# Universe has no end because

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If we took two lines leaving a single point and then some where in far distance the outer sides of each would come together. Could this be how the universe is racing away because its pull itself into itself in the distance place?

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Your question is about the shape of the universe itself. Here are a few of the top ideas: https://phys.org/news/2019-11-universe-rethink-cosmos.html

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Now our new paper, published in Nature Astronomy, has come to a conclusion that may unleash a crisis in cosmology—if confirmed. We show that the shape of the universe may actually be curved rather than flat, as previously thought—with a probability larger than 99%. In a curved universe, no matter which direction you travel in, you will end up at the starting point—just like on a sphere.

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If it is positively curved the parallel lines will obviously converge at distance.
Expansion has the effect of 'smoothing out' curvature; curvature does not cause, or give the illusion of, expansion.
The measured curvature is exceedingly small, almost flat, indicating that the Universe may be much, much larger than the observable universe.

Can't access the actual paper but it would be interesting to see the error in the curvature data, INow.

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No matter where you go you are still at the center of expansion.  Where is the end of a balloon for  a 2-D being (forget the nozzle and hope our universe doesn’t have one.

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Posted (edited)
On 6/26/2020 at 11:34 AM, MigL said:

If it is positively curved the parallel lines will obviously converge at distance.
Expansion has the effect of 'smoothing out' curvature; curvature does not cause, or give the illusion of, expansion.
The measured curvature is exceedingly small, almost flat, indicating that the Universe may be much, much larger than the observable universe.

If positively curved, does that mean that if you follow a straight line you cannot help but arrive at your starting point?

If the universe was flat, wouldn't it also necessarily be larger than the observable universe?  I thought it didn't matter if the universe was flat or curved, the observable portion would always be the smaller.

Edited by Airbrush

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40 minutes ago, Airbrush said:

If positively curved, does that mean that if you follow a straight line you cannot help but arrive at your starting point?

Correct. But it looks like it may take a veeerrry long time, as curvature is exceedingly small.
A positively curved universe ( no matter how small the curvature ) is necessarily finite, but unbounded, and, since curvature is so small, it may be many orders of magnitude larger than the observable universe.

42 minutes ago, Airbrush said:

If the universe was flat, wouldn't it also necessarily be larger than the observable universe?

If the universe is actually flat, or negatively curved ( saddle shape ), it is necessarily infinite.
So, again correct, any infinite universe is unbounded, and larger than the observable universe.

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Posted (edited)
On 8/28/2020 at 10:58 AM, MigL said:

Correct. But it looks like it may take a veeerrry long time, as curvature is exceedingly small.
A positively curved universe ( no matter how small the curvature ) is necessarily finite, but unbounded, and, since curvature is so small, it may be many orders of magnitude larger than the observable universe....

Why would positive curvature always be along the same arc?  Maybe the curvature is random.

Or maybe there is an "onion analogy" where the "straight" line curves around but misses the starting point and ends up on another layer of the onion?

Edited by Airbrush

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1 hour ago, Airbrush said:

Why would positive curvature always be along the same arc?  Maybe the curvature is random.

Or maybe there is an "onion analogy" where the "straight" line curves around but misses the starting point and ends up on another layer of the onion?

Which of which, would lead to an end???

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1 hour ago, dimreepr said:

Which of which, would lead to an end???

What I'm saying is, just in case you were thinking of taking a big round trip around the universe, traveling in a straight line, you might not return home.  You might end up on another layer of an onion, or the universe curves randomly and the curvature we see only applies to our observable universe.

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What layer ?
The surface of the sphere is space-time, albeit reduced to only two dimensions.
The positive curvature is intrinsic, so you CANNOT end up in 'other layers' as there is no inside or outside of that surface.
Feel free to ask questions if still not understood.

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Posted (edited)
On 6/26/2020 at 8:34 PM, MigL said:

If it is positively curved the parallel lines will obviously converge at distance.
Expansion has the effect of 'smoothing out' curvature; curvature does not cause, or give the illusion of, expansion.

As to what concerns the OP's question, I totally agree with this. +1

The question of planarity of the observable universe, for all I know, is about spatial flatness and has nothing to do with the rate of expansion.

On 6/26/2020 at 8:34 PM, MigL said:

Can't access the actual paper but it would be interesting to see the error in the curvature data, INow.

Preprint is here:

10 hours ago, Airbrush said:

Why would positive curvature always be along the same arc?  Maybe the curvature is random.

Or maybe there is an "onion analogy" where the "straight" line curves around but misses the starting point and ends up on another layer of the onion?

I think a wildly-varying curvature would have noticeable effects in the aberration of light. I'm not sure I understand your onion picture but, wouldn't that have problems with isotropy/homogeneity at large scale? Although I agree that not even in a spherical universe would you necessarily move in periodic orbits. Only if you followed geodesics you would.

Edited by joigus

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22 hours ago, joigus said:

I think a wildly-varying curvature would have noticeable effects in the aberration of light. I'm not sure I understand your onion picture but, wouldn't that have problems with isotropy/homogeneity at large scale? Although I agree that not even in a spherical universe would you necessarily move in periodic orbits. Only if you followed geodesics you would.

Maybe the curvature we can detect is only within the observable universe.  Outside the observable, maybe the curvature changes?  I thought massive objects bend space, but they are located randomly, so how can any curvature be constant?

Can all the isotropy/homogeneity be within the observable universe?  We can't see outside that.

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1 hour ago, Airbrush said:

Maybe the curvature we can detect is only within the observable universe.  Outside the observable, maybe the curvature changes?  I thought massive objects bend space, but they are located randomly, so how can any curvature be constant?

Can all the isotropy/homogeneity be within the observable universe?  We can't see outside that.

The 'observable universe' is an observer-dependent  limitation centred around where they are located. Why should things be different outside of that field of view?

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The graphics INow posted on Jul26 are 2D reductions of 4D space-time.
( in 4D those three graphics would be expanding in size also as time passes )
The 'observable' universe for a 'flatlander' living in one of those three universes, would be a circle, NOT a sphere.
The curvature depicted by those graphics is 'global' curvature, NOT 'local' as you would have in the neighbourhood of planets, stars, galaxies or even clusters.
Homogeneity/isotropy means the same and in all directions; if it changes outside the observable universe then it can't be homogeneous and isotropic. But as Stringy says, why would it ?

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10 hours ago, MigL said:

The graphics INow posted on Jul26 are 2D reductions of 4D space-time.
( in 4D those three graphics would be expanding in size also as time passes )
The 'observable' universe for a 'flatlander' living in one of those three universes, would be a circle, NOT a sphere.
The curvature depicted by those graphics is 'global' curvature, NOT 'local' as you would have in the neighbourhood of planets, stars, galaxies or even clusters.
Homogeneity/isotropy means the same and in all directions; if it changes outside the observable universe then it can't be homogeneous and isotropic. But as Stringy says, why would it ?

My favorite subject on this web site is Astronomy & Cosmology.  Too bad there is not more conversation here.  The idea of a "positively-curved" universe, where if you really travel in a totally straight line, you may end up at your starting point is the most difficult idea I have yet encountered.  It is impossible for me to think in "flat-lander."☹️  Anyone have a more understandable analogy for global curvature?😃  Or at least explain how 3-d space can be so curved.

Edited by Airbrush

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A map is a two dimensional representation of the three dimensional spherical world.
So, just as you might see the Eastern part of Siberia west of Alaska at the far left hand side of the map, while the rest of western Siberia is on the far right side of the map, the implication is that the surface is unbounded. IOW, travelling in the outward direction off one side of the map, brings you to the other side of the map, just as on the real spherical world.

Local curvature is caused by the local accumulation of mass-energy ( momentum ), and give rise to 'dimples' on the two dimensional representation of space-time. Global curvature is caused by the total mass-energy of the universe ( including dark matter and dark energy ); if above a threshold value, then there is enough gravity to curve the universe back on itself ( positive curvature ), and parallel lines will converge and meet at some point. At the threshold value the universe is flat, and below the threshold value curvature is negative, and in neither case do parallel lines converge, so the universe is open and infinite

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16 hours ago, Airbrush said:

Maybe the curvature we can detect is only within the observable universe.  Outside the observable, maybe the curvature changes?  I thought massive objects bend space, but they are located randomly, so how can any curvature be constant?

Can all the isotropy/homogeneity be within the observable universe?  We can't see outside that.

Everything we can detect is within the observable universe. Beyond the last scattering surface, the universe is opaque to radiation and we can't see anything, except maybe with gravitational waves; and beyond that, not even with gravitational waves. I agree that local objects bend space, but on average the universe looks very (spatially) flat. Around black holes and very massive objects you can detect local curvature, like e.g. Einstein rings, but the sphere of the sky looks pretty flat overall. What the paper that @iNow has linked to seems to imply is that within the observable universe the telescopes have detected large-scale lensing that must have to do with curvature within the horizon. That's what's very surprising to me. I'd like to follow up on that.

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2 hours ago, Airbrush said:

My favorite subject on this web site is Astronomy & Cosmology.  Too bad there is not more conversation here.  The idea of a "positively-curved" universe, where if you really travel in a totally straight line, you may end up at your starting point is the most difficult idea I have yet encountered.  It is impossible for me to think in "flat-lander."☹️  Anyone have a more understandable analogy for global curvature?😃  Or at least explain how 3-d space can be so curved.

Even a straight line can be twisted about its own axis.

That is called torsion.

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10 minutes ago, studiot said:

Even a straight line can be twisted about its own axis.

That is called torsion.

unless one is screwed...

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You are telling me I can't say I'm a flat universer now? How tragic... T.T

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Sure you can say it.
Free speech.

And the jury is still out so you are not wrong either.

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