smaller than the observable universe, then light has had time to circumnavigate it since the big bang, producing multiple images of distant objects in the sky. Cornish et al looked for such an effect at scales of up to 24 gigaparsecs (78 billion light years) and failed to find it. 24 gigaparsecs is simply the upper limit of the search space of this study; it has no physical significance.
• 156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (or rather a circumference), the doubled figure is meaningless even in its original context. This figure was very widely reported.
• 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by incorrectly adding 15% to the incorrect figure of 156 billion light-years.
PS: DanishDynamite, welcome to the BAUT Forum.

4. "If the universe is smaller than the observable universe, then light has had time to circumnavigate it since the big bang, producing multiple images of distant objects in the sky. Cornish et al looked for such an effect at scales of up to 24 gigaparsecs (78 billion light years) and failed to find it."
Seems like that would be an awfully difficult search. Obviously the light from the image that had "circumnavigated" the universe would be of a different time than the image we see more directly, so apparently you've got to account for evolution... in both images. Tough study!

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If we care to tread in the semantical quagmire by positing that there are observers all over the place, it's all observable....but not to a single observer.

If we care to tread in the semantical quagmire[...]
Not worth the treading, in that it ignores what "observable universe" means: centered on an observer, not all observers.

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There is no requirement for space to be limited to the 13.7 billion light years that you get from the Hubble constant. Observations of galaxies are made to redshifts like ~6 (I don't know what the record is now) which means that we can already see 6x2 counting both directions times the 13.7 billion light years.

If the CMBR is accepted as being created in a big bang then it has a redshift of about 2000 times, and so what we see right now is from a place that is currently 2000*13.7 billion light years away now by the path that the light took, or 27 trillion light years.

8. Originally Posted by rtomes
There is no requirement for space to be limited to the 13.7 billion light years that you get from the Hubble constant. Observations of galaxies are made to redshifts like ~6 (I don't know what the record is now) which means that we can already see 6x2 counting both directions times the 13.7 billion light years.

If the CMBR is accepted as being created in a big bang then it has a redshift of about 2000 times, and so what we see right now is from a place that is currently 2000*13.7 billion light years away now by the path that the light took, or 27 trillion light years.
Wow now that is impressive, I had wondered what age the universe could be if the CBR redshift was calculated.

My best lay person explanation for co-moving expansion is adding a second liquid to one already present. The second liquid spreads faster than if it was spreading on a surface by itself because it also spreads the second liquid.

I guess in that case the first 'liquid' was space and the second was the inflation.

9. Originally Posted by Cougar
Seems like that would be an awfully difficult search. Obviously the light from the image that had "circumnavigated" the universe would be of a different time than the image we see more directly, so apparently you've got to account for evolution... in both images. Tough study!
Perhaps they are oversimplifying what the study really did-- maybe the real point was that a closed universe could have multiple paths of nearly the same length that connect us to objects in certain special locations. Otherwise, I agree with you-- it would be pretty preposterous to be able to identify the same object with light that has travelled a 10 billion light-year different path length, as on a "second pass" around the universe!

10. Originally Posted by rtomes
If the CMBR is accepted as being created in a big bang then it has a redshift of about 2000 times, and so what we see right now is from a place that is currently 2000*13.7 billion light years away now by the path that the light took, or 27 trillion light years.
With that kind of "funny math", you should be working for the Pentagon. Warning to naive readers: the above calculation does not represent a correct result of any remotely acceptable theory of cosmology.

11. Originally Posted by BISMARCK
This answer won't help all that much, because I have a hard time understanding it myself, but the cosmos has more than one horizon as far as observation goes. There's a universal event horizon, the particle horizon, and the Hubble distance, I think.

Basically, we can see the light of objects that are farther away from us than 14 billion light years because the space between us is expanding, and the space near an object that far away/back in time is expanding incredibly quickly.
You are right, that didn't help at all.

I guess my problem (well, one of my problems) is with the term "observable". I mean, all we can really observe is light, right? And light moves at the constant speed c in a vacuum. And the oldest light we could possibly observe would be light which has been underway since the beginning of the Universe.

Ergo, we cannot possibly "observe" or have direct information about anything further away from us than "age of Universe" X "light speed". At least, that's how it seems to me.

Yes, we can certainly conjecture about what has since happened to that object or process which gave rise to the light we now observe, but to include this conjecture into the sphere of "observable", as if this conjecture was as much fact as the photon we detected, seems wrong.

Or maybe I just don't get what's being said.

12. Originally Posted by 01101001
I did start to read it but got a bit bogged down. I'll try and have another go at it.

I read this bit but I didn't really see how it negated my view. Also, look at this excerpt from the article:
For example, the cosmic microwave background radiation that we see right now was emitted about 13.7 billion years ago by matter that has, in the intervening time, condensed into galaxies. Those galaxies are now about 46 billion light-years from us...
How does he know that this matter condensed into galaxies and furthermore, that these galaxies are now 46 billion light-years from us??

I mean, yes, this might be true if one assumes a bunch of stuff, but to talk about this as fact and include this "fact" as part of what can be observed, seems a bit rich for me.
PS: DanishDynamite, welcome to the BAUT Forum.
Thanks!

I've read a few of the Bad Astronomers bloggs and he's amazing. Explains stuff incredibly well and in an entertaining fashion.

And from the few threads I've read here so far, there seems to be quite a number of very capable astronomers/physicists/etc here.

Looking forward to spending some time here.

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## Really Big Shew

Originally Posted by 01101001
Not worth the treading, in that it ignores what "observable universe" means: centered on an observer, not all observers.
I'm happy with all observers everywhere - unless we're doing math. Then it's centered, for simplicity. And flat. And infinite. And unbounded. And Euclidian in small doses. And for the same reason, 13.7 works fine.

K.I.S.S.

Albert said "Keep everything as simple as possible, but do not simplify." A wonderful guiding principle.

14. Originally Posted by DanishDynamite
I guess my problem (well, one of my problems) is with the term "observable".
I think your problem is that you seem to want there to be only one definition of cosmic distances, and are upset that someone would use one you don't pick. As long as one states what type of distance they mean, where's the problem?

That was the case in the Wikipedia: Observable universe article you first cited. The author(s) plainly state the size is:

The comoving distance from the Earth to the edge of the visible universe is about 46.5 billion light-years in any direction; this is the comoving radius of the visible universe. It is sometimes quoted as a diameter of 92.94 billion light-years
Here are yet some more methods, you won't like, of defining cosmological distances: Wikipedia: Distance measures (cosmology).

15. Originally Posted by rtomes
There is no requirement for space to be limited to the 13.7 billion light years that you get from the Hubble constant.
I agree completely. I have little doubt that the Universe is much larger, possibly even infinite. I just have a problem with the way that article defines the observable Universe.
Observations of galaxies are made to redshifts like ~6 (I don't know what the record is now) which means that we can already see 6x2 counting both directions times the 13.7 billion light years.
Could you explain a bit more? I know the concept of redshift, but isn't this just a property of the light received? I mean, the photons themselves didn't originate from further away than at most some 14 billion light-years from us, did they?
If the CMBR is accepted as being created in a big bang then it has a redshift of about 2000 times, and so what we see right now is from a place that is currently 2000*13.7 billion light years away now by the path that the light took, or 27 trillion light years.
Yes, it is quite possibly true that this "place" is now some trillions of light-years away, but we can't really observe whether this is the case or not, can we? It therefore doesn't seem right to me to include this "place" as part of our observable Universe.

16. Originally Posted by 01101001
I think your problem is that you seem to want there to be only one definition of cosmic distances, and are upset that someone would use one you don't pick. As long as one states what type of distance they mean, where's the problem?
No problem at all. I was just wondering if that article really gave the generally accepted view on what "the observable Universe" meant. It was the very first time I'd ever seen or heard a suggestion that the observable Universe was bigger than about 14 billion light-years, so I was a bit taken aback.

No doubt its a failing on my part, but I just wondered if this was really the accepted view now.
That was the case in the Wikipedia: Observable universe article you first cited. The author(s) plainly state the size is:
Certainly they do, in that particular quote. But later they go on to sort of ridicule the 13.7 billion light-year interpretation and, well, this is Wikipedia, the source of all knowledge! () If the article is an opinion piece and not the generally accepted view, it should say so.
Here are yet some more methods, you won't like, of defining cosmological distances: Wikipedia: Distance measures (cosmology).
I'll have a look. Thanks.

17. My first post to 01101001 seems not to have appeared. Sorry, 01101001, but I have no control over when my posts appear. Whenever I post, I get a message that my post must first be approved by a mod, and apparently some mods are faster than others. I assume that's why my posts are not appearing in chronological order. And I assume this "mod approval" is a consequence of me being a total newbie.

18. Originally Posted by DanishDynamite
My first post to 01101001 seems not to have appeared. Sorry, 01101001, but I have no control over when my posts appear.
You can probably avoid the delay by not using actual live URL links. Possibly, just leave off the "http://" part. The posting software limits live links for new posters in a tactic to lessen spam. It will pass after some number of articles.

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## Interesting

Originally Posted by DanishDynamite
My first post to 01101001 seems not to have appeared. Sorry, 01101001, but I have no control over when my posts appear. Whenever I post, I get a message that my post must first be approved by a mod, and apparently some mods are faster than others. I assume that's why my posts are not appearing in chronological order. And I assume this "mod approval" is a consequence of me being a total newbie.
Last is probably true. They've had some startling experiences with newcomers. Spamming, flaming, sock puppets, you name it. I like your question, though.
Last edited by John Mendenhall; 2007-Sep-11 at 06:22 PM. Reason: typo

20. This is very strange. I have now made two replies to 01101001 and neither has appeared yet! Perhaps if I reply to you by just using the "Reply" button, instead of the "Quote" button, it will work? Anyway, here goes...
By 01101001
Yes I did, but I got a bit bogged down I must admit. I'll have a second go at it.
By 01101001
It adresses the view but only to pour scorn upon it. I don't really understand the argument that because spacetime is curved, this makes the idea of what is "observable" open to include stuff which is not observable by my understanding of the term.
By 01101001
PS: DanishDynamite, welcome to the BAUT Forum.
Thanks!

I've read a few bloggs by the Bad Astronomer and he is awesome! Explains things in such a way that even I think I understand them and does so in a very entertaining way as well.

By reading a few threads here it also seems to me that there are some very capable people, within the astronomy/physics area, posting here.

I very much look forward to spending some time here.

21. Wow! This seems to work. Don't understand why and it is very time-consuming, but if I just use "Reply" my message apparently comes through immediately.

So, here is a new reply to 01101001...
By 01101001
I think your problem is that you seem to want there to be only one definition of cosmic distances, and are upset that someone would use one you don't pick. As long as one states what type of distance they mean, where's the problem?
No problem at all. I was just wondering if the view given in the Wiki article was the generally accepted view now. It is certainly the first time I've come across the idea that the observable Universe is bigger than some 14 billion light-years. No doubt this is a failing on my part, but I was so taken aback that I tried to look for some confirmation that this was now the accepted view.
By 01101001
That was the case in the Wikipedia: Observable universe article you first cited. The author(s) plainly state the size is:
Yes they did, in that particular quote. They later went on to not distinguish much. And this is a Wikipedia article, the source of all knowledge! () If this article was an opinion piece, they should say so. Hence my question as to whether this was the generally accepted view now.
By 01101001
Here are yet some more methods, you won't like, of defining cosmological distances: Wikipedia: Distance measures (cosmology).
I'm sure they are fine. I was just asking about the definition of the observable Universe.

22. By 01101001
You can probably avoid the delay by not using actual live URL links. Possibly, just leave off the "http://" part. The posting software limits live links for new posters in a tactic to lessen spam. It will pass after some number of articles.
Ahh, OK. Thanks, I'll try that.

23. By John Mendenhall
Last is probably true. They've had some startling experiences with newcomers. Spamming, flaming, sock puppets, you name it.
Most boards have and I don't blame them at all.
By John Mendenhall
Thanks!

24. So, does this bring us to the answer... No, not really.
I see the finite but unbounded sticking its head up here.
And I'm not so sure that is absolutely correct.
Just the closest to logical conclusion as yet...
The issue for me is that we can not see what is 'now'. What we see has been up to 13.7 billion years in getting here. So it can not be the whole story.
I always get the impression that its risky to draw conclusions with only part of the picture.
The word infinite seems to trouble me as it seems that this has not been put away as a possibility of the universes actual reality.
If the expansion is running away at the rate as sagested...
Will we ever know?
Can I be convinced?
Will I listen...?

25. Originally Posted by astromark
The issue for me is that we can not see what is 'now'. What we see has been up to 13.7 billion years in getting here.
Even if we could see the now we could not see the future.

Originally Posted by astromark
So it can not be the whole story. I always get the impression that its risky to draw conclusions with only part of the picture.
It seems we would never see the "whole story" until the end of the Universe, so "conclusions" drawn now can't be all that bad.

Tommorrow there will be new knowledge and the conclusions will get better.

26. Originally Posted by DanishDynamite
I was just wondering if the view given in the Wiki article was the generally accepted view now.
I haven't done the artihmetic, but I think the article is saying the same thing as you, with different words. You are using different methods of describing distance. The Wikipedia article defines the method of distance to go with their number. You should do the same, saying something like: the light-travel distance of the radius of the observable universe is about 13.7 billion lightyears.

Edit: Ned Wright: Why the Light Travel Time Distance should not be used in Press Releases provides an assumption-dependent calculator for converting between methods.

27. Originally Posted by DanishDynamite
I'd always thought that the size of the observable universe, its radius if you will, was equivalent to the age of the universe in years times the distance light travels in a year, i.e. about 14 billion light years.
You just forget that while this light is traveling, the space between here and the source of the light has been expanding all the while. So when the light finally gets here, we are seeing an object that is significantly farther away than the distance light could travel in 14 billion years if space wasn't expanding. Since this light is reaching us, it is obviously part of our visible universe.

Originally Posted by DanishDynamite
...
Is that you, Casper?

28. By 01101001
I haven't done the artihmetic, but I think the article is saying the same thing as you, with different words. You are using different methods of describing distance. The Wikipedia article defines the method of distance to go with their number. You should do the same, saying something like: the light-travel distance of the radius of the observable universe is about 13.7 billion lightyears.
The article is explaining the meaning of "Observable Universe". But it does so in a way quite different from my understanding and in a way quite hostile to that understanding. Fair enough if my understanding is no longer the accepted understanding of the term. But is this the case?

29. Originally Posted by DanishDynamite
The article is explaining the meaning of "Observable Universe". But it does so in a way quite different from my understanding and in a way quite hostile to that understanding.
Again: you are talking about the same observable universe, of the same size, but using different methods of distance to attach a number to its size.

In a way, it's like feet and meters. It doesn't matter, as long as the number is accompanied by the method.

30. By Cougar
You just forget that while this light is traveling, the space between here and the source of the light has been expanding all the while. So when the light finally gets here, we are seeing an object that is significantly farther away than the distance light could travel in 14 billion years if space wasn't expanding. Since this light is reaching us, it is obviously part of our visible universe.
The light is certainly reaching us but the fate of the object producing the light is unknown. We can speculate as to what happened to it, but we cannot observe, even in principle, what happened to it. In what way is it then fair to say that this object is still part of our Observable Universe?
By Cougar
Is that you, Casper?
Nope, sorry.

I've explained the origin of my handle in the "Explain your avatar" thread in the "Off-Topic babbling" forum.

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