What happens to spacetime during the expansion of the Universe?

[MikeeMiracle]

TL;DR Summary

What happens to the fabric of spacetime during the expansion of the universe?

What happens to the fabric of spacetime during the expansion of the universe? Does it stretch or expand? If it does not stretch or expand, does new spacetime form to "fill the gap" as such?

Hypethotically speaking, I have two celestial objects separated by a gap 1 mile wide. Due to the expansion of the universe those objects are now separated by 2 miles. What happened to the fabric of spacetime between them? Did it stretch or expand or did new spacetime form to "fill the gap" as such? It might be better to think of this spacetime at the smallest possible level / quanta of spacetime. Did any of those quanta of spacetime expand or just move?

Before anyone says it, I know the above example of not a great analogy as of course those celestial objects mentioned are also "in" spacetime as opposed to "displacing spacetime" as a ball would displace water in a pool, I was just using that example to try and better explain the core question.

What are PF users view on this as I am yet to find a suitable answer to this question?

 

[PeroK]

Summary: What happens to the fabric of spacetime during the expansion of the universe?

What happens to the fabric of spacetime during the expansion of the universe? Does it stretch or expand? If it does not stretch or expand, does new spacetime form to "fill the gap" as such?

Hypethotically speaking, I have two celestial objects separated by a gap 1 mile wide. Due to the expansion of the universe those objects are now separated by 2 miles. What happened to the fabric of spacetime between them? Did it stretch or expand or did new spacetime form to "fill the gap" as such? It might be better to think of this spacetime at the smallest possible level / quanta of spacetime. Did any of those quanta of spacetime expand or just move?

Before anyone says it, I know the above example of not a great analogy as of course those celestial objects mentioned are also "in" spacetime as opposed to "displacing spacetime" as a ball would displace water in a pool, I was just using that example to try and better explain the core question.

What are PF users view on this as I am yet to find a suitable answer to this question?

There is no "fabric" of spacetime. That is a term used only in popular science.

When space expands the distance between points increases. There is nothing to stretch.

 

[MikeeMiracle]

I generally get answers that fall into 2 categories, the 1st category is like you say it's just empty space. The 2nd category is that we do not really know.

I am not comfortable with your suggestion, if you are correct, then how does "empty space" have a geometry that can be warped as per General Relativity? How does a gravitational wave propagate in empty space? Surely there must be such a thing as a "unit" of spacetime at the very smallest scales. What happens to this"unit" of spacetime according to our current theoretical understanding?

For example at the hypothetical "edge of the universe" which is expanding, assuming for a minute it is like a wave traveling in the ocean, as it travels is it just creating new "empty space" behind it? Your answer seems to suggest that it should do but this seems counter-intuative to me.

 

[PeroK]

I am not comfortable with your suggestion, if you are correct, then how does "empty space" have a geometry that can be warped as per General Relativity? How does a gravitational wave propagate in empty space? Surely there must be such a thing as a "unit" of spacetime at the very smallest scales. What happens to this"unit" of spacetime according to our current theoretical understanding?

For example at the hypothetical "edge of the universe" which is expanding, assuming for a minute it is like a wave traveling in the ocean, as it travels is it just creating new "empty space" behind it? Your answer seems to suggest that it should do but this seems counter-intuative to me.

This site deals with physics as an established academic subject and in this case the theory of GR (General Relativity). In GR spacetime is a 4D continuum and is not divided into discrete units. The geometry of spacetime is determined by the presence of stress-energy. There's no physical mechanism as such and no fabric being shaped.

Note that Electromagnetic radiation can propagate through empty space. And likewise gravitational waves.

There is no edge of the universe and the universe isn't "expanding" into anything. That's a common misconception. There's a useful Insight on that here:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
PS THis one as well.

https://www.physicsforums.com/insights/balloon-analogy-good-bad-ugly/

 

[Buzz Bloom]

Hypethotically speaking, I have two celestial objects separated by a gap 1 mile wide. Due to the expansion of the universe those objects are now separated by 2 miles.

Hi Mike:

The quote above is another misconception. The "expansion" of the distance between objects depends on the distance and the mass of the objects. The Hubble relationship acts like a repelling force which is larger as the distance is larger. It acts against the gravitational attraction between the objects. If the attraction is larger than the repelling, then the objects are gravitationally bound to each other and the expansion does not happen. Stuff within a galaxy does not move apart by the "expansion". Galaxies in a group do not move apart. I am not sure about groups of galaxies groups in a super-cluster. I did some calculations about this some years ago, and I forget some of the details. As I vaguely remember, I think that super-clusters of galaxies do move apart due to the expansion, but I am not sure I am remembering this correctly.

Regards,
Buzz

 

[MikeeMiracle]

Thank you for the replies so far. I feel I am creating confusion by using my over-simplistic examples. Buzz I fully comprehend that the objects in my example would be gravitationaly bound a lot stronger than any expanding effects. I am also totally comfortable with the idea that space expands from all points at the same time and why the "baloon analogy" is a bad analogy but a good starter for those how have problems "visualising" the expansion.

I admit that I am an "armchair" astrophysisist but I am also slightly autistic so I can visualise things the laymen has trouble with hence why I am asking the questions as I feel I can understand more than most but I lack the necesary "groundwork" and am totaly self taught. I love the PBS Spacetime youtube channel. The first link provided by Perok is interesting and although I can't work the equations out I believe I understand most of the principles.

I still feel that my question is valid though and I struggle to comprehend that there is no edge of the universe even though we can never see it. Alas, perhaps I am at the end of my imagination and that I can go no further without an actual astrpophysics education.

 

[Buzz Bloom]

I struggle to comprehend that there is no edge of the universe

Hi Mike:

I am aware that some people who try to visualize the unbounded universe have difficulty, but I have difficulty in understanding the reason for the difficulty. If the universe is finite, I get that the false image that comes to mind is the following: a spherical shape with all the stuff inside a spherical surface, and that the expansion is the enlarging of the sphere. This false idea makes the surface of the sphere a boundary. But for an infinite universe, I do not understand the difficulty. If the universe of stuff is infinite, and for very large distances it is everywhere approximately similar, where could a boundary be?

Regards,
Buzz

 

[PeroK]

I still feel that my question is valid though and I struggle to comprehend that there is no edge of the universe even though we can never see it. Alas, perhaps I am at the end of my imagination and that I can go no further without an actual astrpophysics education.

There is no substitute from learning the material from an authentic academic source. The issue with using your intuition is that no one knows whether their intuition aligns with the universe we inhabit. It's not a reliable guide to 20th Century physics.

 

[MikeeMiracle]

Buzz: If the universe is infinite then yes there is no boundary. The fact that we are expanding suggests there is a boundary somewhere beyond the observable universe. Unless it is only our observable universe is expanding which seems unlikely. Is there something wrong with this logic?

Perok: I agree mostly with your comment about intuition not being a reliable guide. Intuition though will likely guide us to the next major breakthrough in our understanding of the cosmos, be it from an astrophysisist or not that is my belief. Sometimes you need a "dreamer" who is not for want of a better word "conditioned" by our current understanding. For example two centuries ago everyone believed in the Aether but that turned out to be false. Faraday was ridiculed in his day as he didn't have the physics/maths background to translate his ideas into mathematical equations for all to understand either. Admitedly that was in the mid 19th century but I feel that it is still true today. I also have several ideas also but as I have read, this forum is for discussions of currently accepted knowledge as opposed to unproven speculation.

One day when I have more time I will maybe start an acedemic course in Astrophysics.

 

[PeroK]

Buzz: If the universe is infinite then yes there is no boundary. The fact that we are expanding suggests there is a boundary somewhere beyond the observable universe. Unless it is only our observable universe is expanding which seems unlikely. Is there something wrong with this logic?

You are limited by your knowledge of mathematics and physics. GR allows an infinite expanding universe and, in fact, the current data support that scenario.

Perok: I agree mostly with your comment about intuition not being a reliable guide. Intuition though will likely guide us to the next major breakthrough in our understanding of the cosmos, be it from an astrophysisist or not that is my belief. Sometimes you need a "dreamer" who is not for want of a better word "conditioned" by our current understanding. For example two centuries ago everyone believed in the Aether but that turned out to be false. Faraday was ridiculed in his day as he didn't have the physics/maths background to translate his ideas into mathematical equations for all to understand either. Admitedly that was in the mid 19th century but I feel that it is still true today. I also have several ideas also but as I have read, this forum is for discussions of currently accepted knowledge as opposed to unproven speculation.

One day when I have more time I will maybe start an acedemic course in Astrophysics.

Physics has progressed a long way since Faraday. Although Faraday, as far as I am aware, was a respected scientific figure of his day and a FRS:

https://en.wikipedia.org/wiki/Michael_Faraday

 

[weirdoguy]

Sometimes you need a "dreamer" who is not for want of a better word "conditioned" by our current understanding.

Can you name anyone who had no proper knowledge of advanced physics and made any significant breakthorugh in physics? Faraday definitely knew physics very well. The same goes with aether, it's not a good example of anything you want it to be an example of...

 

[Buzz Bloom]

​

If the universe is infinite then yes there is no boundary. The fact that we are expanding suggests there is a boundary somewhere beyond the observable universe. Unless it is only our observable universe is expanding which seems unlikely. Is there something wrong with this logic?

Hi Mike:

I m trying to understand how you are thinking about an expanding universe. From the above quote I am guessing you are assuming the universe is finite but larger than the observable universe.

Based on the value of
Ω_k = 0.005 +/- ~0.165

(see https://arxiv.org/pdf/1502.01589.pdf pg. 39 eq. 49),​

I calculated that there is an approximate 2/3 probability that the univese is infinite and a 1/3 probability that it is finite.

(See https://www.physicsforums.com/threads/is-infinity-possible.970502/ post #6.)​

Thus, based on this analysis there is a plausible possibility that the universe may well be finite but larger than the observable universe.
NOTE: There is also another way to calculate probabilities which can estimate (using a completely different approach) the probability that the universe in not only infinite but approximately flat as well (Euclidean except for local distortions due to local mass distributions). Flat means that Ω_k = 0 exactly. I am working on trying to do that calculation, but the math may be too difficult for me.

I am also guessing that you do not accept the standard cosmological assumption that all of the universe (finite or infinite) at a very large scale is statistically similar with respect to matter distribution, and that you also have conceptual difficulty with a finite universe without boundaries.

If I am correct in my guesses, I can try to make these issues clearer to you.

Regards,
Buzz

 

[Vanadium 50]

Sometimes you need a "dreamer" who is not for want of a better word "conditioned" by our current understanding.

Especially for surgery. I want my doctor to be a free thinker, unshackled by any of that pesky medical school!

 

[Vanadium 50]

Faraday was ridiculed in his day

First, I don't believe this is true. (Jane Davy notwithstanding) But even if it were, "They laughed at Faraday" can be countered with "They also laughed at the Three Stooges".

 

[MikeeMiracle]

Not sure how to do quotes but in response to Buzz's comments

I am also guessing that you do not accept the standard cosmological assumption that all of the universe (finite or infinite) at a very large scale is statistically similar with respect to matter distribution, and that you also have conceptual difficulty with a finite universe without boundaries.

If I am correct in my guesses, I can try to make these issues clearer to you.

I do actually agree with the matter distribution being generally even everywhere in the universe. As to the boundaries, I struggle with a "flat" (like a sheet of paper) universe having no boundaries. As far as I am aware the universe appears flat to us. This does not rule out a curvature so big as to appear flat to us at a local (observable) level but could in fact loop back in on itself, like say we are on the surface of an extremely large baloon or donut. If that were the case then yes it could "appear" to be flat and have no percievable boundaries to ourselves as "travellers" on this flat surface.

That would mean this baloon/donut is also growing into "something," while it is expanding. If we live in the surface of an expanding baloon then while we may never be able to leave the surface (our perceived 3 dimensional universe,) that does not mean the baloon itself cannot expand, even if it is expanding in another dimention.

Basically, you can't just expand, you have to expand into "something" is my point. Whether that "something" is percievable to ourselves is another matter, but it must be happening somewhere at some scale.

 

[weirdoguy]

Basically, you can't just expand, you have to expand into "something" is my point.

And that point is wrong as general relativity shows.

This does not rule out a curvature so big as to appear flat to us at a local (observable) level but could in fact loop back in on itself, like say we are on the surface of an extremely large baloon or donut.

Surface of a baloon or a donut does not have boundaries either. You really need to read a proper textbook, because your intuitions are flat out wrong.

 

[PeroK]

Basically, you can't just expand, you have to expand into "something" is my point. Whether that "something" is percievable to ourselves is another matter, but it must be happening somewhere at some scale.

This is simply not true, mathematically or physically.

 

[MikeeMiracle]

Well I guess we have reached the limit of my intuative understanding.

Thank you all very much for the responses :)

 

[Buzz Bloom]

That would mean this baloon/donut is also growing into "something," while it is expanding.

Hi Mike:

When you expand a balloon it's surface gets bigger in a dimension that is not part of the balloon's surface. That is the point you seem to be missing. If our universe is finite, and the finite curved 3D universe is expanding, it may also be conceptually imagined to be expanding in a 4th spatial dimension that is not part of the 3D universe. The statements by the other PF contributors are trying to explain that this imaginary 4th dimension is not needed for the math or the physics about the expanding universe. If you can grasp these points, then I think it is OK to use whatever mental aids work for you to understand the phenomena. However, when you learn to work with the math, you will likely not need that imaginary crutch any longer.

Regards,
Buzz

 

[MikeeMiracle]

Hi Buzz

Ok, question 1 my original question. I can accept that we nothing is being stretched, case closed.
Question 2 regarding the expansion of the universe expanding into something...I agree, the baloon could be expanding in a dimension that is not part of the baloons surface. As far as I can interprit from the responces so far, they refer to explanations of the baloon's surface. The physics and maths you mention are for working out stuff on the surface which may be finite and which is why the physics and maths work out, perhaps the expansion is not on the surface. I am curious as to where this expansion might be but I guess there is currently no method of answering that question given our available data / theoretical understanding.

There are two excellect video's in the links below that take us beyond the edge of our observable universe mathematically:




In the second video from around 12:30 for 2 minutes regarding a holographic universe. They take a compacted hyperbolic spacetime and split it into slices to represent slices of time. They stack the slices to create a cylindrical space, the surface of the cylinder is one spacetime, the slices another spacetime. Perhaps my answer is that it is this cylindrical space that is constantly expanding as time passes.

I guess ultimately I am stating that there must be something "more" to the universe out there outside the observable universe than just more "normal" universe which carries on indefinately.

 

[PeroK]

I guess ultimately I am stating that there must be something "more" to the universe out there outside the observable universe than just more "normal" universe which carries on indefinately.

This is the problem with trying to learn some very advanced physics through a few videos, without the mathematical background. The first video is good; but, it's dealing with some very advanced differential geometry. Ironically, you are citing it as evidence for a naive geometrical view that "the universe must have an edge and be expanding into something". The geometry and physics described in the video is a long way more advanced than that. And that's not what it's saying.

In fact, as far as I am able to follow the mathematics, both the Penrose diagrams and the hyperbolic compactification are only valid if the universe is infinite and "normal" outside the observable universe. I.e. If the universe is "different" in some way at some region outside the observable universe, then the mathematics you are quoting would NOT be valid.

In other words, everything in that video is dependent on the standard models from GR, which predict an infinite homogeneous universe. What they are doing is employing some advanced mathematical techiques to study those standard universes, including mapping the "infinite future" onto a surface.

What that video is not doing is proposing an alternative model for the universe.

 

[PeroK]

@MikeeMiracle here's a bit of maths for you. From what I remember this is very similar to the idea behind the Penrose diagrams.

Suppose you take the real numbers: the set of all numbers, denoted by $(-\infty, \infty)$. Now, that is an infinite set with no boundary.

But, suppose you use the good old $\tan$ function from trigonometry and map every real number to an angle between $-\pi/2$ and $\pi/2$.

Now, you have effectively "compactified" the real line by mapping the set of "coordinates" $(-\infty, \infty)$ to the set of coordinates $(-\pi/2, \pi/2)$.

And, of course, the set $(-\pi/2, \pi/2)$ has a boundary, which is the two-point set $\{-\pi/2, \pi/2 \}$. Note that a 1D set - the real line - has been given a 0D boundary - just two points.

The compactifications in the video is doing a similar thing, although much more sophisticated, by mapping an infinite (homogeneous) $3+1$ spacetime manifold in such a way as to define a $2+1$ spacetime manifold as its surface.