Galaxy Expansion or Attraction?

  • #1
Andy_K
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First of all, sorry for my naive question here which likely doesn't make sense.

The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively "expanding", but instead being gravitationally attracted towards a supermassive black hole (or perhaps, "Big Crunch") that is forming at a faraway point beyond our visible universe?

Could our observable universe be accelerating towards another point of the hypersphere, and while galaxies appear to all "recede away" from us, we are actually all moving in the same direction?

Instead of dark energy causing the expansion, could it be immense gravity from a distant source that is drawing our galaxies over?

Would we be able to distinguish between "expansion" and "attraction"?

Thank you for enlightening me.
 

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  • #3
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In brief, answer is "no" - expansion of Universe as we know it is mostly isotropic
What if the universe had a boundary which is attractive? It would be outside the observable range, attraction would be greater the farther away galaxies are, and it would be an isotropic movement. I admit this sounds very strange, but so does dark energy. Can this be ruled out by our current data?
 
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  • #4
russ_watters
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What if the universe had a boundary which is attractive? It would be outside the observable range, attraction would be greater the farther away galaxies are, and it would be an isotropic movement. I admit this sounds very strange, but so does dark energy. Can this be ruled out by our current data?
Wouldn't that be like being inside a spherical shell? E.G. no net gravitational force at all?
 
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  • #5
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Wouldn't that be like being inside a spherical shell? E.G. no net gravitational force at all?
I'm not sure about the math here, but shouldn't the attraction be greater near the boundary as the "other side" is too far away for a significant contribution? I don't want to defend this "idea", I just thought, it is what the OP was asking and an argument to rule it out would be nice.
 
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  • #7
PeroK
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I'm not sure about the math here, but shouldn't the attraction be greater near the boundary as the "other side" is too far away for a significant contribution? I don't want to defend this "idea", I just thought, it is what the OP was asking and an argument to rule it out would be nice.
The problem would be to get an isotropic linear relationship between distance from Earth and relative velocity from Earth.

You could try it with a 2D universe on the surface of a sphere. Where would the attraction be for everything to move away from every other point? And what sort of force would create a linear relationship between distance and relative velocity in any case?
 
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  • #8
Andy_K
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Thank you everyone for your great feedback and references.

You could try it with a 2D universe on the surface of a sphere. Where would the attraction be for everything to move away from every other point? And what sort of force would create a linear relationship between distance and relative velocity in any case?
If we imagine it on a 2D surface of a sphere, is it possible that the galaxies are being attracted from one "pole" (a.k.a. Big Bang) to the opposite "pole" (a.k.a. Big Crunch), just like the movement in a magnetic field?

Let's say 2 galaxies are receding from Earth at 30km/s. Would it be possible that all are just accelerating towards the same destination, where the relative velocities make it appear that the galaxies on both sides are receding away from us?

As for why galaxies "parallel" to us are moving away (since they would be headed towards the same destination), is it possible that because we are all moving on a curved plane, so two parallel points actually get further away as we move along the same direction? At least, until we cross the middle point, upon which all galaxies would then close back in.

I tried piecing together some simple illustrations, hope it makes sense. Thank you.

galaxy.png
 

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  • #9
PeroK
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If we imagine it on a 2D surface of a sphere, is it possible that the galaxies are being attracted from one "pole" (a.k.a. Big Bang) to the opposite "pole" (a.k.a. Big Crunch), just like the movement in a magnetic field?

Let's say 2 galaxies are receding from Earth at 30km/s. Would it be possible that all are just accelerating towards the same destination, where the relative velocities make it appear that the galaxies on both sides are receding away from us?
One problem is that to get the same acceleration, you need to the same force. For example, if the Earth is far from the source of the attraction, then a galaxy a long way behind ours would not be receding from ours very much. It would be even further, so the difference in the force on Earth and the force on that galaxy would be small. In order for a galaxy far in front of us to be receding from us, it would need to be much stronger force.

And a constant or near constant force would not produce the differences in recessional velocity that we see.

You could imagine a sequence of objects falling towards a star. The motion of each object would be slow-slow-slow-little bit faster - little bit faster ... very fast. The objects closer to the star are always falling faster away from you than the objects behind, at the same distance.

There's no force equation that would produce that pattern - acceleration doesn't work like that. But, spatial expansion does work like the data shows.

Finally, as you admit, you have only shown a linear model. There's no explanation for galaxies in the other two dimensions receding from you, let alone proportional to their distance from you.
 
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  • #10
PeterDonis
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What if the universe had a boundary which is attractive?
What would this mean? How can a boundary be attractive?

I admit this sounds very strange, but so does dark energy.
Dark energy is just a cosmological constant--no problem at all with the Einstein Field Equation. A "boundary which is attractive" doesn't even seem like a valid solution to the EFE at all.
 
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  • #11
PeterDonis
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Could our observable universe be accelerating towards another point of the hypersphere, and while galaxies appear to all "recede away" from us, we are actually all moving in the same direction?
No. If this were true, we would not see the apparent expansion as being isotropic. In such a model, where all the galaxies we can see are a big cloud of matter free-falling towards a very far distant supermassive black hole, we would see apparent expansion along only one direction, towards the hole, and apparent contraction along the other two. This is a simple consequence of the effects of tidal gravity on a free-falling cloud in the gravitational field of a massive object.

(Note, btw, that this is true whether or not you postulate the overall spatial geometry to be a hypersphere, i.e., closed, or flat or open.)
 
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  • #12
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What would this mean? How can a boundary be attractive?
I just thought that we cannot be sure about the actual topological form the universe has, and a boundary far out should at least be thinkable. Now if there is one, it might as well be equipped with additional properties simply because it is of a different dimension and thus of a different nature, and we do not know what or how such a boundary would be. I admit, that a) this is speculative, which is why I didn't want to defend it, b) that this might have been the idea behind the OP's question, at least as I read it, and c) the Shell theorem has answered it. Any argument with EFE already assumes that they can be extrapolated. But for sure, we only know for the inner of the universe.
 
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  • #13
PeterDonis
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I just thought that we cannot be sure about the actual topological form the universe has, and a boundary far out should at least be thinkable.
What kind of boundary?

If you are talking about a boundary between our universe and some other region of spacetime, yes, there could be.

If you are talking about a boundary of spacetime itself, there can't be one--at least, not without violating the Einstein Field Equation. Unless you are talking about something like the singularity inside a black hole, where the actual "boundary" is not part of the manifold, only the open region having the boundary as a limit is.
 
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  • #14
PeterDonis
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Any argument with EFE already assumes that they can be extrapolated.
If we are not allowed to extrapolate our current laws of physics, then we have no laws at all and we can't make any claims whatsoever. The only reasonable context in which to answer a question like the OP's is to use our best current laws of physics.
 
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If we are not allowed to extrapolate our current laws of physics, then we have no laws at all and we can't make any claims whatsoever. The only reasonable context in which to answer a question like the OP's is to use our best current laws of physics.
Sure, and the inner of a manifold is almost everything, so there is no contradiction here. But it does not allow conclusions about its boundary. I know that we do not assume the universe to have one, but can it be ruled out?
 
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  • #16
PeterDonis
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it does not allow conclusions about its boundary. I know that we do not assume the universe to have one, but can it be ruled out?
It can if the EFE is valid. A boundary to spacetime would violate the EFE--basically because the EFE requires every point in spacetime to have an open neighborhood (so that the conservation laws will work). A point on a boundary would not satisfy that requirement.
 
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  • #17
Andy_K
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One problem is that to get the same acceleration, you need to the same force. For example, if the Earth is far from the source of the attraction, then a galaxy a long way behind ours would not be receding from ours very much.
Thank you for taking the time to illustrate it in detail. I now have a better idea on how the expansion works (and doesn't work).


No. If this were true, we would not see the apparent expansion as being isotropic. In such a model, where all the galaxies we can see are a big cloud of matter free-falling towards a very far distant supermassive black hole, we would see apparent expansion along only one direction, towards the hole, and apparent contraction along the other two. This is a simple consequence of the effects of tidal gravity on a free-falling cloud in the gravitational field of a massive object.
Thanks Peter for your clarifications and analysis.

Appreciate everyone's guidance. Figuring out why something doesn't work is as interesting as knowing why something works!
 
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  • #18
The "other side" is much further away, but also much larger;
https://en.m.wikipedia.org/wiki/Shell_theorem
This guy is correct. If it was an attractive spherical shell, there would be no attractive force within it. Gauss's Law tells us that the electric force within a charged hollow sphere is equal to zero at every point within the sphere. The mathematical proof of this is difficult yet quite beautiful. We can extend this idea to mass and gravitation (since both gravitation and electric force are proportionate to 1/r^2) and imagine the earth as a hollow shell of a planet. If it is hollow with only the outer shell proving gravitation, then any point within the shell has a net gravitational pull of 0.
 
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  • #19
First of all, sorry for my naive question here which likely doesn't make sense.

The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively "expanding", but instead being gravitationally attracted towards a supermassive black hole (or perhaps, "Big Crunch") that is forming at a faraway point beyond our visible universe?

Could our observable universe be accelerating towards another point of the hypersphere, and while galaxies appear to all "recede away" from us, we are actually all moving in the same direction?

Instead of dark energy causing the expansion, could it be immense gravity from a distant source that is drawing our galaxies over?

Would we be able to distinguish between "expansion" and "attraction"?

Thank you for enlightening me.
Dark energy is weird and we really don't know anything about it. I like to think that the universe itself is expanding, and that expansion causes us to see galaxies moving away. Like baking raisin bread. Bread expands in the oven when baking. The bread is the space expanding and the raisins are the galaxies moving away. The raisins aren't actively moving within the bread, but they do move away from each other. Its the actual expansion itself causing the movement of raisins, rather than the movement of raisins causing the perception of expansion.

So rather than there being any particular force pushing or pulling the galaxies away, its a simple matter of space itself expanding. Each individual galaxy doesn't feel that it is moving or accelerating in any particular direction. It feels as if it is staying in place and sees every other galaxy as accelerating away from it due to the fact that the space itself between galaxies is expanding.
 
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I believe another issue with an idea such as this one is that it would require earth's position in the universe to be special. The most likely scenario is that there is nothing special about our location in the universe and any idea based on it being special is almost certainly false.

I was wondering if a portion of the Redshift we measure though could be accounted by actual velocities in addition to the universal expansion. The reason being that if you were to run a simulation of x neutral particles scattered in empty 3d space and assign them random velocities, then they would over time spread out in a similar pattern that we see when observing galaxies. from the perspective of any particle in the simulation almost every other particle will be moving away and the relationship between how fast they are moving and and how far away they currently are would be a linear relationship. But, if the process of the big bang did not cause galaxies to have arbitrary velocities relative to one another then everything would be mostly at rest relative to each other and the receding velocities would be almost entirely universal expansion. Do we know for a fact there wasn't an explosion piece as part as of the big bang?
 
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  • #21
PeterDonis
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I was wondering if a portion of the Redshift we measure though could be accounted by actual velocities in addition to the universal expansion.
For individual galaxies, this is true; some of the measured redshift is due to their peculiar velocity (which means their velocity relative to a comoving object at the same location), and the rest is due to universal expansion. When collecting data from many galaxies, this shows up as a spread in the distribution of redshifts for galaxies at a given distance from us.
 
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  • #22
kimbyd
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First of all, sorry for my naive question here which likely doesn't make sense.

The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively "expanding", but instead being gravitationally attracted towards a supermassive black hole (or perhaps, "Big Crunch") that is forming at a faraway point beyond our visible universe?

Could our observable universe be accelerating towards another point of the hypersphere, and while galaxies appear to all "recede away" from us, we are actually all moving in the same direction?

Instead of dark energy causing the expansion, could it be immense gravity from a distant source that is drawing our galaxies over?

Would we be able to distinguish between "expansion" and "attraction"?

Thank you for enlightening me.
This kind of model has actually been examined in detail. It is in principle possible to replicate an accelerated expansion if it just so happens that our galaxy is extremely close to the center of a large region that is less dense than the surrounding universe. Current evidence suggests that this model doesn't fit our universe:
https://arxiv.org/abs/1007.3725
 
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  • #23
PeterDonis
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It is in principle possible to replicate an accelerated expansion if it just so happens that our galaxy is extremely close to the center of a large region that is less dense than the surrounding universe.
I don't think this is the kind of model the OP is describing. The apparent accelerated expansion in this model is not due to all of the galaxies we can see being attracted to some object beyond our observable universe.
 
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  • #24
kimbyd
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I don't think this is the kind of model the OP is describing. The apparent accelerated expansion in this model is not due to all of the galaxies we can see being attracted to some object beyond our observable universe.
Perhaps, but it's the closest thing to a real theory that approximates the OP's question.
 
  • #25
PeterDonis
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it's the closest thing to a real theory that approximates the OP's question
I don't think so. The OP specified a closed universe with some massive object in a region beyond the part of the closed universe that is visible to us. The "real theory" that would approximate that would be a closed FRW model perturbed by a massive object in some particular spatial region.
 

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