# Boarder of universe

what makes the boarder of a universe and why and can it change shapes and size

chroot
Staff Emeritus
Gold Member
The observable universe has no real "border." The "edge" of the universe is just defined as the furthest distance from which light has had time to reach us. It's about 47 billion light-years away.

- Warren

JesseM
chroot said:
The observable universe has no real "border." The "edge" of the universe is just defined as the furthest distance from which light has had time to reach us. It's about 47 billion light-years away.

- Warren
According to this article, the furthest point in space from which light has had time to reach us would currently be about 78 billion light-years away (although of course it wasn't that far away at the time the light was actually emitted).

chroot
Staff Emeritus
Gold Member
Oops, I guess I remembered the wrong number -- or perhaps the right number, but for the wrong question.

- Warren

ohwilleke
Gold Member
whatzzupboy said:
what makes the boarder of a universe and why and can it change shapes and size

I am a boarder of the universe and so are you, because we all live here. I change shapes and sized when I eat too much. :surprised

ok then why cant we interact with other universe or even see signs that they exisit by like nuclear radiation or radio transmitions?

hellfire
chroot said:
Oops, I guess I remembered the wrong number -- or perhaps the right number, but for the wrong question.

- Warren
No, you were right. In an universe with OL = 0.73, Om = 0.27 and H0 = 71, the (theoretical) radius of the observable universe is equal to 46 Gly.

The press article is wrong. It refers to the paper of N. Cornish in which the possibility of a finite universe was analized according to possible matches of circles in the CMB. http://arxiv.org/astro-ph/0310233" [Broken] concludes that we can exclude a universe smaller than 24 Gpc (78 GLy) in diameter. Note that the press acticle claims that "The universe is at least 156 billion light-years wide", which is wrong.

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Chronos
Gold Member
I just want to scrape my fingernails across the chalkboard every time I hear this argument. Eeeeek... the observable universe is the only universe that is scientifically relevant. Until we have a workable explanation of what is observed, the rest is meaningless. I agree all things are possible. I do not agree things that are inherently unobservable are meaningful.

turbo
Gold Member
Chronos said:
I just want to scrape my fingernails across the chalkboard every time I hear this argument. Eeeeek... the observable universe is the only universe that is scientifically relevant. Until we have a workable explanation of what is observed, the rest is meaningless. I agree all things are possible. I do not agree things that are inherently unobservable are meaningful.
Dear Chronos: To you, I am inherently unobservable. I promise you that I exist. The fact that you cannot see me does not negate my existence, nor make my existence less meaningful (at least to me!). I make this point for a reason:

Until the invention of some pretty nice optical instruments, only the brighter planets were known, and until the invention (and insightful use) of some much better instruments (telescopes, spectroscopes, etc), it was thought that Andromeda and other "nebulae" were clouds of gas in our galaxy. The existence and natures of these planets and galaxies were in NO way affected by our ability or inability to see them or to understand their natures. They did not miraculously come into existence or change their attributes when we managed to perceive them more accurately. Our viewpoint and perception do NOT matter to the universe.

Speaking of Andromeda, that galaxy is about 3 million light-years from us. An observer presently living in that galaxy has a "visible universe" that is just a bit different from ours. It is a spherical volume (just like ours), but it is offset by 3Mly. That observer can see things (opposite the direction of the Milky Way) 3M light years more distant than we can see. Looking back in our direction, that observer's visible universe will be foreshortened by 3M light years, compared to our visible universe. Does that limitation (Andromeda observer cannot see quite as far as we do in the direction of the MW) have any effect on OUR perception of the universe, or on the nature of the universe in total? No. That would confer a special nature to the frame of reference of the Andromeda observer, and that is forbidden by the Standard Model (and by simple logic). Similarly, OUR frame of reference has no effect on the existence or nature of the parts of the universe that may be unobservable to us.

The "observable universe" is an artifact of the limitations posed by our particular location and by the speed of light. The observable universe is a subset of the Universe. The Universe cannot be constrained by our particular limitations (no special reference frame), so one cannot posit a finite Universe on the basis of our observable universe - it is forbidden by the Standard Model. Any model in which our frame of reference can impose limitations on the frames of reference of distant observers, or on the Universe as a whole, is a non-starter. Talk about fingernails on the chalk-board!

Chronos
Gold Member
Turbo-1. There is a difference between unobserved and unobservable. You may be unobserved, but you are not unobservable. Assuming BB is correct, every observer in the universe will conclude the universe is NOW 13.7 Gy old. No observer, NOW or in the future, will ever conclude it is observationally older than we do. How does that confer a priveleged position to any observer? The universe may be metaphysically infinite, but not observationally infinite.

turbo
Gold Member
Dear Chronos:

If we decide that the visible universe is the only valid framework for scientific inquiry, and that no inference can be made about the extent or nature of the of the universe as a whole because we cannot see past our surface of last scattering, THAT particular choice confers upon us a "special" frame of reference.

The idea that only the visible universe is fair game for scientific inquiry is a Special Relativity view of the universe. It will give us problems, though, when we try to address the GR/FRW universe of the standard model. The concepts of simultaniety and "no special reference frame" are central to cosmology. They allow cosmologists to envision overlapping light cones, estimate the present size of the visible universe based on apparent lookback time and expansion rate, etc.

Our recent exchange on this (see post 41 and follow-ups) probably highlights the differences between the SR view and the GR/FRW view as well as can be expected.

Chronos
Gold Member
turbo-1 said:
Dear Chronos:

If we decide that the visible universe is the only valid framework for scientific inquiry, and that no inference can be made about the extent or nature of the of the universe as a whole because we cannot see past our surface of last scattering, THAT particular choice confers upon us a "special" frame of reference.
How is that? Every other observer in this universe sees exactly the same thing.
turbo-1 said:
The idea that only the visible universe is fair game for scientific inquiry is a Special Relativity view of the universe.
What other universe is 'fair game'? What does SR have to do with that?
turbo-1 said:
It will give us problems, though, when we try to address the GR/FRW universe of the standard model. The concepts of simultaniety and "no special reference frame" are central to cosmology. They allow cosmologists to envision overlapping light cones, estimate the present size of the visible universe based on apparent lookback time and expansion rate, etc.
What problems?
turbo-1 said:
Our recent exchange on this (see post 41 and follow-ups) probably highlights the differences between the SR view and the GR/FRW view as well as can be expected.
I don't think it is relevant to this discussion, but, I see no conflict between SR and GR. Please explain what GR predictions conflict with SR.

turbo
Gold Member
turbo said:
If we decide that the visible universe is the only valid framework for scientific inquiry, and that no inference can be made about the extent or nature of the of the universe as a whole because we cannot see past our surface of last scattering, THAT particular choice confers upon us a "special" frame of reference.
Chronos said:
How is that? Every other observer in this universe sees exactly the same thing.
If you consider the reference frame of a theoretical observer presently existing in a galaxy that we can see at the edge of our detectable limit (about 13Gly away), you will see that over half of his observable universe lies outside of our observable universe. To say that our observable universe is the only universe that is scientifically relevant...
Chronos said:
Eeeeek... the observable universe is the only universe that is scientifically relevant.
is to deny the validity of the reference frame of the distant observer.

turbo said:
The idea that only the visible universe is fair game for scientific inquiry is a Special Relativity view of the universe.
Chronos said:
What other universe is 'fair game'? What does SR have to do with that?
The universe, as envisioned by most cosmologists, includes the condition of simultaneity, which allow us to consider the reference frames of theoretical distant observers as equivalent to our own. In SR, gauging the simultaneity of events is not possible, and it is possible to get trapped into characterizing the distant observer's reference frame based on whether or not he can communicate with us, and how long the communication would take, etc.

turbo said:
It will give us problems, though, when we try to address the GR/FRW universe of the standard model. The concepts of simultaneity and "no special reference frame" are central to cosmology. They allow cosmologists to envision overlapping light cones, estimate the present size of the visible universe based on apparent lookback time and expansion rate, etc.
Chronos said:
What problems?
For example, the problem of considering only our visible universe (with its limitations arising from our position and the speed of light) as the only "real" universe worthy of scientific inquiry.

turbo said:
Our recent exchange on this (see post 41 and follow-ups) probably highlights the differences between the SR view and the GR/FRW view as well as can be expected.
https://www.physicsforums.com/showth...34&page=3&pp=15 [Broken]
Chronos said:
I don't think it is relevant to this discussion, but, I see no conflict between SR and GR. Please explain what GR predictions conflict with SR.
Here is a fairly illuminating exchange regarding the conflicts that can arise if we insist on applying SR's "no simultaneity" to the GR/FRW universe.

turbo:
Now for the logical proof:
Choose a quasar or galaxy at an apparent distance of 13Gly. Given the concordance assumptions of homogeneity, isotropy, and no special frame of reference, what can we say with certainty about a theoretical observer "B" who exists at that distant position right NOW?
Chronos:
We can say that observer B's 'now', is in our future.

turbo:
1. Since the universe is homogeneous and isotropic, and because "B's" frame of reference is no more or less special than ours, our theoretical observer looks out at his universe and sees a universe that is identical in its basic qualities to the one we see. He sees his own neighborhood, and due to the finite speed of light, he sees distant objects as they appeared in the past.
Chronos:
He sees distant objects as they appeared to be in his past, not ours.
(we are certainly in agreement, here - turbo)

turbo:
Like us, he can only see objects out to about 13 billion light years distant.
Chronos:
Incorrect. Observer B's universe is younger and smaller.
(Oops! Suddenly observer B's present visible universe has shrunk because WE perceive him to be in our past?? We do not have a special reference frame and cannot invalidate his reference frame by imposing the limitations of ours on him. - turbo)

turbo:
and he is looking at his surface of last scattering, just like we look out at our own. Just like us, "B" has a visible universe about 27 billion light years in diameter.
Chronos:
By the time we receive observer B's report on the size of the universe, it will be 12 billion years older than we perceive it to be.
(We do not need for him to report to us in order to posit the validity of his reference frame. We can assume that an observer presently existing at location B will exist in a universe that is just as valid as our own and shares the same essential features. - turbo)

turbo:
We are on one edge of his visible universe, just as he is on one edge of our visible universe.
Chronos:
But we are in his future and he is in our past. There is no simultaneity.
The above exchange (additional comments in parentheses) arose from a proof I gave that a flat or open FRW universe must be infinite (unless we impose a non-trivial topography on it). If I am not mistaken, each of the objections you raised to the proof was based on the SR notion of "no simultaneity" and the insistence on communication to validate the reference frame of the theoretical presently-existing distant observer.

I did a little searching to find someone who can explain this SR/cosmology problem better than I can, and I found this archived post by Marcus replying to a question by "confutatis maledictis":
https://www.physicsforums.com/archive/topic/t-16465_Confutatis_maledictis_(text_of_Latin_requiem).html [Broken]

Marcus said:
no professional community is completely consistent in every detail of every claim. You have to pick whom to listen to.

there is an idea of simultaneity in cosmology
and an idea of universal rest frame
A universal meaning is given to the present moment
by means of the FRW metric which they all use
as their standard distance-measure (Friedmann-Robertson-Walker metric)

cosmology uses General Relativity which is different from Special Rel.
The space of SR does not expand. there is no way of defining absolute rest or simultaneity in it. But SR is also not quite realistic, being unable to expand and contract, so when people preach Special Rel sermons and maintain there is no idea of simultaneity, it has a limited applicability.

In cosmology (with the Friedmann model) you can say what it means to be at rest. At rest with respect to the expansion of the universe.
And you can define simultaneous events at widely separated points.

the FRW metric is not 100 percent accurate but it is a heck of a lot more accurate and closer to reality than the Minkowski metric of SR.

Ned Wright and Charles Lineweaver are two worldclass cosmologists.
Try google with their names. Wright has a website tutorial on cosmology and Lineweaver has at least one good article with clear diagrams and not-to-hard formulas. There are a bunch of others. Sean Carroll, Eric Linder, Michael Turner. I dont know the best to recommend but if you stick close to reputable working cosmologists and keep away from people popularizing Special Rel then you can probably avoid the worst confutatuses.

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Isn't it possible that galaxy "B" that we observer to be 13Gly away and in the past is now so far away that it is racing away from us faster than light so that we will never see what it is like at the present moment? Isn't this most likely the case since the expansion is accelerating?

If distant galaxies are leaving behind a cosmological event horizon, then what does that say about the entropy/information content of the observable universe? Perhaps it is more than a coincidence that the universe started to expand about the same time that life arose. Perhaps the complexity of life is compensation for information disappearing behind the cosmological event horizon. What do you think? Thanks.

turbo
Gold Member
Mike2 said:
Isn't it possible that galaxy "B" that we observer to be 13Gly away and in the past is now so far away that it is racing away from us faster than light so that we will never see what it is like at the present moment? Isn't this most likely the case since the expansion is accelerating?
Not only possible, but required, if we live in a BB universe with accelerating expansion. Galaxies will be redshifted into the cosmic microwave background and become undetectable, beginning with the most distant ones.

Mike2 said:
If distant galaxies are leaving behind a cosmological event horizon, then what does that say about the entropy/information content of the observable universe?
The galaxies are not crossing a real boundary. Each observer in an accelerating BB universe will have his/her/its own visible universe, each perceiving (given enough time) that more distant objects are being red-shifted into undetectability. These visible universes may or may not overlap, and have no real boundary. The "boundary" is merely an observational limitation faced by each observer.

Mike2 said:
Perhaps it is more than a coincidence that the universe started to expand about the same time that life arose. Perhaps the complexity of life is compensation for information disappearing behind the cosmological event horizon. What do you think? Thanks.
Is anybody giving serious consideration the idea that there is an "information budget" for each observer's visible universe that somehow has to be conserved? I haven't heard that one before, nor the idea that the universe started to expand when life arose on Earth. Do you have links or references? (I'm spoiled by the Internet, obviously. )

turbo-1 said:
Not only possible, but required, if we live in a BB universe with accelerating expansion. Galaxies will be redshifted into the cosmic microwave background and become undetectable, beginning with the most distant ones.
...
Is anybody giving serious consideration the idea that there is an "information budget" for each observer's visible universe that somehow has to be conserved? I haven't heard that one before, nor the idea that the universe started to expand when life arose on Earth. Do you have links or references? (I'm spoiled by the Internet, obviously. )

What difference is there between disappearing behind the event horizon of a black hole and disappearing behind the cosmological event horizon? Both have objects being accelerated so fast that we lose all contact with it. So if we have concerns about where the information of objects go when it falls into a black hole, then I would assume that there must be the same concerns about objects falling behind the cosmological event horizon as well.

I wonder if there is not a type of Hawking radiation associated with the cosmological event horizon as there is for the event horizon of a BH. Perhaps this is the source of zero point energy of space a.k.a, the cosmological constant. Since everywhere is the cosmological event horizon of somewhere else, the Hawking radiation of the cosmological event horizon would have to exist everywhere as well, right?

turbo
Gold Member
Mike2 said:
When I do that, I get lots of Dembski links. I encourage everyone to try this once, and get it out your systems. Pure crap. Remember the name.

turbo-1 said:
When I do that, I get lots of Dembski links. I encourage everyone to try this once, and get it out your systems. Pure crap. Remember the name.
I think his method might be flawed. But the question still remains. To repeat myself:

What difference is there between disappearing behind the event horizon of a black hole and disappearing behind the cosmological event horizon? Both have objects being accelerated so fast that we lose all contact with it. So if we have concerns about where the information of objects go when it falls into a black hole, then I would assume that there must be the same concerns about objects falling behind the cosmological event horizon as well.

JesseM
Mike2 said:
What difference is there between disappearing behind the event horizon of a black hole and disappearing behind the cosmological event horizon? Both have objects being accelerated so fast that we lose all contact with it. So if we have concerns about where the information of objects go when it falls into a black hole, then I would assume that there must be the same concerns about objects falling behind the cosmological event horizon as well.
If black holes didn't evaporate through Hawking radiation, then there would be no information loss paradox, because you could assume the information was still present, but just hidden behind the horizon. There's no Hawking radiation with cosmological event horizons, and we're certainly never going to see the rest of the universe beyond the horizon evaporate away to nothing, so again, no information loss paradox.

Dembski is a creationist crackpot, by the way. Read this for a nice refutation of his arguments.

Chronos
Gold Member
Let me try a different approach. A universe that is observationally finite to all observers may be huge, but still finite. An endless sum of finite quantities is still finite.

turbo
Gold Member
Mathematically, you can divide the interval between zero and one ad infinitum. In other words, you can visualize a infinite number of intermediate values between one and zero, although the total span is one unit. In this way, you can have a "degree of infinity" bounded by a finite value. Fractals have this intriquing property.

The idea you propose - summing an infinite number of finite values - is fundamentally different, and yields an infinite result. Infinity multiplied by any finite but non-zero value equals infinity.

JesseM
Chronos said:
Let me try a different approach. A universe that is observationally finite to all observers may be huge, but still finite. An endless sum of finite quantities is still finite.
Not if there are an infinite number of observers in different locations, each seeing their observable universe as being the same size.

Chronos
Gold Member
Let's say I get on a bus. The bus next stops at NYC, and I get off. How many miles did I travel? How long have I been on the bus? Do my questions even make any sense?

Chronos said:
Let's say I get on a bus. The bus next stops at NYC, and I get off. How many miles did I travel? How long have I been on the bus? Do my questions even make any sense?

Exactly. The questions are meaningless, because WHO are you asking?

Do we define the universe as everything that CAN be observed? Then observed by whom? If we say "yes, observed by US", then we have a universe. This, as far as my knowledge goes, is how we view things.

Say there exists at our current measurement of time another observer at the edge of our "universe". Is he at the edge of HIS universe? No. His light cones, which define his observeable universe, are completely different and seperate from ours. He is in a different observeable universe. Does anyone dispute this?

The question then arises, is there then conservation of energy in an observable universe? I would have to say "no" then, unless theres some physics of GR that proves this. It would seem that observable energy is independent of observable universe.

These are not my theories or any such stuff, just questions arising from what I understand our current beliefs to be.

JesseM said:
If black holes didn't evaporate through Hawking radiation, then there would be no information loss paradox, because you could assume the information was still present, but just hidden behind the horizon. There's no Hawking radiation with cosmological event horizons, and we're certainly never going to see the rest of the universe beyond the horizon evaporate away to nothing, so again, no information loss paradox.

Dembski is a creationist crackpot, by the way. Read this for a nice refutation of his arguments.
This article states that cosmological event horizons do contrain the information in the universe. So as the cosmological event horizon gets smaller with acceleration, the potential information loss is getting smaller and smaller. It also states that there is a Hawking radiation associated with this cosmological event horizon.

http://www.arxiv.org/multi?archive=...sses&subj_physics=-->+physics+subject+classes