# Thread: Size of the observable universe?

1. This link is a very good guide to sorting out all the different distances involved:

The Distance Scale of the Universe

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.

2. Originally Posted by 01101001
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".

3. Originally Posted by speedfreek
This link is a very good guide to sorting out all the different distances involved:

The Distance Scale of the Universe

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?

4. Originally Posted by DanishDynamite
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 (ATM) subforum.

5. Originally Posted by 01101001
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 (ATM) subforum.
I really just wanted a clear answer to my question.

6. Finally!

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

7. Originally Posted by DanishDynamite
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.

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Originally Posted by Ken G
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.

9. Originally Posted by 01101001
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.

10. Originally Posted by DanishDynamite
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.

11. Originally Posted by rtomes
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.

12. Originally Posted by DanishDynamite
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.

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?

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.

Sean Carroll's Preposterous Universe Blog: Doing away with dark energy? 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.

Scientific American: Misconceptions about the Big Bang 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.
Last edited by 01101001; 2007-Sep-12 at 03:50 PM.

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Originally Posted by DanishDynamite
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 ...

14. Originally Posted by ToSeek
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
Originally Posted by the current version of Wikipedia anyway
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.

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## Ah, Let Me See, Then . . .

Originally Posted by Nereid
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?

16. John, I thought GR predicts the speed of gravity to be exactly the speed of light?

Originally Posted by Wiki article on "speed of gravity"
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.

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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

18. 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.

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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: "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) is a link to a PDF document, the Scientific American, March 2005 article; it's aimed at a more general audience.

20. Originally Posted by rtomes
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.

21. Originally Posted by Cougar
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".

22. Originally Posted by 01101001
Maybe not accepted by you, but accepted by a highly respected cosmologist.

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?

Or, accepted by another highly respected cosmologist.

Sean Carroll's Preposterous Universe Blog: Doing away with dark energy? March 22, 2005

It's promoted by a popular-science publication.

Scientific American: Misconceptions about the Big Bang 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".

23. Originally Posted by Nereid
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".

24. Originally Posted by speedfreek
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?

25. 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!"

26. Originally Posted by speedfreek
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!"
"The check's in the mail". Not usually accepted by the Physics bookkeeping department.

27. 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.

28. Yes, sorry Ken, you are correct. I might have been muddying the waters there a bit.

29. Originally Posted by Ken G
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?

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Originally Posted by DanishDynamite
Originally Posted by Nereid
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.

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