# How does the observable Universe have meaning?

Why does the term observerable universe have any meaning outside of observers on earth?

From earth, the observable universe is the universe 13.8 billion light years away in every direction which is large but a finite distance. Say there's a galaxy near the edge of our observable universe. Wouldn't you have to say that the observable universe from that point is 13.8 billion light years away in each direction therefore part of the universe from that point in space has to be outside of our observable universe?

It would seem that the universe would have to be infinite because where is the edge of the Big U Universe meaning all of space vs. the Little U universe which is just space we can observe. Just because I can only see so far down the street doesn't mean there's no more houses and roads beyond where I can see.

What's wrong with this picture?

Delta2

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mfb
Mentor
Every observer has a different region of the universe they call observable universe. The observable universe as seen by Earth is quite important to us for obvious reasons.

The universe might be infinite but it doesn't have to be. One possible alternative is a hypersphere: Like the surface of Earth but with one more dimension. Finite but without an edge.

etotheipi and phinds
Every observer has a different region of the universe they call observable universe. The observable universe as seen by Earth is quite important to us for obvious reasons.

The universe might be infinite but it doesn't have to be. One possible alternative is a hypersphere: Like the surface of Earth but with one more dimension. Finite but without an edge.
I agree, the observable universe is important to observers on earth but it's not important to all of space. An observer on Saturn would have a slightly different observable universe than observers on earth.

A planet in Andromeda Galaxy has a different observable universe than earth. Part of the observable universe for that planet has to be outside of our observable unverse. So there's all of space which includes space outside of our observable universe and our observable universe which is just saying there's a limit of how many objects we can see light from because we have a limit to how much space we can see from earth but that's different on a planet a million light years away.

From earth, the observable universe is the universe 13.8 billion light years away in every direction which is large but a finite distance.
Radius of observable universe is about 46.5 billion light years, due to expansion.

I am not sure what exactly is so disturbing to you about the term observable universe. Of course it depends on location of the observer, but you seem to fully understand that.

mfb
Mentor
I agree, the observable universe is important to observers on earth but it's not important to all of space.
No one claims it would be.

Fervent Freyja
phinds
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etotheipi, davenn and weirdoguy
Not only is the size and shape observable universe dependent on where your are, your velocity also impacts it. At near c (relative to the CMB) the Hubble horizon (where expansion over distance = c) is much closer ahead than behind.

phinds
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Not only is the size and shape observable universe dependent on where your are ...
No, it is not. The observable universe, NO MATTER WHERE YOU ARE, is always a sphere, centered on your left eyeball when your right eye is closed, that always has the same radius. That radius does change slowly over time but that has nothing to do with where you are. Exactly what is IN that sphere changes by an utterly trivial amount whenever you change positions.

What you say about movement may be true but that's a separate issue.

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etotheipi
Why does the term observerable universe have any meaning outside of observers on earth?
Because observations can be made at places other than on earth, and the purpose of the term "observable universe" -- which is used to limit the scope of the user's comment to apply only to an observable portion of the universe as opposed to an unobservable portion -- is also appropriate at other observation locations.
From earth, the observable universe is the universe 13.8 billion light years away in every direction which is large but a finite distance. Say there's a galaxy near the edge of our observable universe. Wouldn't you have to say that the observable universe from that point is 13.8 billion light years away in each direction therefore part of the universe from that point in space has to be outside of our observable universe?
Yes, and the fact that there are multiple perspective locations that have their own portion of the universe that is "observable" disproves your own assertion that perspectives other than from earth have no meaning.
It would seem that the universe would have to be infinite because where is the edge of the Big U Universe meaning all of space vs. the Little U universe which is just space we can observe. Just because I can only see so far down the street doesn't mean there's no more houses and roads beyond where I can see.
No, the universe does not necessarily have to be infinite merely because there is a portion of it that you can't see. To use your analogy: Just because you can't see all of the street doesn't mean that the street never ends.

phinds
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It would seem that the universe would have to be infinite because where is the edge of the Big U Universe meaning all of space vs. the Little U universe which is just space we can observe. Just because I can only see so far down the street doesn't mean there's no more houses and roads beyond where I can see.
As has already been pointed out, this is wrong. Think of the surface of the Earth and consider the horizon you see as your Observable Universe. Now, move anywhere on the planet and you still have an Observable Universe different than the others. Is the surface of the Earth infinite?

The universe MAY be infinite but not following your logic. It could also be a hyperspace that is not infinite but has no center and no edge, just like the surface of the Earth.

What we see at the edge of "our" observable universes is 13+ billion years. We must suppose that those objects now are either extinct or have been replaced by more modern galaxies. Those galaxies would have an observable universe that would see our galaxy at the beginning of it's existence 13 billion years in the past. Looking the other direction they would see another 13 billions years to the edge of the their observable universe. And what would a galaxy at that extremis see? Possibly another 13 billion years, and so on. It could very well be infinite in the purest sense of the word. If a light year were different all these observable universes would be longer or shorter depending.

phinds
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What we see at the edge of "our" observable universes is 13+ billion years. We must suppose that those objects now are either extinct or have been replaced by more modern galaxies. Those galaxies would have an observable universe that would see our galaxy at the beginning of it's existence 13 billion years in the past. Looking the other direction they would see another 13 billions years to the edge of the their observable universe. And what would a galaxy at that extremis see? Possibly another 13 billion years, and so on. It could very well be infinite in the purest sense of the word. If a light year were different all these observable universes would be longer or shorter depending.
No. See post #10. Your logic is flawed. The universe may indeed be infinite in extent but not even remotely because of your logic. If a light year were "different" all bets are off since it would mean physics as we know it is incorrect. Not likely. Incomplete for sure, but THAT wrong. Nope. Don't think so.

No, it is not. The observable universe, NO MATTER WHERE YOU ARE, is always a sphere, centered on your left eyeball when your right eye is closed, that always has the same radius. That radius does change slowly over time but that has nothing to do with where you are. Exactly what is IN that sphere changes by an utterly trivial amount whenever you change positions.

What you say about movement may be true but that's a separate issue.
Right, the shape only depends on velocity.

My point was all we see at the edge of our observation is very old pre-galaxies as they were 13.5 billion years ago. We have absolutely no idea what is going on out there now. Imagine if you're in a galaxy that's `10 billion light years from us. Is there observable universe only 3.5 billion light years looking away from us? It's probably the same 13.5 billion years in all directions to the point where the light is red-shifted beyond the visible light spectrum. What other things would they see? Thinking of the universe as a sphere doesn't work for me. It's no different than folks thought of the stars in our galaxy being on a sphere. I keep hearing that there's no "center" in the universe. The sphere analogy implies a center. I can't imagine a sphere without one.

phinds
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The sphere analogy implies a center.
No it absolutely does not. The sphere is an analogy, and there is NOT a center to the surface of the Earth. You are making a 3D analogy out of what is actually a 2D analogy.

So are we inside this sphere? Or are we outside? Or is the analogy completing passing me by?

Bandersnatch
Honestly, I don't quite get what @phinds is objecting to. Post #11 seems like a perfectly valid observation. In essence, all it says is that there are as many observable universes as there are observers. Maybe the meaning of that bit about light years being different is unclear, but otherwise it looks kosher.

Excuse me for my ignorance but does this discussion relate to practical or philosophical matters related to syntac. It appears we had enough problems describing the nature of things we can observe without expending creative energy trying to figure out what we can observe with either eye or any planet. I suspect the word observable universe is more of a metaphor rather than a scientific principle.

Syntax, my apologies. The spell checker flagged it as erroneous.

Bandersnatch
No, it has a specific, well-defined meaning. It's the particle horizon - a sphere centred on the observer, with its radius equal to the proper distance a photon has travelled through expanding space in the time between recombination and now.
It's no more philosophical of a concept than how far a thunder can be heard x seconds after a lightning strikes.

Motore
phinds
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So are we inside this sphere? Or are we outside? Or is the analogy completing passing me by?
As for the analogy, there is no sphere, just a 2D surface. As for the hypothesized hypersphere (or other finite topology) we are of course INside and there IS NO outside, no center, no edge.

phinds
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Honestly, I don't quite get what @phinds is objecting to. Post #11 seems like a perfectly valid observation. In essence, all it says is that there are as many observable universes as there are observers. Maybe the meaning of that bit about light years being different is unclear, but otherwise it looks kosher.
I object to the "logic" chain that says just because there are different observable universes as you move through the universe, there therefore HAS to be infinite extent. As I said, there may BE infinite extent but not at all because of that flawed logic and I used the surface of a sphere as an analogy to show why.

We don't get a true view were looking into the past we are observing what was not how it is now. Its a distorted view of the universe. We know where a galaxy was 13.8 billion years ago but not were it is today. So what you observe isn't even there anymore. Its an illusion created by time.

phinds