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DanishDynamite
2007-Sep-10, 11:33 PM
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 (http://en.wikipedia.org/wiki/Visible_universe) 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?

BISMARCK
2007-Sep-11, 01:19 AM
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.

01101001
2007-Sep-11, 01:43 AM
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 (http://en.wikipedia.org/wiki/Visible_universe) that the radius is in fact closer to 46.5 billion lightyears!

Did you read the nicely linked article on Comoving distance (http://en.wikipedia.org/wiki/Comoving_distance) there?

Observable universe :: Size :: Misconceptions (http://en.wikipedia.org/wiki/Visible_universe#Misconceptions) seems to address your former belief about size.


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.

Cougar
2007-Sep-11, 02:54 AM
"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!

GOURDHEAD
2007-Sep-11, 03:01 AM
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.

01101001
2007-Sep-11, 03:12 AM
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.

rtomes
2007-Sep-11, 04:48 AM
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.

Michael Noonan
2007-Sep-11, 11:26 AM
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.

Ken G
2007-Sep-11, 04:58 PM
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!

Ken G
2007-Sep-11, 05:02 PM
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.

DanishDynamite
2007-Sep-11, 05:17 PM
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. :lol:

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.

DanishDynamite
2007-Sep-11, 05:31 PM
Did you read the nicely linked article on Comoving distance (http://en.wikipedia.org/wiki/Comoving_distance) there?
I did start to read it but got a bit bogged down. I'll try and have another go at it. :)


Observable universe :: Size :: Misconceptions (http://en.wikipedia.org/wiki/Visible_universe#Misconceptions) seems to address your former belief about size.
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.

John Mendenhall
2007-Sep-11, 05:33 PM
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.

01101001
2007-Sep-11, 05:34 PM
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 (http://en.wikipedia.org/wiki/Visible_universe) article you first cited. The author(s) plainly state the size is:


The comoving distance (http://en.wikipedia.org/wiki/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) (http://en.wikipedia.org/wiki/Distance_measures_%28cosmology%29).

DanishDynamite
2007-Sep-11, 05:50 PM
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.

DanishDynamite
2007-Sep-11, 06:02 PM
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 (http://en.wikipedia.org/wiki/Visible_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) (http://en.wikipedia.org/wiki/Distance_measures_%28cosmology%29).
I'll have a look. Thanks.

DanishDynamite
2007-Sep-11, 06:10 PM
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. :)

01101001
2007-Sep-11, 06:15 PM
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.

John Mendenhall
2007-Sep-11, 06:17 PM
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.

DanishDynamite
2007-Sep-11, 06:50 PM
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
Did you read the nicely linked article on Comoving distance there?
Yes I did, but I got a bit bogged down I must admit. I'll have a second go at it. :)

By 01101001
Observable universe :: Size :: Misconceptions seems to address your former belief about size.
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. :)

DanishDynamite
2007-Sep-11, 07:08 PM
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.

DanishDynamite
2007-Sep-11, 07:11 PM
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.

DanishDynamite
2007-Sep-11, 07:16 PM
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
I like your question, though.
Thanks!

astromark
2007-Sep-11, 07:42 PM
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...?

pzkpfw
2007-Sep-11, 08:04 PM
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.



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.

01101001
2007-Sep-11, 08:33 PM
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 (http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html) provides an assumption-dependent calculator for converting between methods.

Cougar
2007-Sep-11, 08:33 PM
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.


...
Is that you, Casper (http://en.wikipedia.org/wiki/Danish_Dynamite)? ;)

DanishDynamite
2007-Sep-11, 09:03 PM
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?

01101001
2007-Sep-11, 09:07 PM
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.

DanishDynamite
2007-Sep-11, 09:10 PM
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.

speedfreek
2007-Sep-11, 09:13 PM
This link is a very good guide to sorting out all the different distances involved:

The Distance Scale of the Universe (http://www.atlasoftheuniverse.com/redshift.html)

A quick summation: The most distant galaxies have redshifts of around z=15, they were only around 2 billion light years away from this point in space when they emitted that light, the light took around 13 billion years to reach us because the space it was travelling through was expanding whilst it travelled, and those most distant galaxies are estimated to be something over 35 billion light years away by now. We often sum this all up by incorrectly stating they are around 13 billion light years away, when that figure has no relevance as a distance, it only has relevance as a measure of time.

DanishDynamite
2007-Sep-11, 09:17 PM
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.
I must disagree. One uses only the constancy of light-speed as the basis for its definition, the other assumes the Big Bang, the Hubble Law and a weird re-definition of "observable".

DanishDynamite
2007-Sep-11, 09:19 PM
This link is a very good guide to sorting out all the different distances involved:

The Distance Scale of the Universe (http://www.atlasoftheuniverse.com/redshift.html)

A quick summation: The most distant galaxies have redshifts of around z=15, they were only around 2 billion light years away from this point in space when they emitted that light, the light took around 13 billion years to reach us because the space it was travelling through was expanding whilst it travelled, and those most distant galaxies are estimated to be something over 35 billion light years away by now. We often sum this all up by incorrectly stating they are around 13 billion light years away, when that figure has no relevance as a distance, it only has relevance as a measure of time.
My highlighting.

Why use a defintion which requires estimating anything?

01101001
2007-Sep-11, 09:22 PM
I must disagree. One uses only the constancy of light-speed as the basis for its definition, the other assumes the Big Bang, the Hubble Law and a weird re-definition of "observable".

You've asked your question and gotten several answers from out of mainstream science. If you wish to assert those answers are weird and wrong, you may find the place for that is the Against the Mainstream (http://www.bautforum.com/against-mainstream/) (ATM) subforum.

DanishDynamite
2007-Sep-11, 09:35 PM
You've asked your question and gotten several answers from out of mainstream science. If you wish to assert those answers are weird and wrong, you may find the place for that is the Against the Mainstream (http://www.bautforum.com/against-mainstream/) (ATM) subforum.
I really just wanted a clear answer to my question.

DanishDynamite
2007-Sep-11, 09:51 PM
Finally!

Just noticed that my first two replies to 01101001 have now arrived. :)

01101001
2007-Sep-11, 10:00 PM
I really just wanted a clear answer to my question.

Sorry, but I disagree, from what I've seen. You want to argue, I think. It seems like our statements are just being met with counter-statements.

Attempts to imply it is wrong to assume the Big Bang and the Hubble Law, are best conducted in a place like ATM.

rtomes
2007-Sep-11, 10:03 PM
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.According to wikipedia the z of the CMBR is 1089. If you consider the path taken by that radiation to reach us, then the current length of that path is 1+z=1090 times 13.7 billion light years according to the big bang theory. You should check your facts rather than insult people.

DanishDynamite
2007-Sep-11, 10:19 PM
Sorry, but I disagree, from what I've seen. You want to argue, I think. It seems like our statements are just being met with counter-statements.

Attempts to imply it is wrong to assume the Big Bang and the Hubble Law, are best conducted in a place like ATM.
I assumed the Wiki article was telling it as it was and that I was just behind the times. Wiki usually tells it as it is, afterall, at least in my experience.

I thought this forum would be a good place to understand where I went wrong and Wiki went right.

If you feel that me arguing my case vs. Wiki is just "wanting to argue", then I'm sorry you feel this way. But I'm not sorry for arguing my case. I'm only sorry that this place did not provide the enlightenment I was expecting.

I am thus still unconvinced that the authors of the Wiki article are giving anything other than their own little biased opinion on what the term "Observable Universe" means. An opinion which is not generally accepted.

Cougar
2007-Sep-12, 02:13 PM
I thought this forum would be a good place to understand where I went wrong and Wiki went right.
Take another look at that link provided by Speedfreek. I don't think it can be explained much better than that.

Note: One cannot (accurately) picture the universe as if you're looking down on a little spherical model. It's the relativity, stupid! (No, not you! That's a take-off from an old Clinton saying.) There's no absolute time. No absolute frame of reference. The light you're seeing now may have been emitted billions of years ago.

ToSeek
2007-Sep-12, 02:59 PM
According to wikipedia the z of the CMBR is 1089. If you consider the path taken by that radiation to reach us, then the current length of that path is 1+z=1090 times 13.7 billion light years according to the big bang theory. You should check your facts rather than insult people.

This is wrong. The "z" indicates (among other things) the factor by which the universe has expanded since the light was emitted. Yes, the universe is 1090 times as large as when the CMBR was generated, but there's absolutely no reason to think that the universe was 13.7 billion light years across at that time.

01101001
2007-Sep-12, 03:21 PM
I am thus still unconvinced that the authors of the Wiki article are giving anything other than their own little biased opinion on what the term "Observable Universe" means. An opinion which is not generally accepted.

Maybe not accepted by you, but accepted by a highly respected cosmologist (http://en.wikipedia.org/wiki/Edward_L._Wright).

Ned Wright's Cosmology FAQ: If the Universe is only 14 billion years old, how can we see objects that are now 47 billion light years away? (http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN)


When talking about the distance of a moving object, we mean the spatial separation NOW, with the positions of both objects specified at the current time. In an expanding Universe this distance NOW is larger than the speed of light times the light travel time due to the increase of separations between objects as the Universe expands. This is not due to any change in the units of space and time, but just caused by things being farther apart now than they used to be.
[...]
The current best fit model which has an accelerating expansion gives a maximum distance we can see of 47 billion light years.

Or, accepted by another highly respected cosmologist (http://en.wikipedia.org/wiki/Sean_M._Carroll).

Sean Carroll's Preposterous Universe Blog: Doing away with dark energy? (http://preposterousuniverse.blogspot.com/2005/03/doing-away-with-dark-energy.html) March 22, 2005


In general relativity, no influence can travel faster than the speed of light. Since there is only a finite time since the Big Bang (14 billion years), there is only a finite piece of universe that possibly could affect what we see today. (The observable universe actually has a radius of about 46 billion light years, not 14 billion light years, because of sneaky expansion effects.)

It's promoted by a popular-science publication (http://en.wikipedia.org/wiki/Scientific_American).

Scientific American: Misconceptions about the Big Bang (http://www.sciam.com/article.cfm?chanID=sa006&articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=5&catID=2&colId=1) Page 5, March 2005


If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.

Nereid
2007-Sep-12, 03:28 PM
I really just wanted a clear answer to my question.Do you now understand at least there is a bit of confusion in your question, or that there is some inconsistency?

If you "use[] only the constancy of light-speed as the basis for [distance] definition", you are using Special Relativity.

Special Relativity is a 'limit' of General Relativity, a limit where mass/energy can be ignored.

The definitions of distance, described in the various webpages you can get to following the links, come from using 'distance' in General Relativity.

As several folk here have indicated, once you accept relativity, everything else posted here follows.

But perhaps this just means that your real question is something like 'why do scientists (astronomers, physicists, etc) think Einstein's theory of (special, general, or both) relativity is right'?

BTW, in one sense of 'observable', we can 'see' back to a much higher 'z' (redshift) than the cosmic microwave background; we can 'see' up to a z of ~10^10 (approx 10 billion! the CMB is 'only' ~1000), by analysing the abundance of the light nuclides hydrogen, deuterium, 3He, 4He, and 7Li. If we are ever able to 'see' the sea of relict neutrinos, then the observable universe will be even bigger (nucleosynthesis ended around 3 minutes (co-moving time); neutrinos decoupled at around 1 second). Finally, details of the CMB angular power spectrum, and its polarisation, allow us to 'see' back in time even further ...

eburacum45
2007-Sep-12, 03:57 PM
This is wrong. The "z" indicates (among other things) the factor by which the universe has expanded since the light was emitted. Yes, the universe is 1090 times as large as when the CMBR was generated, but there's absolutely no reason to think that the universe was 13.7 billion light years across at that time.

In fact the visible universe was only 40 million light years in radius when it first became transparent, according to Wikipedia. From here
http://en.wikipedia.org/wiki/Visible_universe#Size

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, but at the time the light was emitted, that matter was only about 40 million light-years away from the matter that would eventually become the Earth.

John Mendenhall
2007-Sep-12, 04:40 PM
Do you now understand at least there is a bit of confusion in your question, or that there is some inconsistency? (snip)

Good post, Nereid, folks, go back and read the snip, I don't want to make a lot of clutter.

. . . as General Relativity lets planets "know" the present location of the Sun, rather than its apparent location (for us, 8.3 minutes delayed, see the numerous threads and Wiki on the Speed of Gravity), then the gravitational influence of the most distant objects should appear to us to come from their present location, at ~35 billion light years, rather than ~13 billion? A startling idea.

I know, the universe being uniform in all directions, on the largest scales the net influence sums to zero. But how about the great attractor? Is there enough information avalable to do calculations?

CodeSlinger
2007-Sep-12, 04:50 PM
John, I thought GR predicts the speed of gravity to be exactly the speed of light?


Where no other theory is specified, discussion of the speed of gravity is normally in reference to general relativity, which predicts it to equal c.

Therefore, gravitational influence of distant objects will not have any discrepancy with the light we receive from them.

neilzero
2007-Sep-12, 05:25 PM
So the Universe is 1090 times bigger now, than when the CMBR (cosmic microwave background radiation) was emitted, And the radius of what we see now was then 40 million lightyears? 1090 times 40 million = 43.6 billion light years = about 3 times what we can see now. Was it perhaps 15 times bigger when the light left the most distant stars we can detect? Neil

speedfreek
2007-Sep-12, 06:45 PM
Yup, that's the theory

If the light from the most distant galaxy we have seen has a redshift of z=15 (I'm not sure of the exact figure!), then the universe is now 15 times larger than when that light was emitted from that galaxy.

So using a figure of 46.5 billion light years, this would mean that our observable universe was only around 3 billion light years in radius when the light left that galaxy and started its journey towards us. Of course, the whole universe could have been any amount larger at that time!

For DanishDynamite: What is it you want to know about the observable universe? How large it is? We have to ask, how large it is when? How large our observable universe was when the light left those objects we see? Or how large the observable universe is now, as their light reaches us? They are two different questions and require two different answers, and both answers are our current best estimates using our current observations and understanding. The first question can be answered through observation (their apparent size), but the second can only be answered through understanding (what has happened to their light during its journey, why is it very dim and why are the emission/absorption lines shifted towards the red end of its spectrum?). Nothing is known for sure, but we are getting more confident of our answers as time goes on. Of course there is always the chance that new observations will change those estimates. :)

Nereid
2007-Sep-12, 07:13 PM
For those interested - and many who've come to BAUT's Q&A section have been - google on (Charles H.) Lineweaver and (Tamara M.) Davis; there are some very good material published by these two, which explain not only the distance things, but also 'how come parts of the universe we can now 'see' are moving away from us at > c?'

For example (http://arxiv.org/abs/astro-ph/0310808): "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe":
We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe. To show the abundance of these misconceptions we cite numerous misleading, or easily misinterpreted, statements in the literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the ``observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. Attempts to restrict recession velocities to less than the speed of light require a special relativistic interpretation of cosmological redshifts. We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23 sigma.At the bottom of Lineweaver's homepage (Publications) (http://www.mso.anu.edu.au/~charley/publications.html) is a link to a PDF document, the Scientific American, March 2005 article; it's aimed at a more general audience.

Ken G
2007-Sep-12, 10:56 PM
According to wikipedia the z of the CMBR is 1089. If you consider the path taken by that radiation to reach us, then the current length of that path is 1+z=1090 times 13.7 billion light years according to the big bang theory. You should check your facts rather than insult people.I did not insult you-- I said your math is wrong. It is wrong, but that's not an insult-- we all make mistakes! My insult was of the Pentagon, which seems to do it on purpose! I did not mean to intimate that you were doing it on purpose-- in fact your mistake was a very easy one to make as clearly explained above by ToSeek. There's already so much confusion about the distance scale I wanted to nip in the bud any proliferation of additional confusion, but I probably should have explained why it was wrong the way ToSeek did, to leave that out did seem a bit harsh on my part.

DanishDynamite
2007-Sep-14, 06:55 PM
Take another look at that link provided by Speedfreek. I don't think it can be explained much better than that.

Note: One cannot (accurately) picture the universe as if you're looking down on a little spherical model. It's the relativity, stupid! (No, not you! That's a take-off from an old Clinton saying.) There's no absolute time. No absolute frame of reference. The light you're seeing now may have been emitted billions of years ago.
Yes, I think I understand the concept of Co-Moving. It is the extrapolation of where things we can actually observe now would probably be now, if we assume these things have moved subsequently in a certain manner.

I find such extrapolation hard to reconcile with the term "observable".

DanishDynamite
2007-Sep-14, 07:07 PM
Maybe not accepted by you, but accepted by a highly respected cosmologist (http://en.wikipedia.org/wiki/Edward_L._Wright).

Ned Wright's Cosmology FAQ: If the Universe is only 14 billion years old, how can we see objects that are now 47 billion light years away? (http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN)

Or, accepted by another highly respected cosmologist (http://en.wikipedia.org/wiki/Sean_M._Carroll).

Sean Carroll's Preposterous Universe Blog: Doing away with dark energy? (http://preposterousuniverse.blogspot.com/2005/03/doing-away-with-dark-energy.html) March 22, 2005

It's promoted by a popular-science publication (http://en.wikipedia.org/wiki/Scientific_American).

Scientific American: Misconceptions about the Big Bang (http://www.sciam.com/article.cfm?chanID=sa006&articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=5&catID=2&colId=1) Page 5, March 2005
Thanks for the info, 01101001. Your links are certainly evidence that more knowledgable people than just the authors of the Wiki article at least ackowledge the concept or even endorse it.

It isn't evidence that this way of looking at the Universe is the generally accepted view of what constitutes "the Observable Universe", though.

I don't know, maybe I'm just being a stick in the mud. It just seems a total defilement of my understanding of the term "observable".

DanishDynamite
2007-Sep-14, 07:16 PM
Do you now understand at least there is a bit of confusion in your question, or that there is some inconsistency?

If you "use[] only the constancy of light-speed as the basis for [distance] definition", you are using Special Relativity.

Special Relativity is a 'limit' of General Relativity, a limit where mass/energy can be ignored.

The definitions of distance, described in the various webpages you can get to following the links, come from using 'distance' in General Relativity.

As several folk here have indicated, once you accept relativity, everything else posted here follows.

But perhaps this just means that your real question is something like 'why do scientists (astronomers, physicists, etc) think Einstein's theory of (special, general, or both) relativity is right'?

BTW, in one sense of 'observable', we can 'see' back to a much higher 'z' (redshift) than the cosmic microwave background; we can 'see' up to a z of ~10^10 (approx 10 billion! the CMB is 'only' ~1000), by analysing the abundance of the light nuclides hydrogen, deuterium, 3He, 4He, and 7Li. If we are ever able to 'see' the sea of relict neutrinos, then the observable universe will be even bigger (nucleosynthesis ended around 3 minutes (co-moving time); neutrinos decoupled at around 1 second). Finally, details of the CMB angular power spectrum, and its polarisation, allow us to 'see' back in time even further ...
I don't think I'm using Special Relativity, or any other theory or law in my understanding of what "observable" means. I think I'm only using the constancy of light-speed, simple math and the scientific definition of "observable".

DanishDynamite
2007-Sep-14, 07:34 PM
For DanishDynamite: What is it you want to know about the observable universe? How large it is? We have to ask, how large it is when? How large our observable universe was when the light left those objects we see? Or how large the observable universe is now, as their light reaches us? They are two different questions and require two different answers, and both answers are our current best estimates using our current observations and understanding.
My question, from the very beginning, was whether the Wiki article I linked to really provided the generally accepted answer to the question: "What is meant by the Observable Universe?"

I found the answer given completely at odds with my own remembered definition as well as it being at odds with my understanding of the term "observable". And I don't just mean in the everyday usage of the term, but in the Physics way of using the term.

Calling the existence of something, whose fate we cannot even in principle observe for 30 billion years, observable, goes against every scientific fiber in my body.

The first question can be answered through observation (their apparent size), but the second can only be answered through understanding (what has happened to their light during its journey, why is it very dim and why are the emission/absorption lines shifted towards the red end of its spectrum?). Nothing is known for sure, but we are getting more confident of our answers as time goes on. Of course there is always the chance that new observations will change those estimates. :)
Let's make a deal. Let's agree to call everything which impinges our sensors or which could in principle do so, observable. Let's call everything which doesn't and couldn't in principle do so....something else. In regard to the question currently explored, I suggest calling things 46 billion light-years away to be part of our "Extrapolated Universe". What do you think?

speedfreek
2007-Sep-15, 05:18 PM
Hey, we don't invent these terms here, nor do they invent them on wikipedia, you know! The definition of "observable" in cosmology is as removed from other fields of science as the definition of expansion! :)

I think the problem you are having is that you are thinking in terms of the "observed" universe, rather than the observable universe.

A better way to think of it might be "all the information that is heading towards us right now, which includes all the light that is coming, but hasn't got here yet!" :whistle:

DanishDynamite
2007-Sep-16, 02:30 AM
Hey, we don't invent these terms here, nor do they invent them on wikipedia, you know! The definition of "observable" in cosmology is as removed from other fields of science as the definition of expansion! :)
Cosmology is almost exclusively within the area of Physics, as far as I know.

I think the problem you are having is that you are thinking in terms of the "observed" universe, rather than the observable universe.

A better way to think of it might be "all the information that is heading towards us right now, which includes all the light that is coming, but hasn't got here yet!" :whistle:
"The check's in the mail". Not usually accepted by the Physics bookkeeping department. :)

Ken G
2007-Sep-16, 05:40 AM
I actually don't think that's the conventional meaning of observable, it's more a synonym to observed. The distinction is that there may be sources that our telescope technology cannot yet detect, but their light is already here. A key issue is that the objects are not "now" as we are seeing them, and their location "now" depends both on how far they were when the light was emitted, and how much it has been redshifted by the subsequent expansion of space.

speedfreek
2007-Sep-16, 05:26 PM
Yes, sorry Ken, you are correct. I might have been muddying the waters there a bit.

DanishDynamite
2007-Sep-17, 08:34 PM
I actually don't think that's the conventional meaning of observable, it's more a synonym to observed. The distinction is that there may be sources that our telescope technology cannot yet detect, but their light is already here. A key issue is that the objects are not "now" as we are seeing them, and their location "now" depends both on how far they were when the light was emitted, and how much it has been redshifted by the subsequent expansion of space.
Tell me, Ken G, do you accept and feel comfortable with the definition given in the Wiki article?

Nereid
2007-Sep-18, 12:37 AM
Do you now understand at least there is a bit of confusion in your question, or that there is some inconsistency?

If you "use[] only the constancy of light-speed as the basis for [distance] definition", you are using Special Relativity.

Special Relativity is a 'limit' of General Relativity, a limit where mass/energy can be ignored.

The definitions of distance, described in the various webpages you can get to following the links, come from using 'distance' in General Relativity.

As several folk here have indicated, once you accept relativity, everything else posted here follows.

But perhaps this just means that your real question is something like 'why do scientists (astronomers, physicists, etc) think Einstein's theory of (special, general, or both) relativity is right'?

BTW, in one sense of 'observable', we can 'see' back to a much higher 'z' (redshift) than the cosmic microwave background; we can 'see' up to a z of ~10^10 (approx 10 billion! the CMB is 'only' ~1000), by analysing the abundance of the light nuclides hydrogen, deuterium, 3He, 4He, and 7Li. If we are ever able to 'see' the sea of relict neutrinos, then the observable universe will be even bigger (nucleosynthesis ended around 3 minutes (co-moving time); neutrinos decoupled at around 1 second). Finally, details of the CMB angular power spectrum, and its polarisation, allow us to 'see' back in time even further ...I don't think I'm using Special Relativity, or any other theory or law in my understanding of what "observable" means. I think I'm only using the constancy of light-speed, simple math and the scientific definition of "observable"."the constancy of light-speed" is the second postulate of Special Relativity:
Second postulate - Invariance of c - The speed of light in a vacuum is a universal constant, c, which is independent of the motion of the light source.It is certainly counter-intuitive that an invariant c can lead to the definition of 'observable' that you are having difficulty with*, but that's just what it is.

So, to rephrase, perhaps your question is more "how does an invariant c lead to such complications and counter-intuitive results, wrt 'observable'?"?

If you've not studied SR before, you have yet to discover some of the remarkable things the follow from just the two postulates (the other one is "The laws of physics are the same in all inertial frames of reference. In other words, there are no privileged inertial frames of reference.")

Would you like some suggestions on where to learn about SR?

*You actually need the more general GR, but the principles are essentially the same.

Ken G
2007-Sep-18, 05:24 AM
Tell me, Ken G, do you accept and feel comfortable with the definition given in the Wiki article?

The definition of observable universe seems fine, but I can't say I think very much of the explanation of the Hubble limit. The article in general has some strange emphases, like mentioning the duplicate image ideas and the lower bound of 78 billion LY so early in the article, which seem pretty sketchy when there are so many very solid observational facts to start with. Then there's this odd claim:

In the aftermath of the Big Bang everything in the universe is flying apart, and due to the fact that the speed of light is constant, farther objects appear to be receding at a higher velocity. Eventually an object will appear to have a velocity which is the speed of light, and an object at this point is known to be at the Hubble Limit.
I have no idea what this is trying to say, but both sentences seem pretty badly muddled to me. I don't see why the speed of light being constant is required for farther objects to be receding at a higher velocity, and I don't see why they would want to suggest that an object at the Hubble limit "appears" to have a velocity which is the speed of light. The Hubble limit is a calculated location where the rate of recession is c right now-- it has nothing at all to do with how an object will "appear", we're not even seeing the objects at the Hubble limit at the moment.

The article does not even seem to be aware that the linear law generally termed the "Hubble law" can only be said to be observed fairly nearby because it requires a fixed Hubble constant H-- at larger distances the Hubble law is entirely extrapolated using the cosmological principle. The objects we actually see at those distances we are seeing well in the past when the Hubble constant (which is constant in space not time) was larger, and an object that we infer to be "now" at the Hubble limit (if the object even still exists) just means it is now receding from us at c-- it does not mean its recessional speed was c when it emitted the light we are now detecting from it.

Cougar
2007-Sep-18, 01:51 PM
In the aftermath of the Big Bang everything in the universe is flying apart, and due to the fact that the speed of light is constant, farther objects appear to be receding at a higher velocity.
OMG, yes, that is terrible!

Ken G
2007-Sep-18, 03:40 PM
Yeah, it really sounds like the old "explosion" misconception, although I'm still not sure what it was intended to mean. That's all we need, one more reputable source to nurture that misconception-- the name Big Bang wasn't bad enough.

John Mendenhall
2007-Sep-18, 05:45 PM
Yeah, it really sounds like the old "explosion" misconception, although I'm still not sure what it was intended to mean. That's all we need, one more reputable source to nurture that misconception-- the name Big Bang wasn't bad enough.

Wiki is a publicly editable encyclopedia. They'll pick up some wrong, or incompetent, or malicious edits. Look at the problems they've had with the evolution article. I love Wiki, it's a great place to start because it's so fast and easy to get to, but keep your skepticism level on high.

With respect to the article in question on 'Observable Universe', I get a distinct feeling of at least two contributors, of very different levels of competence. I don't want to tackle fixing that unclear section; there are better people aplenty at SR and GR than me, right here on BAUT. Publius?

01101001
2007-Sep-18, 06:07 PM
Thinking it's better to light a candle, though not being a cosmologist, I took a stab at editing away the Hubble limit (http://en.wikipedia.org/wiki/Observable_universe#Hubble_limit) rationale, leaving the Hubble articles to do the heavy lifting:


Hubble limit
The Hubble limit or Hubble radius is a concept in physical cosmology that is related to the Big Bang Theory and Hubble's law. It refers to the distance at which objects receding from the observer are receding at the speed of light. It is named after the astronomer Edwin Hubble, who was the first to discover that objects on a galactic scale are moving away from us, and that more distant objects appear to be receding at a higher velocity. There is a distance from the observer at which an object will appear to have a velocity which is the speed of light, and an object at this point is known to be at the Hubble limit.

Yes, I'm sure it still needs work. I can only nudge it towards goodness.

The original flying-apart rationale seemed to have come from some hobbyist site: Taylormade.com: The age of the Universe (http://www.taylormade.com.au/billspages/relativity/relativity5.html). It didn't seem worth preserving that reference.

Edit: too late I notice I retained the "appear to have a velocity" wording that Ken G objected to. Oh, well. Not enough passion or research here. Anyone can be an editor. My words eagerly await your reshaping.

Ken G
2007-Sep-19, 05:11 PM
Yup, you caught it-- the Hubble limit has naught to do with appearances, it is a purely theoretical distance. One should probably mention the "cosmological principle", because that is the source of the "Hubble law", whenever said law is applied to the meaning of "distance now". Distance now is a theoretical construct that requires the cosmological principle, and said principle all by itself is all you need for a Hubble law. But what some people mean by "Hubble law" is the local version found by Hubble, over scales where everything you are seeing is coming from the same epoch of the expansion and the same Hubble constant applies. The global Hubble law also uses a single constant, but as it is not physically connected to the value that constant would have attained for the objects being seen, and the objects being seen are not seen at the time we conceptualize their distance "now", the global Hubble law (that you need for the Hubble limit) is theoretical not observable.

DanishDynamite
2007-Sep-19, 07:57 PM
"the constancy of light-speed" is the second postulate of Special Relativity:
Yes it is, but the constancy of light-speed is not Special Reletavity.

It is certainly counter-intuitive that an invariant c can lead to the definition of 'observable' that you are having difficulty with*, but that's just what it is.

So, to rephrase, perhaps your question is more "how does an invariant c lead to such complications and counter-intuitive results, wrt 'observable'?"?
Relativity, Special or General, leads to results which seem counter-intuitive, yes. But one doesn't need relativity theory to define the term "observable". Physics certainly doesn't. Hence my opposition at the Wiki article.

If you've not studied SR before, you have yet to discover some of the remarkable things the follow from just the two postulates (the other one is "The laws of physics are the same in all inertial frames of reference. In other words, there are no privileged inertial frames of reference.")

Would you like some suggestions on where to learn about SR?
I think I already have a decent grasp of SR. I honestly don't see how it relates to the question at hand, though.

DanishDynamite
2007-Sep-19, 08:20 PM
The definition of observable universe seems fine, but I can't say I think very much of the explanation of the Hubble limit.
I guess it must just be me then.

Ken G
2007-Sep-20, 01:42 AM
But one doesn't need relativity theory to define the term "observable". Physics certainly doesn't. Hence my opposition at the Wiki article.

I don't understand what you mean, the Wiki's definition of "observable" did not reference any relativity in any way. It is a general meaning that will work in any situation-- a sphere of matter such that, in the age of the universe (be it finite or infinite, though it's all pretty moot if infinite), there has been time for light from that material to reach the Earth by now. All it assumes is that we do our observing via light, a pretty good assumption at the moment. However, when you want to calculate its size, you then need physics and observations, and they do that to arrive at the current mainstream result.

DanishDynamite
2007-Sep-22, 12:36 AM
I don't understand what you mean, the Wiki's definition of "observable" did not reference any relativity in any way. No it didn't, but Nereid did.

It is a general meaning that will work in any situation-- a sphere of matter such that, in the age of the universe (be it finite or infinite, though it's all pretty moot if infinite), there has been time for light from that material to reach the Earth by now.
YES! That is exactly the definition that I agree with. But that is not the definition being used in the article.

The article, instead, looks at where, according to a bunch of assumptions, that matter might be NOW, and calls this speculation "observable"!

All it assumes is that we do our observing via light, a pretty good assumption at the moment. However, when you want to calculate its size, you then need physics and observations, and they do that to arrive at the current mainstream result.
The only thing we can observe is light arriving in our detectors now. How do we know how old this light is? Which physical laws must we use to arrive at this age?

Van Rijn
2007-Sep-22, 12:55 AM
The article, instead, looks at where, according to a bunch of assumptions, that matter might be NOW, and calls this speculation "observable"!


It's far from "speculation." There are a number of related issues derived from evidence and related theory, and those are used for the predictions. Given the speed of light, and a determination of distance, you can calculate how long the light has been travelling. Given a determination of relative velocity, you can calculate how far away an object would be now.



The only thing we can observe is light arriving in our detectors now. How do we know how old this light is? Which physical laws must we use to arrive at this age?

I'm sure you're aware of various ways to determine the distance to the sun or another planet, and methods for determining the speed of light. Once you know, for example, the distance of the sun, and given the speed of light, you can determine how long ago the light left the sun.

DanishDynamite
2007-Sep-22, 01:11 AM
It's far from "speculation." There are a number of related issues derived from evidence and related theory, and those are used for the predictions. Given the speed of light, and a determination of distance, you can calculate how long the light has been travelling. Given a determination of relative velocity, you can calculate how far away an object would be now.
We are looking at the ultra basic concept of "observable". Something is observable if it impinges our sensors or could in principle do so. One requirement for something which could in principle impinge our sensors, is that it not be farther away from us than the age of Universe allows that a photon could have travelled from it to our sensors.

I'm sure you're aware of various ways to determine the distance to the sun or another planet, and methods for determining the speed of light. Once you know, for example, the distance of the sun, and given the speed of light, you can determine how long ago the light left the sun.
Please, if you can, answer my question directly. It is important, as perhaps there is a relationship I'm not aware of.

Van Rijn
2007-Sep-22, 01:28 AM
Please, if you can, answer my question directly. It is important, as perhaps there is a relationship I'm not aware of.

It was a direct answer to this question:


The only thing we can observe is light arriving in our detectors now. How do we know how old this light is? Which physical laws must we use to arrive at this age?

DanishDynamite
2007-Sep-22, 02:02 AM
It was a direct answer to this question:
Sorry, but it wasn't. Here was my question:

The only thing we can observe is light arriving in our detectors now. How do we know how old this light is? Which physical laws must we use to arrive at this age?
So, how do we know how old this light is?

And which physical laws must we use to arrive at this age?

Ken G
2007-Sep-22, 02:41 AM
YES! That is exactly the definition that I agree with. But that is not the definition being used in the article.
Actually, it is. But the article then goes on to try and determine what distance that is now. I see your objection with calling that observable, but that gas is out there somewhere, is it not? Perhaps "observed universe" is better than "observable universe", I take them as virtual synonyms. There's no good solution, we see that material, but not as it is now, or where it is now-- we have to make some inferences.


The only thing we can observe is light arriving in our detectors now. How do we know how old this light is? Which physical laws must we use to arrive at this age?
We have a host of physical laws at our disposal to reach those inferences, and are beginning to see a consistent picture emerging, though we can't explain what dark matter or dark energy are yet. Come back in 50 years!

Van Rijn
2007-Sep-22, 04:07 AM
Sorry, but it wasn't. Here was my question:

So, how do we know how old this light is?


In general, you determine when light was emitted by dividing the distance of the source of the light by the speed of light, as with my sun example in my earlier post.



And which physical laws must we use to arrive at this age?

A key item is the (verified by numerous experiments) the invariable speed of light. There are many methods for determining distance, some used for nearer objects, some for more distant. On the cosmological scale, type 1a supernovas are used. Here is an article on standard candles and distance measurement. (http://www.astrophysicsspectator.com/topics/overview/DistanceExtragalactic.html)

publius
2007-Sep-22, 04:24 AM
I don't want to tackle fixing that unclear section; there are better people aplenty at SR and GR than me, right here on BAUT. Publius?

You rang? I just got around to reading this thread. Unfortunately, don't ring for me when it comes to Cosmology. I know just enough to be dangerous, and it's best for me to do more listening than speaking. :) If I see something I can handle, I'll jump in.

Many of these questions are "mere coordiante issues". You've got a "metric expansion" metric going on, and you coordinatize it someway. How one "explains things" depends on that as much as it does "real physics". :)

So until I master the metric expansion background and understand it in my own weird way and know what are coordinates and what is invariant and all the ways one can do that, I just remain quiet.

-Richard

Cougar
2007-Sep-22, 04:55 PM
One requirement for something which could in principle impinge our sensors, is that it not be farther away from us than the age of Universe allows that a photon could have travelled from it to our sensors.
There could be a problem with that perceived requirement. Cosmic inflation (http://en.wikipedia.org/wiki/Cosmic_inflation).

speedfreek
2007-Sep-24, 07:05 PM
Well it seems to me that, if we are strictly speaking, the observable universe should be defined as "everything it is possible for us to see or measure" - all that we can observe. If we have the most sensitive instruments possible, the sum of everything we can detect would be our observable universe. Unfortunately, I feel this strict definition is of little use.

Currently, our observable universe is a volume of space around us, filled with sources of radiation that we detect in various ways over various frequencies. When we compare different detection methods for the same piece of sky, we get different ranges of results depending on which object we look at in that area. Some galaxies have an apparently large angular diameter, but are very dim and spectroscopy shows that their emission/absorption lines have been apparently shifted across their spectrum, towards the red. Some other galaxies have an apparently much smaller angular diameter, but are quite bright and spectroscopy shows little apparent shifting of their spectrum. We reach a point where we can detect no dimmer galaxies. We also measure an almost uniform background radiation, with a very high apparent shifting of its spectrum, a hundred times more shifted than any galaxy we have measured.

For the purposes of this post, those are our observations. Everything other than the above is interpretation of the data. On its own, our definition is of little use. It is describing what we see, without asking us to think about what we see or to try to understand what we see. We need some way to interpret those observations.

So, we see galaxies of similar structure, which we consider to be of similar absolute size. But some of them are incredibly dim and the smallest of the incredibly dim galaxies has a relatively large angular diameter. We consider the smallest of the dimmest galaxies of this structure type, to be the furthest away, as we infer that their light has been travelling for the longest time. These dimmest galaxies have the highest redshifts.

Now we look at data for a lot brighter galaxies of the same structure type. The smallest of these brighter galaxies is actually smaller than the smallest of the very dim! (Remember I am talking about apparent angular size here, and assuming the galaxies are of similar actual or absolute size). These galaxies all have smaller redshifts than the dimmest ones.

We interpret the difference in the angular diameter of apparently similar objects as a difference in distance when their light was emitted. We interpret the dimness and redshift as an effect on their light that seems to increase over time.

So, the smallest of the dimmest, highest redshift, (furthest in time) galaxies is larger than the smallest of the brighter, lower redshift, (nearer in time) galaxies. This would imply that, furthest back in time, when the light was emitted from that smallest dimmest galaxy, that galaxy was closer to this region of space at that time, than the smallest of the brighter galaxies was when it emitted the light we see. (A galaxy at redshift z=7 has an angular diameter that puts it around 3-4 billion years light years away when the light was emitted, but its light is around 13 billion years old whereas a galaxy of z=2 looks to be over 5 billion light years away when the light was emitted, but its light is only 10 billion years old).

This tells us that in this model the oldest galaxies are 13 billion years old, and were up to something around 3 billion light years away when they emitted the light we see. Light from more recent times, 3 billion years later, comes from objects that were around 5 billion light years away at the time. We assume that light doesn't "overtake" other light, so the light from the dimmest furthest galaxies was also around 5 billion light years away, 10 billion years ago, still making it's journey towards us. This would mean that, during the first 3 billion years after galaxies formed, the light from the most distant galaxies was actually receding from us on its journey! How can this be? How can light that is moving towards us become further away?

We then assume the metric expansion of space. The earliest, most distant galaxies were around 3 billion ly away from the point in space where our galaxy formed when their light started its journey towards us, but the point in space that our galaxy formed in was, at the time, receding from those galaxies at a speed faster than light due to the metric expansion of space! 3 billion years later, the light from those distant galaxies found itself 5 billion light years away from us! In the end, it took another 10 billion years to get here after that. (We actually theorise that redshifts of z>~1.46 mean recession faster than light, which represents something over 9 billion years ago).

So, from our point of view, for the first 3 billion years or so, the light from the most distant objects that we can now see was actually receding from us, but for the last 9 billion years or so, all the light that is reaching us now has indeed actually been moving towards us!

The earliest, dimmest galaxies we see were only 3 billion light years away when they emitted the light we can now see, so at that time the radius of our observable universe was around 3 billion light years. There might have been galaxies that were 6 billion light years away at that time, but they are outside of our observable universe, so we have no knowledge of them. If there were galaxies 6 billion ly away, then right now their light would just be reaching our most distant galaxies!

So we theorise that our observable universe was around 3 billion light years in radius when galaxies first formed. We theorise that the light from that most distant galaxy in our observable universe took 13 billion years or so to reach us. We use metric expansion to explain how that light took so long to reach us, and infer that those most distant galaxies would now be up to 46 billion light years away due to that expansion. (Which means, for the galaxy what might have been 6 billion ly away when the galaxies formed, that it's light is now around 46 billion light years away from us!)

The metric expansion of space is most often defined as a process where the metric that defines distance changes over time. An easy example of metric expansion at a constant rate is to say that when one distance doubles in size, all distances double in size - in the time it took a metre to expand to become double the size, a light year also doubled in size and the radius of the universe also doubled in size. But it seems that in the case of our observable universe, the rate at which the "metric that defines distance changes" has been changing. During the earliest times, the distance at which an object would be receding from us at the speed of light was close to us, and as time went on and the expansion slowed, that distance, that Hubble limit, receded. That distance is currently considered to be over 9 billion light years away. We interpret these various observations and interpretations into a picture where the rate of expansion was extremely fast early on, and until recently we thought it had been decelerating ever since. Now it seems there is some evidence that the rate of expansion has started to increase again.

Should we really try to combine all these concepts into a definition of the observable universe, and if so, how? If not, should we just leave it at "all we can see", and then use current theories to interpret the definition elsewhere? And I didn't even mention anything about a description of how our observable universe may evolve - what we might expect to see in the future - would we ever see those galaxies that might have been 6 billion ly away when the galaxies first formed?

*All figures for distance or age are of course very rough approximations. And sorry for the rambling interpretation, at least it proves that I am certainly not the person to be coming up with a concise description of the observable universe! ;)