Universe Vantage Points

[KobiashiBooBoo]

TL;DR Summary

If the universe is infinite, each vantage point beyond our horizon should look like the image below in 3 dimensions. Is this accurate? Just like on a foggy day you can only see a certain distance ahead of you.

 

[Ibix]

Depends what you mean by 'accurate'. Yes, every point in the infinite universe sees the same temperature microwave background and has the same range of visibility. However the expansion of the universe means the circles you've drawn have a radius of around 45 billion light years, since the stuff at the limits of our visible universe is now a lot further away than it was when it emitted the light we now see. And it isn't really like a foggy day, because that light can't have reached us rather than it got absorbed on the way (but I see what you're getting at - that individual personal dome centered on you).

 

[anorlunda]

You may find this PF Insights article interesting.
https://www.physicsforums.com/insights/radius-observable-universe-light-years-greater-age/

Also, don't forget that those circles can overlap. If I stand 1m away from you, then my observable universe's center is 1m away from your center, no matter how big the radius is.

But it is a very good topic to raise. Many (or perhaps most) people don't understand the significance of "observable" in "the observable universe", so they ignore it and think the meaning is the same as "the universe." That is a very common misconception.

 

[KobiashiBooBoo]

Thanks - so if we think of "Now" at the edge of the line - the universe has expanded and the what we see as background radiation has fully formed into galaxies (we assume). Basically how to describe and infinite yet finite universe, if we "hop" from bubble to bubble - eventually we run out of new bubbles - and get back to where we started?

 

[anorlunda]

and get back to where we started?

Not necessarily.

 

[Ibix]

Thanks - so if we think of "Now" at the edge of the line - the universe has expanded and the what we see as background radiation has fully formed into galaxies (we assume).

Yes. Although it's a bit stronger than an assumption, since models based on the cosmological principle ("everything is the same everywhere") seem to produce accurate descriptions of what we can see. It's possible those models are wrong, but I don't think we have any credible models that don't work like this.

Basically how to describe and infinite yet fintite universe, if we "hop" from bubble to bubble - eventually we run out of new bubbles - and get back to where we started?

I don't understand what you are trying to ask. As far as we are aware the universe is spatially infinite. You can never reach the edge of your observable universe, so you can't "hop from bubble to bubble", but our current best models say that there is no end to space so you can't get back to where you started.

 

[KobiashiBooBoo]

So let's imagine all "bubbles" as they exist "now" - there are infinite amounts in all directions as space is infinite. To roll back the tape, each observer in each "bubble" sees space expanding from a big bang. So this concept has no real analog, except the "raisins in a bread" or "dots on a balloon" which leaves the idea of any multiverse existing outside our 3 dimensional framework. Thoughts?

 

[PeroK]

So let's imagine all "bubbles" as they exist "now" - there are infinite amounts in all directions as space is infinite. To roll back the tape, each observer in each "bubble" sees space expanding from a big bang. So this concept has no real analog, except the "raisins in a bread" or "dots on a balloon" which leaves the idea of any multiverse existing outside our 3 dimensional framework. Thoughts?

I think your idea of separate "bubbles" is ill-conceived. The universe on a large scale is homogeneous and isotropic. There are no "bubbles" as such.

An analogy would be observation towers on the surface of the Earth. Each tower would have a different observable part of the Earth's surface, but these parts do not represent anything physical in terms of the Earth's surface itself.

 

[KobiashiBooBoo]

I think your idea of separate "bubbles" is ill-conceived. The universe on a large scale is homogeneous and isotropic. There are no "bubbles" as such.

An analogy would be observation towers on the surface of the Earth. Each tower would have have a different observable part of the Earth's surface, but these parts do not represent anything physical in terms of the Earth's surface itself.

Yes, I do not mean that they are separate regions of space, just the vantage points. Thank you for clarifying.

 

[KobiashiBooBoo]

You may find this PF Insights article interesting.
https://www.physicsforums.com/insights/radius-observable-universe-light-years-greater-age/

Also, don't forget that those circles can overlap. If I stand 1m away from you, then my observable universe's center is 1m away from your center, no matter how big the radius is.

But it is a very good topic to raise. Many (or perhaps most) people don't understand the significance of "observable" in "the observable universe", so they ignore it and think the meaning is the same as "the universe." That is a very common misconception.

I'm trying, this may be a bit advanced for me - thank you!

 

[phinds]

Yes, I do not mean that they are separate regions of space, just the vantage points.

But there is nothing special about picking "vantage points" that happen to be separated from each other by the diameter of the observable universe so your concept might be misleading you in some way.

 

[KobiashiBooBoo]

Yes, relativity tells us there is no special "vantage point" or "observation tower" etc. So we think of space as infinite, and each observer existing "now" with 45 BLY of observable space - extending in all directions forever. Sound right?

 

[phinds]

Yes, relativity tells us there is no special "vantage point" or "observation tower" etc. So we think of space as infinite, and each observer existing "now" with 45 BLY of observable space - extending in all directions forever. Sound right?

Not quite. The fact that there is no special vantage point is does not make it conclusive that the universe is infinite. A universe could be finite (as the universe we live in could still be) but still have no special vantage points. There are several topologies that allow this.

 

[anorlunda]

Another, potentially helpful, PF article.

https://www.physicsforums.com/insights/balloon-analogy-good-bad-ugly/

Bear in mind that all the analogies have flaws. So try to avoid pushing any analogy too far.

 

[elcaro]

TL;DR Summary: If the universe is infinite, each vantage point beyond our horizon should look like the image below in 3 dimensions. Is this accurate? Just like on a foggy day you can only see a certain distance ahead of you.

View attachment 315846

You picture these horizon circles just touching each other, but basically every point is a vantage point that describes it's own horizon, so they could be overlapping or completely disjoint.
And also, the radius is not 13.9 billion light years, since the observable universe has expanded, so the actual radius is more like 46 billion light years.

 

[kimbyd]

TL;DR Summary: If the universe is infinite, each vantage point beyond our horizon should look like the image below in 3 dimensions. Is this accurate? Just like on a foggy day you can only see a certain distance ahead of you.

View attachment 315846

A better analogy than a foggy day is the horizon on the Earth: you can only see bits of the ocean so far away, because the Earth is curved. It's not quite the same because you can see tall things further away, whereas in the case of the universe nothing can be visible beyond the horizon.

 

[KobiashiBooBoo]

A better analogy than a foggy day is the horizon on the Earth: you can only see bits of the ocean so far away, because the Earth is curved. It's not quite the same because you can see tall things further away, whereas in the case of the universe nothing can be visible beyond the horizon.

If this analogy works, and if the universe is finite yet unbounded, would you eventually come back to the same part of the ocean? Forgive my ignorance.

 

[Hornbein]

Yes, relativity tells us there is no special "vantage point" or "observation tower" etc. So we think of space as infinite, and each observer existing "now" with 45 BLY of observable space - extending in all directions forever. Sound right?

It's 45 BLY today but the number is always increasing.

 

[PeroK]

If this analogy works, and if the universe is finite yet unbounded, would you eventually come back to the same part of the ocean? Forgive my ignorance.

Yes, if the universe has a positive curvature, then it is analagous to the surface of a sphere and curves back on itself. If our universe is like this, then it must be very large as there is no evidence of significant curvature even across the extent to the observable universe.

 

[phinds]

If this analogy works, and if the universe is finite yet unbounded, would you eventually come back to the same part of the ocean?

Yes, as @PeroK has said, the analogy is apt, BUT ... "the same part" is somewhat misleading in the case of the universe because, as far as we can tell, if the universe IS finite, it is still HUGE (WAY more than the observable universe), and since you can't go faster than light, by the time you got back to "the same place" (after probably many tens of trillions of years) it would be "the same place" only in the most technical sense and would not look anywhere near the same as when you left.

In fact, assuming expansion continues (and there is no reason not to assume that), it would actually be a totally empty "observable universe" except for small amounts of weak radiation.

EDIT: it occurs to me now that if the "tens of trillions" of light years were all it took, then most of the black holes that were in the Local Group would still be there since there would not have not been enough time for Hawking Radiation to cause them to evaporate.

 

[KobiashiBooBoo]

Yes, as @PeroK has said, the analogy is apt, BUT ... "the same part" is somewhat misleading in the case of the universe because, as far as we can tell, if the universe IS finite, it is still HUGE (WAY more than the observable universe), and since you can't go faster than light, by the time you got back to "the same place" (after probably many tens of trillions of years) it would be "the same place" only in the most technical sense and would not look anywhere near the same as when you left.

In fact, assuming expansion continues (and there is no reason not to assume that), it would actually be a totally empty "observable universe" except for small amounts of weak radiation.

EDIT: it occurs to me now that if the "tens of trillions" of light years were all it took, then most of the black holes that were in the Local Group would still be there since there would not have not been enough time for Hawking Radiation to cause them to evaporate.

"the same part" as instantly hopping - this is a thought experiment, no "real" travel required!

 

[phinds]

"the same part" as instantly hopping - this is a thought experiment, no "real" travel required!

Understood. I just wanted to make sure you understood the ramifications of real travel

 

[kimbyd]

If this analogy works, and if the universe is finite yet unbounded, would you eventually come back to the same part of the ocean? Forgive my ignorance.

That analogy doesn't follow, because the curvature of the Earth is spatial curvature, and the curvature of space-time which causes our universe's horizon is in space-time. The analogy only really works locally.

Now, it is possible, as others have noted, that the universe wraps back on itself, but it would be a different effect than the one causing this horizon. Because of the horizon, we can't really say, because we can't see far enough. Various tests have tried to find effects of the universe wrapping back on itself, but so far there's no evidence of this. This indicates that if it does wrap, it's at a distance far beyond the horizon. Nobody knows how to say how likely or unlikely this is.

Maybe someday if we figure out the physics of the very early universe, e.g., what caused inflation, then maybe we could say definitively if the universe wraps back on itself (some models for the early universe only allow universes that wrap back on themselves). But we're at best decades away from learning that.

 

[anorlunda]

I always remind myself that even if we don't know if the universe is flat, or whether it has positive or negative curvature, it is approximately flat. Therefore, in all hypothetical cases, the best guess is to presume that it is actually flat.

 

[phinds]

I always remind myself that even if we don't know if the universe is flat, or whether it has positive or negative curvature, it is approximately flat. Therefore, in all hypothetical cases, the best guess is to presume that it is actually flat.

I agree, but the best guess does not prohibit discussion of other possibilities, just a realization that they are less likely.

 

[kimbyd]

I always remind myself that even if we don't know if the universe is flat, or whether it has positive or negative curvature, it is approximately flat. Therefore, in all hypothetical cases, the best guess is to presume that it is actually flat.

That's potentially a stretch. The universe being actually flat requires infinite fine tuning. Specifically, it requires an exact balance between the rate of expansion and the average density.

 

[phinds]

That's potentially a stretch. The universe being actually flat requires infinite fine tuning. Specifically, it requires an exact balance between the rate of expansion and the average density.

On the other hand consider that out of all of the infinite values it COULD take on, it is very nearly exactly flat. What an amazing coincidence if it isn't actually flat.

EDIT: Actually, "infinite values" is not an accurate description of what it could be since if inflation is true (and there is every indication that it is) then the range of values is much more limited.

 

[jbriggs444]

On the other hand consider that out of all of the infinite values it COULD take on, it is very nearly exactly flat. What an amazing coincidence if it isn't actually flat.

Keep in mind that the surface of the Earth is almost flat as well.

 

[KobiashiBooBoo]

This is all very helpful. As this is outside our human experience, analogies are helpful, even if not exactly reflecting the scope and reality. If you think of the expansion of the universe as "the space between things" instead of a point consisting of entire universe (big bang) it may help make more sense, if only in a fleeting way. Then you start getting into branes and such...

 

[PeroK]

This is all very helpful. As this is outside our human experience, analogies are helpful

Mathematics is even more helpful!

 

[kimbyd]

On the other hand consider that out of all of the infinite values it COULD take on, it is very nearly exactly flat. What an amazing coincidence if it isn't actually flat.

EDIT: Actually, "infinite values" is not an accurate description of what it could be since if inflation is true (and there is every indication that it is) then the range of values is much more limited.

I don't think this follows. There are basically three possibilities:
1) There is some kind of physical process that drives the universe towards flatness to an extreme degree. Cosmic inflation is an example of one such process.
2) Most universes are actually close to flat and we're just not thinking about it correctly (see https://arxiv.org/abs/1406.3057 for an argument along these lines).
3) Some kind of symmetry exists which demands the universe be exactly flat.

So far, lines of argument (1) and (2) have more potential than (3) in my opinion, as nobody has come up with a symmetry along the lines demanded by (3). Also I believe that the near scale invariance of the primordial power spectrum guarantees some small deviation from flatness within the observable universe.

 

[anorlunda]

That's potentially a stretch. The universe being actually flat requires infinite fine tuning. Specifically, it requires an exact balance between the rate of expansion and the average density.

Excuse this late post. This morning, I was viewing an Alan Guth lecture on Inflationary Cosmology. He touched on this point, and argues that because galaxies formed, Ω must be one to 15 decimal places. That is indeed pretty fine tuning.

Guth argues further that inflation would flatten any curvature that existed pre-inflation.

The slides are from
https://ocw.mit.edu/courses/8-286-the-early-universe-fall-2013/resources/mit8_286f13_lec01/

 

[pbuk]

by the time you got back to "the same place" (after probably many tens of trillions of years) it would be "the same place" only in the most technical sense and would not look anywhere near the same as when you left.

That's not how expansion works (as we understand it). Light emitted now from Earth will only ever reach objects that are less than about 16.5bn light years away - inside the cosmic event horizon. So either we can already see what it used to look like "here" (we can see things that are now almost 47bn light years away), or we never will, no matter how fast we travel or for how long.

(there is a third alternative which is that the expansion of the universe stops accelerating but we don't have a model that predicts that).

 

[KobiashiBooBoo]

As a simple thought experiment you can conceive of observers in the most distant galaxy from us, looking at us "now" and seeing us as a dim red-shifted galaxy.

SO - take a galaxy halfway between us and the most distant galaxy we can observe. THEIR cosmological horizon will overlap both.

Extrapolate this and it goes on forever...

?

 

[phinds]

@KobiashiBooBoo, Yes (assuming the universe is infinite or has a "wraparound" topology if finite). So, why the question marks?