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Old 02-11-2018, 12:09 PM
Chimera Chimera is offline
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The Edge of the Universe

Assuming the Big Bang, that the Universe popped out like a rapidly expanding bubble in all directions, what would the edge of the Universe look like?

I'm thinking that even as mass coalesced from the primorial quantum ooze and formed stars, green clovers and galaxies, even if that expansion of the Universe slowed, wouldn't there still be what essentially amounts of an expanding particle wavefront of neutrinos, photons and other high energy, light speed particles expanding at light speed in every direction? And even if these particles are virtually massless, wouldn't that still be a tremendous amount of mass/energy pushing outward in all directions at light speed?
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Old 02-11-2018, 12:17 PM
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The Big Bang did not happen at a single point in preexisting space. It happened everywhere. The expanding universe is not a bubble with an edge in the way that you envision.
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Old 02-11-2018, 12:26 PM
k9bfriender k9bfriender is offline
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There is an expanding particle wavefront of neutrinos, photons and other high energy, light speed particles expanding at light speed in every direction, and that wavefront is passing by us right now.
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Old 02-11-2018, 12:30 PM
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And was also passing us yesterday, and a billion years ago, and ten billion years ago, and will also be passing us ten and a hundred billion years from now.
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Old 02-11-2018, 12:33 PM
Chimera Chimera is offline
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The Big Bang did not happen at a single point in preexisting space. It happened everywhere. The expanding universe is not a bubble with an edge in the way that you envision.
That would be a wrong assumption. Everything that ever was, is and will be is in that bubble. I'm just asking what is happening at the edges.

So let's back off the "you're wrong" and start providing actual, productive answers, eh?
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Old 02-11-2018, 12:37 PM
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There is an expanding particle wavefront of neutrinos, photons and other high energy, light speed particles expanding at light speed in every direction, and that wavefront is passing by us right now.
But describing it as an "expanding particle wavefront" may tend to reinforce the misconception in the OP. The origin of this radiation is a sphere centered on us, not a point. This radiation originated everywhere in the universe, not at a single point in preexisting space (and somewhat later than the Big Bang, but that's another matter). The source of the radiation that happens to be passing us at a given moment is just a function of our time and place in the universe.

Last edited by Riemann; 02-11-2018 at 12:37 PM.
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Old 02-11-2018, 12:41 PM
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That would be a wrong assumption. Everything that ever was, is and will be is in that bubble. I'm just asking what is happening at the edges.

So let's back off the "you're wrong" and start providing actual, productive answers, eh?
If you think there are edges to the expansion of space, your conception of cosmology is indeed wrong.

I am trying to search out some prior threads with good explanations rather than just telling you your are wrong, but give me a few minutes - the search function on here is not the best.
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Old 02-11-2018, 12:43 PM
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But describing it as an "expanding particle wavefront" may tend to reinforce the misconception in the OP. The origin of this radiation is a sphere centered on us, not a point. This radiation originated everywhere in the universe, not at a single point in preexisting space (and somewhat later than the Big Bang, but that's another matter). The source of the radiation that happens to be passing us at a given moment is just a function of our time and place in the universe.
I get that, and Chronos subsequently and correctly added to what I said. But it is true that the "explosion" of the big bang is passing by us right now, and as he said, yesterday and tomorrow, and all the yesterdays and all the tomorrows.
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Old 02-11-2018, 12:59 PM
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Assuming the Big Bang, that the Universe popped out like a rapidly expanding bubble in all directions, what would the edge of the Universe look like?
There is no edge or center of the universe in any privileged frame of reference. There is the cosmological (or comoving) horizon which defines the limits of the observable universe for us on Earth, but somewhere out there could be an intelligent observer for whom our planet and sun are at the limit of their obsevable universe, approximately 46.5 Bly away in comoving distance (as the interstellar crow flies, so to speak). They wouldn’t literally see our solar system because they’d be seeing light from over 13 Byr ago, but they could see the precursor galaxies forming to generate the heavier elements that would one day make up our galaxy and planetary system.

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Old 02-11-2018, 01:07 PM
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There is no edge or center of the universe in any privileged frame of reference. There is the cosmological (or comoving) horizon which defines the limits of the observable universe for us on Earth, but somewhere out there could be an intelligent observer for whom our planet and sun are at the limit of their obsevable universe, approximately 46.5 Bly away in comoving distance (as the interstellar crow flies, so to speak). They wouldn’t literally see our solar system because they’d be seeing light from over 13 Byr ago, but they could see the precursor galaxies forming to generate the heavier elements that would one day make up our galaxy and planetary system.

Stranger
So waving to them is pointless then?
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Old 02-11-2018, 01:26 PM
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So waving to them is pointless then?
Not to whomever’s around 13 billion years from now.
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Old 02-11-2018, 01:38 PM
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If you think there are edges to the expansion of space, your conception of cosmology is indeed wrong.

I am trying to search out some prior threads with good explanations rather than just telling you your are wrong, but give me a few minutes - the search function on here is not the best.
So the region before the big bang was unbounded?
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Old 02-11-2018, 01:48 PM
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Not to whomever’s around 13 billion years from now.
That's the downside. As the universe expands, more and more of what is now the visible universe will pass beyond the cosmological horizon. the light leaving our planet now will never reach them.
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Old 02-11-2018, 01:53 PM
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I get that, and Chronos subsequently and correctly added to what I said. But it is true that the "explosion" of the big bang is passing by us right now, and as he said, yesterday and tomorrow, and all the yesterdays and all the tomorrows.
And add to that that it applies equally everywhere in the universe. Everywhere sees the BB (or more exactly, the light emitted 380,000 years after the BB) in all directions. And everywhere has always (well, since it was emitted, of course) seen it and will always see it in all directions. No exceptions, there's no edge where they see it from less than every direction.
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Old 02-11-2018, 02:01 PM
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And add to that that it applies equally everywhere in the universe.
Awww, I thought we were special
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Everywhere sees the BB (or more exactly, the light emitted 380,000 years after the BB) in all directions. And everywhere has always (well, since it was emitted, of course) seen it and will always see it in all directions. No exceptions, there's no edge where they see it from less than every direction.
That is not to say that we know in any way definitively what lies beyond what we can see. It's pretty sure that it is pretty much the same for some distance beyond the cosmological horizon, but it is possible that this happened to be a patch of the universe that inflated in a smooth and flat fashion, further out could have played out differently.
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Old 02-11-2018, 02:21 PM
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So the region before the big bang was unbounded?
I'm not considering any notion of "before the big bang".

Many descriptions of cosmology fail to clarify the distinction between our observable universe and the entire universe. The observable universe is indeed bounded by a sphere, centered on us, that shrinks back to a point if we look back in time to the limit of the Big Bang. This leads to the misconception that the entire universe started at a point, and is bounded by a spherical edge, expanding in preexisting space.

Although the entire universe may not be infinite, I think it's best to start with an assumption that it is - because if your conception of cosmology can't accommodate that, you probably have some misconceptions in there.

I think the 1-dimensional "ants on a stretchy band" model is probably the best analogy. The expansion of space is the band stretching, meaning that ants that are locally stationary on the band move further apart. But assume that the band is infinite, just goes on forever without any ends. It's not entirely clear quite what that means, but I think it clarifies some aspects of the model. Looking forward in time, the band expands, making points further apart; looking back in time, it contracts, making points closer together. But it has no ends, it just disappears off into infinity however close together or far apart things become.

Now, if you consider the observable universe for a particular ant standing at one particular place on the band, that just represents the line-segment of the band centered on that ant. And if you look back in time, that segment shrinks, tending to the limit of a point centered on that ant at the Big Bang. But there's nothing special about that one point - it's just defined by where that one ant happens to be standing. More generally, looking back in time toward the Big Bang is not the process of shrinking the entire band to any one particular point, it is the process of all points on this infinitely long band tending to become mutually closer to one another - while the total extent of the band remains infinite, it has no ends. And the Big Bang itself is the undefinied limit where the distance between any two points on the infinitely long band approaches zero.

A singularity is not the same thing as a "point in space". A singularity is the infinity-times-zero limit where the model breaks.

The bottom line is that if you haven't somehow got to grips with
(a) there are no edges to the expansion of space;
(b) the Big Bang happened everywhere
then you need to keep working on your understanding to find an analogy that works for you.

Last edited by Riemann; 02-11-2018 at 02:24 PM.
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Old 02-11-2018, 02:43 PM
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That's the downside. As the universe expands, more and more of what is now the visible universe will pass beyond the cosmological horizon. the light leaving our planet now will never reach them.
This both is and isn't correct:

The particle horizon (aka cosmological horizon) which is the limit of the observable Universe, expands faster than a comoving volume meaning it takes in more galaxies. This is a generic feature of particle horizons*

Galaxies are leaving the cosmological event horizon which is the limit of what happens at the current cosmological time what we will be able to see in the future. This is a generic feature of cosmological event horizons*

*If the Universe is compact several copies of the same galaxy may be observable and it is one of those copies that enters/leaves. The Universe needn't have either type of horizon and the existence of one horizon isn't dependent on the other's existence
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Old 02-11-2018, 02:53 PM
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This both is and isn't correct:

The particle horizon (aka cosmological horizon) which is the limit of the observable Universe, expands faster than a comoving volume meaning it takes in more galaxies. This is a generic feature of particle horizons*

Galaxies are leaving the cosmological event horizon which is the limit of what happens at the current cosmological time what we will be able to see in the future. This is a generic feature of cosmological event horizons*

*If the Universe is compact several copies of the same galaxy may be observable and it is one of those copies that enters/leaves. The Universe needn't have either type of horizon and the existence of one horizon isn't dependent on the other's existence
Sorry, I did mean the cosmological event horizon, but I was focused on the difference between it and the black hole event horizon that I neglected to put that word in.
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Old 02-11-2018, 03:27 PM
Chimera Chimera is offline
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The bottom line is that if you haven't somehow got to grips with
(a) there are no edges to the expansion of space;
(b) the Big Bang happened everywhere
then you need to keep working on your understanding to find an analogy that works for you.
You know, this is why I've struggled with the idea of asking this on this board. Because instead of being gentle and informative, we have people like you on this board who fall back to telling people they're wrong and coming all too close to being insulting, mocking and completely less than helpful.

Do better.
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Old 02-11-2018, 03:28 PM
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I think there's a theory that there was a "before the big bang" in a five dimensional universe now forever closed to us, and that we are the "before the big bang" for numerous new one-dimensional universes that, from our point of view, begin at the bottom (to speak somewhat poetically) of the black holes in our universe. In this theory universes cascade below one another through black holes and each descending generation has two fewer geometrical dimensions than the one above. They are truly "below" in the sense that matter passes from one layer to the next by falling into black holes. This part is probably wrong, because it just sounds too neat and simple, but I vaguely recall the whole deal started with an eleven dimensional universe.

I wondered if expanding space today corresponds to stuff continuing to fall into the black hole above us. But that sounds about as sophisticated as that false equivalency between solar systems and atoms. Anyway, all the turtles on the way down have fewer and fewer dimensions.
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Old 02-11-2018, 03:32 PM
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That would be a wrong assumption. Everything that ever was, is and will be is in that bubble. I'm just asking what is happening at the edges.

So let's back off the "you're wrong" and start providing actual, productive answers, eh?
Actually, Chimera, Reimann isn't the one who said "wrong", you are. He's been polite and did take the trouble to write out some pretty detailed actual, productive answers -- and, though I got my astronomy degree decades ago, he seems to me to be correct.

"Bubble" is an analogy, but it breaks down when one considers "edges". There really aren't any.
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Old 02-11-2018, 03:33 PM
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You know, this is why I've struggled with the idea of asking this on this board. Because instead of being gentle and informative, we have people like you on this board who fall back to telling people they're wrong and coming all too close to being insulting, mocking and completely less than helpful.

Do better.
I think perhaps you should reread my comments and reconsider this response. Where was I remotely insulting or mocking? I pointed out quite simply and neutrally that your premise of the expansion having edges is a misconception, which it is - a common one. I have then taken the trouble to write out an extensive follow-up comment describing an analogy that I know has helped others in the past, and that I thought might be helpful to you.

What, exactly, are you looking for?

Perhaps it's your own attitude that needs some rethinking here?

Last edited by Riemann; 02-11-2018 at 03:37 PM.
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Old 02-11-2018, 03:37 PM
Chimera Chimera is offline
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Perhaps it's your own attitude that needs some rethinking here?
Your first response was to tell me my premise was wrong. Unhelpful when you don't explain why. The second one is to tell me I'm wrong and you'll respond later.

Honestly, both responses are unhelpful. You could have waited until you had that third response ready.
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Old 02-11-2018, 03:39 PM
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There's a pretty good article on Wikipedia entitled "Big Bang" that lays the whole thing out simply enough. Here's a quote:

"The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. In other words, the Big Bang is not an explosion in space, but rather an expansion of space."
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Old 02-11-2018, 03:42 PM
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Your first response was to tell me my premise was wrong. Unhelpful when you don't explain why. The second one is to tell me I'm wrong and you'll respond later.

Honestly, both responses are unhelpful. You could have waited until you had that third response ready.
Well, then, I apologize for taking the time to try to give you a helpful explanatory response, but not doing it quite on a schedule that suited you.

Again, where exactly was I mocking or insulting, please?
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Old 02-11-2018, 03:44 PM
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So let's back off the "you're wrong" and start providing actual, productive answers, eh?
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You know, this is why I've struggled with the idea of asking this on this board. Because instead of being gentle and informative, we have people like you on this board who fall back to telling people they're wrong and coming all too close to being insulting, mocking and completely less than helpful.

Do better.
Moderator Note

Chimera, as far as I can see the responses you have received so far have been polite and informative. I think you need to be less defensive and less snarky if you are going to ask questions here. Dial it way back.

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Old 02-11-2018, 03:59 PM
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Yes, if a question contains an inherent misconception, then it is impossible to answer the question without first correcting the misconception. Which Riemann did.
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Old 02-11-2018, 04:02 PM
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Here's a good simple article about the concept of the "edge of the universe":
https://www.space.com/33005-where-is...dge-op-ed.html
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Old 02-11-2018, 04:04 PM
Chimera Chimera is offline
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There is no edge or center of the universe in any privileged frame of reference. There is the cosmological (or comoving) horizon which defines the limits of the observable universe for us on Earth, but somewhere out there could be an intelligent observer for whom our planet and sun are at the limit of their obsevable universe, approximately 46.5 Bly away in comoving distance (as the interstellar crow flies, so to speak). They wouldn’t literally see our solar system because they’d be seeing light from over 13 Byr ago, but they could see the precursor galaxies forming to generate the heavier elements that would one day make up our galaxy and planetary system.
Ok, this makes sense and tells me where my premise is wrong.

Are there models where there is a larger, older "Universe" and our singularity/Universe is merely a new manifestation spreading out into that existing universe? I know there's a couple of scifi universes where they posit this. I'm blanking on some of them but I know the Well of Souls series is essentially like this, in that when they destroy the universe to reboot it, there are remnants of an older universe still around.

I also recall reading something last year where they posited something like this, where if a new universe were to spring into existence near us, we wouldn't detect it until it hit us, potentially rewriting our local laws of physics and the like.

I'm not suggesting that is the way it works, just asking about the idea and if that's just science fiction or some actual theory somewhere.
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Old 02-11-2018, 04:06 PM
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Reimann,

I still don't like the idea of being told I'm wrong twice without any actual answer*, but thank you for the one you did provide. Yes, it does help.
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Old 02-11-2018, 04:14 PM
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Quoth Chimera:

Are there models where there is a larger, older "Universe" and our singularity/Universe is merely a new manifestation spreading out into that existing universe?
Eternal inflation models are something like that, but it's more that our universe is a region which (almost) stopped expanding, relative to the background which is still expanding ludicrously quickly. It's actually quite an elegant little model, which is unfortunately completely impossible to test experimentally or observationally.
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Old 02-11-2018, 04:27 PM
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Right, it's kind of like those stars they find that they say appear to be older than the observable universe. You can't really prove that they are, as the means you're using to determine that could be flawed and the star properties you're using to determine that could be compromised by other things in its past.
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Old 02-11-2018, 04:29 PM
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Reimann,

I still don't like the idea of being told I'm wrong twice without any actual answer*, but thank you for the one you did provide. Yes, it does help.
If you're incapable of taking a step back and discerning how inappropriate your reactions have been here, all I can suggest is that you come back and reread the thread tomorrow with a fresh eye. In the meantime, I suggest we return to the topic at hand.
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Old 02-11-2018, 04:30 PM
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Here's a good simple article about the concept of the "edge of the universe":
https://www.space.com/33005-where-is...dge-op-ed.html
Thanks. More ¯\_(ツ)_/¯, but still informative.
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Old 02-11-2018, 04:36 PM
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Reimann,

I still don't like the idea of being told I'm wrong twice without any actual answer*,
Moderator Note

As I said, your responses here were inappropriate. Let's drop any further discussion of the matter.

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Old 02-11-2018, 04:46 PM
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nm

Last edited by rbroome; 02-11-2018 at 04:49 PM. Reason: Oops. I didn't see the mod note about dropping this
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Old 02-11-2018, 05:15 PM
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It's actually quite an elegant little model, which is unfortunately completely impossible to test experimentally or observationally.
Also, if anyone thinks the size of the solar system or the galaxy or the observable universe is humbling, they ain't got nothing on eternal inflation. Relative to the rest of the universe, the observable universe is shrinking exponentially at a rate of perhaps 1060 per second. For me at least, this is somehow more disturbing than a truly infinite universe.
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Old 02-11-2018, 05:27 PM
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Also, if anyone thinks the size of the solar system or the galaxy or the observable universe is humbling, they ain't got nothing on eternal inflation. Relative to the rest of the universe, the observable universe is shrinking exponentially at a rate of perhaps 1060 per second. For me at least, this is somehow more disturbing than a truly infinite universe.
Just to make us feel a bit better, remember, that you are always the center of an infinite universe.
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Old 02-11-2018, 05:28 PM
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That's the downside. As the universe expands, more and more of what is now the visible universe will pass beyond the cosmological horizon. the light leaving our planet now will never reach them.
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That is not to say that we know in any way definitively what lies beyond what we can see. It's pretty sure that it is pretty much the same for some distance beyond the cosmological horizon, but it is possible that this happened to be a patch of the universe that inflated in a smooth and flat fashion, further out could have played out differently.
Does this mean that the furthermost-away things that earth's astronomers can observe now are becoming farther away, so that eventually future generations of astronomers will no longer be able to observe them?

That seems to be implied by an ever-expanding universe, but I don't think it's correct. There seems to constantly be more information discovered about farther-away things. Is that just a function of us developing better quality instruments?
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Old 02-11-2018, 05:30 PM
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Awww, I thought we were special
So did Pope Paul V.
And got rather upset when Galileo told him it wasn't so.

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Old 02-11-2018, 05:38 PM
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Does this mean that the furthermost-away things that earth's astronomers can observe now are becoming farther away, so that eventually future generations of astronomers will no longer be able to observe them?
Yes, right now there is a galaxy far far away that is just crossing that threshold. When we look up into the night sky, we will not be able to see it ever again. In the far far future, we will be back to having only one galaxy in the observable universe, and future astronomers will have a hard time telling there was a big bang.
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That seems to be implied by an ever-expanding universe, but I don't think it's correct. There seems to constantly be more information discovered about farther-away things. Is that just a function of us developing better quality instruments?
And that is definitely the case. As they get farther away, the get dimmer, but they also get red shifted. That's why the James Webb telescope will not be operating in visible light, but infrared, to see these further away galaxies that are shifted out of the spectrum that hubble can see.
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Old 02-11-2018, 07:23 PM
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Old 02-11-2018, 07:58 PM
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Old 02-11-2018, 08:20 PM
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Yes, right now there is a galaxy far far away that is just crossing that threshold. When we look up into the night sky, we will not be able to see it ever again. In the far far future, we will be back to having only one galaxy in the observable universe, and future astronomers will have a hard time telling there was a big bang.


And that is definitely the case. As they get farther away, the get dimmer, but they also get red shifted. That's why the James Webb telescope will not be operating in visible light, but infrared, to see these further away galaxies that are shifted out of the spectrum that hubble can see.
This goes back to my point earlier though: in theory as time progresses we will be able to observe more and more galaxies as the observable Universe gets bigger faster than the the volume of space it contains. However practically it becomes harder to view galaxies due to red shift.

This Penrose diagram of the evolution of the Universe is probably the easiest way to see what is going on (note the distance measure is comoving which means that expansion is suppressed and the time measure is conformal which means its been re-scaled to map onto flat spacetime)

https://imgur.com/PynJnbj
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Old 02-11-2018, 10:45 PM
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Stars away from us are moving away faster ( by Hubble’s law).

So at a certain distance from us, stars will get to the speed of light ? Won’t this distance be the edge ?
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Old 02-11-2018, 11:04 PM
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Stars away from us are moving away faster ( by Hubble’s law).

So at a certain distance from us, stars will get to the speed of light ? Won’t this distance be the edge ?
The "speed limit" only applies locally, to movement through space. The recession velocity of distant galaxies attributable to the expansion of space can and does exceed c.

Last edited by Riemann; 02-11-2018 at 11:05 PM.
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Old 02-11-2018, 11:48 PM
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I'm not considering any notion of "before the big bang".

Many descriptions of cosmology fail to clarify the distinction between our observable universe and the entire universe. The observable universe is indeed bounded by a sphere, centered on us, that shrinks back to a point if we look back in time to the limit of the Big Bang. This leads to the misconception that the entire universe started at a point, and is bounded by a spherical edge, expanding in preexisting space.

Although the entire universe may not be infinite, I think it's best to start with an assumption that it is - because if your conception of cosmology can't accommodate that, you probably have some misconceptions in there.

I think the 1-dimensional "ants on a stretchy band" model is probably the best analogy. The expansion of space is the band stretching, meaning that ants that are locally stationary on the band move further apart. But assume that the band is infinite, just goes on forever without any ends. It's not entirely clear quite what that means, but I think it clarifies some aspects of the model. Looking forward in time, the band expands, making points further apart; looking back in time, it contracts, making points closer together. But it has no ends, it just disappears off into infinity however close together or far apart things become.

Now, if you consider the observable universe for a particular ant standing at one particular place on the band, that just represents the line-segment of the band centered on that ant. And if you look back in time, that segment shrinks, tending to the limit of a point centered on that ant at the Big Bang. But there's nothing special about that one point - it's just defined by where that one ant happens to be standing. More generally, looking back in time toward the Big Bang is not the process of shrinking the entire band to any one particular point, it is the process of all points on this infinitely long band tending to become mutually closer to one another - while the total extent of the band remains infinite, it has no ends. And the Big Bang itself is the undefinied limit where the distance between any two points on the infinitely long band approaches zero.

A singularity is not the same thing as a "point in space". A singularity is the infinity-times-zero limit where the model breaks.

The bottom line is that if you haven't somehow got to grips with
(a) there are no edges to the expansion of space;
(b) the Big Bang happened everywhere
then you need to keep working on your understanding to find an analogy that works for you.
I can understand if the so called Big Bang started from an infinite region that the current universe could be infinite with no spatial bound. I have a hard time with a finite Big Bang inflating to an infinite size.
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Old 02-12-2018, 01:13 AM
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I can understand if the so called Big Bang started from an infinite region that the current universe could be infinite with no spatial bound. I have a hard time with a finite Big Bang inflating to an infinite size.
I agree, if infinity means anything, it's hard to imagine how it could arise from anything that was ever finite in the past. When I described the ants-on-a-stretchy-band model as infinite, it's not intended to be an argument that the universe is infinite, more that I think maybe it helps dispel certain misconceptions about the expansion of space if you can sort-of get your head around how things would work if it were infinite. But part of a correct conception of the expansion of space under the Big Bang model is that it should only work (looking back in time) up until just before we reach the start of the universe. The starting point itself is undefined in the model.

The Big Bang is often portrayed as an "event", some well-defined thing that actually happened. That's misleading. In fact, what we have is a Big Bang model, which describes everything after the starting point pretty well, but not the starting point itself. Looking backwards in time, the "moment of creation" of the universe is the state that the contraction of space tends towards, the limit at which the model breaks. So the Big Bang is often popularly taken to mean the moment of creation; but, ironically, the Big Bang model describes everything except the moment of creation.

I'm sure you're aware that there are other models (inflation) that are hypothesized to govern what happens when (looking backwards) the simple Big Bang model breaks down near the beginning of the universe.

Last edited by Riemann; 02-12-2018 at 01:15 AM.
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Old 02-12-2018, 01:28 AM
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I can understand if the so called Big Bang started from an infinite region that the current universe could be infinite with no spatial bound. I have a hard time with a finite Big Bang inflating to an infinite size.
Concur with yourself and Riemann on this - the idea of something growing larger until it is infinite, does not make any sense in the physical world.

There is never a moment where something that physically exists is so big, that adding just a little more to it (or even doubling it, or whatever) will render it infinite in size - the concept there is just 'bigger', which is easy for the man in the street to casually conflate with infinity, but not by any means the same thing.

The most grasp-able concept of a universe with no edges, for me, is the 'finite but boundless' one - where the curvature of the universe itself means there are no edges, but it is possible to travel in (what appears to be) a straight line indefinitely - like the surface of the Earth, or the 'universe' inside the old-school Asteroids arcade game (which is topologically the surface of a toroid - things that go off the top of the screen appear at the bottom, etc).

The trouble with all of those analogies is that in likening the finite-but-boundless universe to a finite-but-boundless entity inside our own experience, they open up the question "yes, but what's outside of that?" - and accepting the answer "there is no outside" (or maybe "there is no valid answer to the question 'what is outside?'"), is hard.

Last edited by Mangetout; 02-12-2018 at 01:29 AM.
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Old 02-12-2018, 08:18 AM
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There's a simple, traditional answer to OP's question which nobody has given yet.

Imagine a spherical balloon. It has no edges; when you inflate it it gets bigger. It is a 2-D topology called the 2-sphere; any creatures living on the balloon would be in 'Flatland', not even knowing their world was curved if it was very big. The interior of the spherical balloon is called a 3-ball. But we needn't posit a 3rd dimension to cope with the 2-sphere: it is complete in itself.

If the universe is finite, it is a 3-sphere, the 3-D analog of the 2-sphere. This may be hard to visualize but just think of the inflating balloon and imagine there's another dimension! (The surface of a 4-D ball is a 3-sphere, but this mayn't help visualization and, anyway, as with the 2-sphere that extra dimension is unneeded.)
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