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Old 14th November 2017, 11:28 AM   #41
HansMustermann
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Also, well, just to clarify my "breaking apart" point. Not that it's needed any more, after Delvo's answer, but just so it's clear what I meant.

The orbit of the Earth nowadays is about 150 million km from the Sun. Some 5 billion years in the future, well, it will still be in that ball park, with some small corrections for the Sun's losing about 0.05% of its mass by then, tidal forces and such. Even though the space keeps expanding, the Earth and Sun are sufficiently gravitationally bound that they're not really being pushed apart.

If you will, it's like having a treadmill representing space expansion. If I put an Earth globe like we had in school on it, it moves with the treadmill band. But now if I tether the globe with a piece of rubber hose (representing gravity) to a big frikken slab of concrete (representing the Sun), then the Earth isn't going anywhere.

In a nutsa... err... nutshell, space expansion doesn't "break apart" systems that are bound strongly enough by gravity. (Well, not until such time as the Big Rip, anyway.)

Which was the source of my confusion. Far as I could tell, things in the early universe were much stronger bound by gravity, yet obviously they still ended up some billions of years apart. Something "broke apart" those gravitationally bound systems.

The current rate of expansion or the current values for dark energy, don't come anywhere near explaining what pushed those early clusters apart hard enough to overcome gravity.

Hence, my asking how did it work waay back when.
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Old 14th November 2017, 12:15 PM   #42
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Originally Posted by HansMustermann View Post
I'm perfectly trusting that GR works and is adequately tested. After all, my GPS wouldn't work if it didn't. Whether GR is right was never the question.

As such, I'm also not very surprised that such simulations work.

I was just asking that someone gives me a simplified version of how/why that works for the early universe. Mostly because I can't seem to find a good explanation of what drove it apart so fast and hard, against some gravity that was much stronger then, on account of everything being closer to everything else.

The thing is, I don't have an alternate crackpot theory to propose. I really believe that the real thing works. I'm not even asking whether it does. I'm just asking how, which would probably also clarify what I got wrong about it.

So just telling me to trust that it works, and there are some simulations that work... well, that still doesn't really answer my question.

I mean, take this as an exact analogy: let's say I asked how something simpler works, let's say a Diesel engine or something. Just telling me that it does work, or to trust in thermodynamics, or that there are very good simulations of Diesel engines, isn't answering the question of how it works, does it? In fact, it doesn't even tell me anything I don't already know. I KNOW that my car's engine works (not as well as VW claimed back then, mind you, but it does work), I have total faith in thermodynamics, etc. But none of that answers the question of how it works.


That said, I'm also perfectly content to leave it at "we don't know", since that's what Delvo tells me. I appreciate a good "we don't know", really. It saves me from trying to figure it out.
Thanks for the clarification.

Depending on what you mean by “the early Universe”, here are some answers:

* not answerable, because there’s no theory of quantum gravity
* inflation, but that’s not vanilla GR, and is a shorthand for dozens of different ideas
* Dark Energy, which plays nicely with GR, but what is it?
* GR alone ensures an expansion, see W.D.Clinger’s post for details

On the last point: what gets “split apart” and what not depends on “local conditions”, or the balance between “expansion” and “gravity from local masses” (note that the “expansion” is also “gravity”, because this is pure GR).

Hope this helps.
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Old 14th November 2017, 12:36 PM   #43
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I stand by my original assertion. If mass/energy is evenly distributed through all of space, why would it collapse into a black hole?

Gravity isn't mass attracting mass. It's mass warping spacetime, and mass's path through spacetime being affected by the shape of spacetime. If everything is evenly distributed, spacetime looks flat locally at any spot you can pick. There is no movement induced by gravity, no downhill, no collapse.
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Old 14th November 2017, 01:11 PM   #44
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Hmm... I can see how that would work. If everything thinks it's in the middle, yeah, Birkhoff would make it flat. Hmm... that would work.

However, wouldn't that only work if the universe is infinite (and it may well be), or wraps around like a bubble, so to speak? If the universe is finite, as in, there's actually nothing past a certain point, not even space, wouldn't the stuff near the edge be more drawn towards the centre than towards the edge?

It's nothing we can observe from where we are, on account that last I heard the actual universe was larger than the currently observable one, but IF it ends abruptly at some point, and more importantly IF it ended abruptly early enough in its expansion, wouldn't that throw a spanner into it?
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Old 14th November 2017, 03:12 PM   #45
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Originally Posted by HansMustermann View Post
Hmm... I can see how that would work. If everything thinks it's in the middle, yeah, Birkhoff would make it flat. Hmm... that would work.

However, wouldn't that only work if the universe is infinite (and it may well be), or wraps around like a bubble, so to speak? If the universe is finite, as in, there's actually nothing past a certain point, not even space, wouldn't the stuff near the edge be more drawn towards the centre than towards the edge?

It's nothing we can observe from where we are, on account that last I heard the actual universe was larger than the currently observable one, but IF it ends abruptly at some point, and more importantly IF it ended abruptly early enough in its expansion, wouldn't that throw a spanner into it?
Not necessarily.

Someone, in this thread or the other (or perhaps the “Farsight” one cited) mentioned a horizon, one to do with causal connection. Also world lines. If the edge is always outside the horizon, there can be no causal connection, right (unless GR does not rule)?

And to phunk’s point, the universe is, obviously, not isotropic and homogenous... which is the basis for at least one of your questions, right? Spherical cows are good for many things, but real cows are not spherical!
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Old 14th November 2017, 03:50 PM   #46
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Originally Posted by HansMustermann View Post



... Whereas the early universe WAS in a small enough space.

...
Was it though? I was under the impression that all the space we see around us now was there at the beginning; it was just folded or compressed or something.
Things were closer together, but they were still separated by vast amounts of space... maybe...
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Old 14th November 2017, 08:09 PM   #47
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Originally Posted by JeanTate View Post
And to phunk’s point, the universe is, obviously, not isotropic and homogenous...
It's not anymore. At the start, it was very close to it. The variations in the CMB are like one part in 100000*. It took a long while for those tiny variations + gravitation to create the structure we see today.


*ETA: And that was after 370000 years of gravity having time to start to move things around.

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Old 14th November 2017, 10:13 PM   #48
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Originally Posted by Brainache View Post
Was it though? I was under the impression that all the space we see around us now was there at the beginning; it was just folded or compressed or something.
Things were closer together, but they were still separated by vast amounts of space... maybe...
Far as I know, not really, no. There actually was less space to go around for everyone.
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Old 14th November 2017, 10:52 PM   #49
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Originally Posted by JeanTate View Post
Not necessarily.

Someone, in this thread or the other (or perhaps the “Farsight” one cited) mentioned a horizon, one to do with causal connection. Also world lines. If the edge is always outside the horizon, there can be no causal connection, right (unless GR does not rule)?

And to phunk’s point, the universe is, obviously, not isotropic and homogenous... which is the basis for at least one of your questions, right? Spherical cows are good for many things, but real cows are not spherical!
The edge of the observable universe IS a horizon, and yeah, for us we're not affected by anything outside it without violating causality. It's kind of like a black hole turned inside out. In a black hole you can's see into it from outside (because no photons can come through the horizon from inside), while for the observable universe we can't see outside from within. The space between us and whatever is outside expands too fast for any photons from outside to ever reach us.

But anything for which the actual edge of the universe is closer than that horizon WOULD see more matter on one side than the other.

Incidentally that's one reason we know the universe must be bigger than what we see. Because the things we see near our horizon, also don't seem to observe an edge.

But my point is this: we don't really know if the universe is infinite, or wraps around or what. The bubble analogy, sure, illustrates how everyone sees everything equally moving away from them like they're in the middle of it all. But so does the analogy of taking a very finite piece of rubber and stretching it. Every point on it will still see everything else moving away from it. Just grab the elastic band of some underpants with both hands and stretch it, and the exact same thing happens: no matter where you are in between, everything moves away from you.

Just the bits near your hand will see less other bits to one side than the other. And while for a point in the middle, sure, it's pulled equally in both directions, for a piece near the end, you have to pull to make it move. Otherwise it will snap right back.

Not seeing a centre from which everything moves away, because everything equally moves away from everything, is not the same thing as there actually being no centre and no edge. The bits of elastic band near your hand would still see everything moving away from them in both directions, just like they're in the centre of it all, but would also still see less of everything in one direction than the other.

And sure, the stretching a piece of rubber is easier done in one dimension , but then we fold all 3 dimensions into 1 on a Penrose diagram anyway. And the universe probably wasn't stretched from the edge, but uniformly from within. And sure, it's not a very good analogy for gravity, since the elastic band pulls proportionally to the distance, while gravity is inversely proportional to the distance squared. So I guess it's not a very good analogy, but you get the idea.

So I'm wondering, does that mean the universe HAS to be infinite?
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Old 15th November 2017, 08:42 AM   #50
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Does the physics even exist to describe how a hypothetical edge of the universe would behave?
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Old 15th November 2017, 08:46 AM   #51
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Originally Posted by Brainache View Post
Was it though? I was under the impression that all the space we see around us now was there at the beginning; it was just folded or compressed or something.
No, I think space is being continually created.
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Old 15th November 2017, 10:32 AM   #52
HansMustermann
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Originally Posted by phunk View Post
Does the physics even exist to describe how a hypothetical edge of the universe would behave?
THAT would have been my question, actually.
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Old 15th November 2017, 12:53 PM   #53
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This seems like a weird question. I’m not even quite sure how to phrase it. Is the observable universe the same everywhere in the observable universe?

Eg in the distant universe we see a galaxy as it existed 500 million years after the big bang. If that galaxy subsequently developed planets and intelligent life would the limits of the observable universe be the same for them as it is for us or would there there be objects that are too far away for the light to ever reach us, but who's light could reach them?
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Old 15th November 2017, 12:59 PM   #54
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Originally Posted by lomiller View Post
This seems like a weird question. I’m not even quite sure how to phrase it. Is the visible universe the same everywhere in the visible universe?
No. Each place in the universe has its own sphere centred on it that is its own visible universe.

It's really like this: each point sees all other points moving away from it. The farther away, the faster they move away. Past a point, essentially it's moving away faster than light, and you're moving away from it faster than light. The sphere where that happens is centered on the point where the observer is.

Originally Posted by lomiller View Post
Eg in the distant universe we see a galaxy as it existed 500 million years after the big bang. If that galaxy subsequently developed planets and intelligent life would the limits of the visible universe be the same for them as it is for us? Could there be objects that are too far away for the light to ever reach us, but who's light could reach them?
No. It's really symmetrical. If we see them moving away from us with speed v, then they see us moving away at speed -v. If we don't see them because v is too high, well, so is -v.
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Old 15th November 2017, 01:16 PM   #55
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Originally Posted by HansMustermann View Post
No. It's really symmetrical. If we see them moving away from us with speed v, then they see us moving away at speed -v. If we don't see them because v is too high, well, so is -v.
I’m not asking about what they see looking towards us, I’m asking about what they would see looking away from us, would they see things that are not observable by us.

Originally Posted by HansMustermann View Post
No. Each place in the universe has its own sphere centred on it that is its own visible universe.
But at some point very soon after the big bang the distance between the two would have been negligible, so the two spheres must be very close to identical.
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Old 15th November 2017, 01:41 PM   #56
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Originally Posted by lomiller View Post
I’m not asking about what they see looking towards us, I’m asking about what they would see looking away from us, would they see things that are not observable by us.
Yes.

Originally Posted by lomiller View Post
But at some point very soon after the big bang the distance between the two would have been negligible, so the two spheres must be very close to identical.
Yes, more things were visible back then than are now. And less things will be visible in the future. At some point in the future, in fact we won't even see any other galaxy than our own.
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Old 15th November 2017, 02:03 PM   #57
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Originally Posted by HansMustermann View Post
Yes.



Yes, more things were visible back then than are now. And less things will be visible in the future. At some point in the future, in fact we won't even see any other galaxy than our own.
Ok, but since the two started out infinitely close immediately after the big bang, the only they can be different is if space expanded much faster than the speed of light in the period following the big bang. If it didn’t the additional distance they could “see” would be tiny. Since the speed at which mas can move in space is limited to the speed of light everything must have been moving apart due to the expansion of space much much faster than gravity could pull it together.

Secondly, and I’m not sure this is even relevant given some of the other answers you received, is that the mass which is observable to them but not us would have excreted a gravitational force in the opposite direction pulling matter away from matter that it part of our observable universe. The result would be that net gravitational force would be reduced and possibly even zero across the observable universe as everything we see is/was being pulled away by mass outside our observable universe.
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Old 15th November 2017, 04:36 PM   #58
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Originally Posted by HansMustermann View Post
How did universe expansion break that kind of a tightly bound system apart, when currently it doesn't break us apart from the great attractor?
The answer is that the universe expansion cannot and so did not break any tightly bound systems apart. Groups of galaxies have always been gravitationally bound.

ETA: The OP mentions the Schwarzschild radius. This is from the Schwarzschild solution in GR for the gravitational field outside of a body. The Earth has a tiny Schwarzschild radius but is not a black hole because all of the mass is not inside that radius. Ditto for the Sun. Ditto for the universe.

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Old 15th November 2017, 05:49 PM   #59
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It’s been said before, but it’s worth saying again...

If GR rules, “expansion” and “gravity” are the same ... remember spacetime tells mass how to move, matter tells spacetime how to curve (if there’s Dark Energy, you could say that it is different, but so far no one has tried to wrinkle out that part due to DE and that due to vanilla GR).

This doesn’t apply for the Planck regime, nor to any inflation.

A narrative that has expansion and gravity as “competing” might have some intuitive resonance, but let’s be clear that it’s something not in the GR-based models. Or, that if it is, we need to point to where, in those models, the distinction is made.

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Old 15th November 2017, 11:20 PM   #60
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Originally Posted by JeanTate View Post
This doesn’t apply for the Planck regime, nor to any inflation.
That is correct.

In the beginning the universe was extremely, unbelievably hot and dense, much denser than usually required to form a black hole, but also very homogeneous. It couldn't form clumps. The usual calculations for gravitational collapse apply to objects in space, but not to expanding space itself.

Alan Guth found that if the universe contained a field that has a positive-energy false-vacuum state, then according to GR it would generate an exponential expansion of space.

This is believed to have come into effect due to the universe undergoing a phase transition 10-36 seconds after the BB. Inflation only lasted up until 10-32 or -33 seconds after the BB, another phase transition. It is believed that things that were less then a nanometer apart before inflation ended up at least more than 10 light years from each other.

Lots about this on wikipedia:

https://en.wikipedia.org/wiki/Chrono...f_the_universe
https://en.wikipedia.org/wiki/Metric_expansion_of_space
https://en.wikipedia.org/wiki/Inflation_(cosmology)

The universe is also believed to have no edge, there is no math to cope with an edge. Only the observable universe has an 'edge', a horizon.
All the latest data seems to say it is also flat, but not necessarily presumed to infinite in size, although very, very large.
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Old 15th November 2017, 11:42 PM   #61
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Well, someone will have to explain to me how it works that:
- the universe is not infinite, but
- the universe has no edge

Unless it wraps around like a bubble, I suppose, but that would involve it being embedded in a universe with more than 3 dimensions. Which is possible, but gets us right back to "why didn't it collapse into a black hole?" because in that universe it's anything but uniform and edgeless.
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Old 16th November 2017, 03:19 AM   #62
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If the universe is not infinite in size, it is presumed to be a closed manifold, that is it is compact and without boundry.
It could have any of many shapes, the simplest being, I think, a 3-torus i.e. a wrap around 'astroids' (the game) type universe. It might also be a 3-sphere, but then space would not be euclidian.
Although the 3-d surface of the 3-torus or 3-sphere has to wrap around to join with itself, it does not imply the existence of a higher dimension for the universe to bend through.

For example the 2d surface of a torus can mathematically be described without reference to a third dimension through which it curves and without stretching or squeezing the surface, the same is done for the 3D surface of the 3-torus.
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Old 16th November 2017, 07:09 AM   #63
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Originally Posted by HansMustermann View Post
Well, that citation also just said that current calculations don't apply. But my question was basically "what DOES apply?" Removing one calculation, as per that quote, still lives potentially N to go before I arrive to the one that does apply. It's a process of elimination, since it just eliminated one. Problem is, I don't even know the other N to go, so it's not even going nowhere fast. It's going nowhere slow

And what the citation "also just said that" it's the expansion of space that directly applies to your question of why no black hole. As Delvo notes above the question then becomes why expansion was so strong then. Which itself would really just go back to why the singularity expands in the first place. Again while we know our current theories (relativity and QFT) are incomparable and insufficient at that scale. Even if we preclude a singularity and just keep the limit at the Planck scale we still come up short. However, once expansion is initiated, by whatever means, it becomes a dominating factor. If recollapse is possible it is then a matter of time when gravity overcomes expansion. So far some almost 14 billion years latter and there is no sign of expansion letting up, in fact it appears to be accelerating again.

So again to your above question "what DOES apply?", as given in the original answer expansion of space still applies. Even today while insufficient to separate gravitationally bound systems it still appears sufficient to keep the universe open (prevent recollapse).

http://enews.lbl.gov/Science-Article...ve/SNAP-3.html
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Old 16th November 2017, 07:18 AM   #64
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Originally Posted by Delvo View Post
In a big enough black hole, the singularity could be so far away that you wouldn't really know about it, even in the most purely simple sense, that of looking at it as a location, a place in space. But there's also another reason you might not see any sign of such a "place". Inside a black hole, the roles & behavior of time and space mix, so the singularity is not a place you move toward but an event in your future timeline. And the future is something we're all being inescapably pulled toward, so, again, how would we know the difference between "the future as a distant singularity as seen from inside the black hole" and "the future as... something else but not that"?
Exactly, if recollapse were indeed inevitable then a singularity would certainly be in the future of the universe. As I mentioned before expansion doesn't have to prevent recollapse entirely it just have to prevent it for some period of time.
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Old 16th November 2017, 08:03 AM   #65
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Originally Posted by HansMustermann View Post
Well, someone will have to explain to me how it works that:
- the universe is not infinite, but
- the universe has no edge

Unless it wraps around like a bubble, I suppose, but that would involve it being embedded in a universe with more than 3 dimensions.
A universe that "wraps around" doesn't require embedding in other dimensions any more than a perfectly flat one does.

Given what we know, the only requirement is that it is so big that the observable part appears flat. In the same way that Kansas appears flat if you only measure a few meters in the middle of a field.
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Old 16th November 2017, 02:15 PM   #66
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Originally Posted by lomiller View Post
Ok, but since the two started out infinitely close immediately after the big bang, the only they can be different is if space expanded much faster than the speed of light in the period following the big bang. If it didn’t the additional distance they could “see” would be tiny. Since the speed at which mas can move in space is limited to the speed of light everything must have been moving apart due to the expansion of space much much faster than gravity could pull it together.
I'm not sure I follow.

1. Let's say we start next to each other and start walking in opposite directions, at a leisurely stroll. About 1m/s. If we keep at it, aftwer 15 billion years, we'll be 100 light years apart. That's still really close by cosmic scales, but nevertheless, you'll see 100 light years more in one direction, and I'll see 100 light years more in the other.

Do that with two chemical rockets that do, say, 10,000 m/s each (quite easy to achieve with today's tech, since it's below even Earth's escape velocity) and they'll be 1 million light years apart after 15 billion years, and each can see 1 million light years more in one direction than the other. That's still not huge, but it's about 10 times the diameter of out galaxy, or almost half the way to Andromeda.

No FTL is needed to have different edges for the observable universe. Hell, you don't even need relativistic speeds.

2. While the universe did expand faster than light, points that were really close to each other did not.

Take a 1ft long piece of rubber and stretch it to 2ft long in one second. Sure, the edges move at 1ft/s apart from each other, but two points that were 1 inch apart will only move at 1 inch/s away from each other.

And in fact still don't. Parts of the universe that are outside the observable limit move faster than light away from us, but Andromeda does not.
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Old 16th November 2017, 02:55 PM   #67
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Originally Posted by HansMustermann View Post
No FTL is needed to have different edges for the observable universe.
That is correct. In theory you and I have different edges for the observable universe.
The observable universe is the part of the entire universe than an observer can detect. Every observer thus has their own observable universe. We call what we see the observable universe because we are basically all in one place. But we know that separated observers will measure different observable universes.
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Old 17th November 2017, 01:07 AM   #68
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Well, first of all, thanks everyone. I'm learning interesting stuff here.

I still have one confusion, though. Does gravity matter after all, or not? (And yeah, I know, GR rules, curved space, etc. I'm still gonna call it gravity because it's shorter.) I mean, on one hand I'm told that it would be all flat(ish) space if matter is everywhere. But on the other hand just about everywhere I find considerations like if there's enough mass in the universe for it to be open or not. Hell, even in the Big Rip calculations, the current density of mass matters in how fast the universe will accelerate apart.

So, err, which of them is it? Does gravity still try to pull the universe back together, or doesn't it? It seems to me like it can't jolly well be both at the same time, or can it?
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Old 17th November 2017, 02:01 AM   #69
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Originally Posted by HansMustermann View Post
Well, first of all, thanks everyone. I'm learning interesting stuff here.

I still have one confusion, though. Does gravity matter after all, or not? (And yeah, I know, GR rules, curved space, etc. I'm still gonna call it gravity because it's shorter.) I mean, on one hand I'm told that it would be all flat(ish) space if matter is everywhere. But on the other hand just about everywhere I find considerations like if there's enough mass in the universe for it to be open or not. Hell, even in the Big Rip calculations, the current density of mass matters in how fast the universe will accelerate apart.

So, err, which of them is it? Does gravity still try to pull the universe back together, or doesn't it? It seems to me like it can't jolly well be both at the same time, or can it?
I think it can be close to flat locally while being curved, perhaps even closed, globally.

So for instance, you could have a situation where two masses close to each other would travel along non-intersecting geodesics over short time scales but you could still have a big crunch after billions of years.
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Old 17th November 2017, 02:57 AM   #70
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That much I can easily understand. My question was does mass still pull the universe together (maybe not as hard as it's being pushed apart by dark energy, mind you) or not. On one hand I'm being told that there is no pulling it together if everything is spread out more or less evenly, on the other hand every calculation I've ever seen seems to assume that, yes, mass still pulls the universe together. I may be just mis-understanding the last part, but I'd like to know if I do, and correct the mis-understanding.
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Old 17th November 2017, 06:47 AM   #71
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Originally Posted by HansMustermann View Post
That much I can easily understand. My question was does mass still pull the universe together (maybe not as hard as it's being pushed apart by dark energy, mind you) or not. On one hand I'm being told that there is no pulling it together if everything is spread out more or less evenly, on the other hand every calculation I've ever seen seems to assume that, yes, mass still pulls the universe together. I may be just mis-understanding the last part, but I'd like to know if I do, and correct the mis-understanding.
As I understand it (and I'm sure those here with more knowledge than me can provide corrections if I'm wrong ), it could be either.

1. If the universe is finite and has no edge (it's a 3-torus, as someone mentioned earlier), and matter is pretty evenly spread out, then it's likely that gravity doesn't matter so much. It still pulls things together, but evenly in all directions, so gravity would cause more local clumping (stars, galaxies, galaxy clusters) but not much universally.

2. Any other case: the universe has an edge, matter isn't evenly spread out, etc, then gravity matters more at universal scales.

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Old 17th November 2017, 06:54 AM   #72
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Gravity would be bringing the universe back to a Big Crunch if left unopposed, by pulling everything inward.

What depends on how evenly the matter is distributed is not that total universal effect but the formation of separate objects inside the universe with emptier space between them. Gravity magnifies unevenness, but if the starting point is distributed evenly enough, the net force can't favor one direction over another at any place, and thus certainly not different directions in different places.
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Old 17th November 2017, 07:06 AM   #73
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Thanks, Delvo. I understand that unevenness is responsible for the formation of galaxies and clusters and so on. I just weren't sure any more about that pulling together, after Phunk's messages (and possibly my misunderstanding of what he was trying to say.)
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Old 17th November 2017, 10:14 AM   #74
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Originally Posted by HansMustermann View Post
So it just dawned upon me that there's a question I should have asked the last time: so... err... why didn't it turn into a black hole anyway?
What do you mean by 'it'?


Originally Posted by HansMustermann View Post
We're not even talking about immediately after it blew up.
What is it that you think 'blew up'?


Originally Posted by HansMustermann View Post
We only need to go back a couple billion years to go back to where it should have been a black hole.
Where what should have been a black hole?


Originally Posted by HansMustermann View Post
Or if that plus/minus goes about 50% on the plus side, it would STILL be a black hole, and we're inside one.
I submit that this 'it' you keep talking about is the actual error leading you to erroneous conclusions or ramifications that are irrelevant.


Originally Posted by HansMustermann View Post
So, what am i missing? If the argument against an explosion in flat space is that it would become a black hole before it can explode, how didn't it turn into a black hole anyway?
There is no 'it'. The Big Bang is about the expansion of space-time, not about gravitational attraction of matter. This then gives you the answer: because the force of expansion was greater than the force of gravitation. Also, space is big (or rapidly got bigger in the expansion), and gravitation is subject to the inverse square law.

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Old 17th November 2017, 11:08 AM   #75
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Well, in a nutsa... err... nutshell, "it" is the universe, and what I was asking about was the force driving the expansion. Precisely because gravity is subject to the inverse square law, I expect it to be a lot higher when the universe was a billion times more densely packed. So, something else must have been pushing really hard the other way around. That's all I was going on about.
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Old 17th November 2017, 11:56 AM   #76
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Originally Posted by Reality Check View Post
That is correct. In theory you and I have different edges for the observable universe.
The observable universe is the part of the entire universe than an observer can detect. Every observer thus has their own observable universe. We call what we see the observable universe because we are basically all in one place. But we know that separated observers will measure different observable universes.
True but my question was slightly different.

Take 2 points 1m apart in the instants after the big bang. 13 billion years later they are 12 billion light years apart. Each has a visible universe based on whether light that was emitted 13 billion years ago, when they were only 1m apart, has had time to reach them or not.

When they look at each other do they see the same stuff as the other would see looking in that same direction just with different age/redshift? Or, do they see mostly different stuff that would never be visible to the other? If the stuff they see is different, what conditions allow that to happen when they started so close together?
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Old 17th November 2017, 12:38 PM   #77
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The interesting case isn't as much when they look at each other, since then they're really looking at the intersection of their visible universes. IMHO the more illuminating case is when they look in the same direction.

Anyway... They were closer back then, but the space expanded in the meantime, so light takes a longer time to get to one than to the other. And since expansion is proportional to distance, whatever source of light they're looking at, is moving faster away from one than from the other, so it may fall outside the visible space for one but not for the other.

Also, I wouldn't skip so lightly over "just with different redshift". Essentially the edge of the observable universe is where redshift becomes infinite. If the two see different redshifts for the same source, well, you can see how for one of them it's closer to the edge than to the other. And possibly not visible to one of them.
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Old 17th November 2017, 06:05 PM   #78
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Originally Posted by HansMustermann View Post
That much I can easily understand. My question was does mass still pull the universe together (maybe not as hard as it's being pushed apart by dark energy, mind you) or not.
Yes, it does. To be precise, the FLRW models explain how mass density affects the rate of expansion and also determines whether the universe will expand forever or collapse into a big crunch.

Originally Posted by HansMustermann View Post
On one hand I'm being told that there is no pulling it together if everything is spread out more or less evenly,
Not everything you've been told is true.
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Old 17th November 2017, 11:07 PM   #79
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Thanks.
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