Expansion of the universe, a poor choice of words?

[windy miller]

I think cosmologists use a poor choice of words when they say the universe is expanding. I don't dispute any of the science behind the "expanding" universe but I do want to dispute the appropriateness of the phrase to the science.
I think people typically understand expanding to mean: getting bigger.
But the size of the universe is unknown. It may be finite or infinite and if its the latter the "expansion" of space does not mean the universe is getting bigger. It may have been infinite in spatial extent even at the big bang.
Many people cannot understand how this could be so, because they think space is expanding in the sense of "getting bigger" hence it could not have been infinite in size at the big bang. Therefore they rule out the infinite universe on the grounds of the poor understanding that was given to them by a bad phrase.
I think the public are literally being misled by the cosmological community. i don't think this is done intentionally by the community. Cosmologists known that what they mean by expanding is the scale factor is increasing over time. But what the public understand by it is not the same.

Cosmologists are familiar with bad phrases being forced upon them by history. For example big bang implies a conventional explosion in space, but I have often heard cosmologists frequently saying big bang is a bad phrase and trying to correct the misunderstanding. I don't think I have ever heard cosmologists trying to say that the universe is not expanding, in the sense of "getting bigger" which is incorrect rather than the scale factor is increasing which is correct.
Consider three phrases to describe the situation in layman terms:
1 the universe is expanding
2 the observable universe is expanding
3 the universe is stretching
To me , 1 is what is used all the time and it is wrong or perhaps not wrong but misleading (after all the universe could be finite we just don't know. But 2 and 3 could easily be used instead without referring to technical terms such as the scale factor and would help understanding.
Interested to see what people think.

 

[Orodruin]

I do not agree, "expansion" is a perfectly fine term in the sense that the distance between two comoving objects is getting bigger with time. Your proposition 2 is strongly misleading as it would be true also in a static universe which had existed for a finite time (and even in a contracting universe as long as it does not contract too fast). I think "stretching" is just as prone to misunderstandings as "expanding" is.

 

[windy miller]

The problem is cosmologists don't say the distance between two coming objects is getting bigger (when they communicate to the public) they say the universe is getting bigger and that implies it could not have been infinite in the past , a statement that is not justified. Also ,are not the Milky way and Andromeda co moving objects? The distance between them is not increasing, it is decreasing. So it is only distant co moving objects where the distant is increasing over time. I will have a think about your statement about stretching perhaps you are right. But why not then just use statement 3 and avoid statement 1?

 

[Orodruin]

The problem is cosmologists don't say the distance between two coming objects is getting bigger (when they communicate to the public) they say the universe is getting bigger and that implies it could not have been infinite in the past , a statement that is not justified.

I think it is perfectly justified, an infinite universe can grow. The standard example of this would be Hilbert's hotel. Your conclusion is therefore wrong.

Also ,are not the Milky way and Andromeda co moving objects?

No, as evidenced by the motion of the Milky way relative to the CMB rest frame.

 

[windy miller]

As I understand Hilberts hotel accommodates more guest by moving the guests not by growing in size. On the second point I stand corrected.

 

[Orodruin]

As I understand Hilberts hotel accommodates more guest by moving the guests not by growing in size.

The same principle applies. There is nothing strange if you are careful with what "size" means. In cosmology, it is the size of a comoving volume which is expanding.

 

[windy miller]

I agree but when cosmologists say to the public the universe is expanding that carefulness of "size" is not communicated with it. It gives the definite impression the universe has a definite finite size that grows with time. This is of course compounded by animations on tv that shows the big bang as a little ball that gets bigger. One can blame popular tv but official sources like WMAP and pPlanck images also show a finite little ball getting bigger . I don't blame these visual representation people need visual representations and there is no easy way to draw an infinitely big universe that has a scale factor increasing (iM not saying the universe was infinitely big but it might have been). Cosmologists need to appreciate these images are out there and will always be out there, and hence that's more reason for a better phrase. The reality is the universe's spatial extent is unknown to cosmologists; it may be finite or infinite. Hence by changing the phrase universe is expanding to observable universe is expanding we aid understanding, whereas simply saying the universe is expanding I maintain gives a false impression.

 

[chasmanian]

I am new here.
hi, I'm Charlie.
what I don't get is this.
logically, anyone is going to think this:
the Universe is expanding.
ok, so far.
13.7 billion years ago, there was the Big Bang.
all of the matter and energy in the observable Universe, was compressed into a small area that was pretty darn dense and hot.
ok, gotcha.
now then, if all of that is true, then, let's rewind from right now present day, and watch the observable Universe contract (the opposite of expand).
how is there somehow, not a center?
I just don't get this.
can anyone please tell me what I am missing?

 

[windy miller]

I think the correct answer is that all of the matter was not compressed into a small area. Rather the density was super high. This is kind of what I ma getting at. when cosmologists say the universe is expanding it creates that impression.

 

[chasmanian]

thank you so much for your reply.
the thought that immediately comes to mind, though is:
if the density was high, then doesn't that mean that all the matter and energy was in a small area?
I must be missing some critical concept.
if you rewind a movie of an expanding Universe back to the Big Bang (or 13 billion years, let's say), how is there possibly not a center?

 

[PeterDonis]

if you rewind a movie of an expanding Universe back to the Big Bang (or 13 billion years, let's say), how is there possibly not a center?

Because rewinding the movie doesn't do what you are imagining. You are imagining the universe as a spherical region of space, and you are imagining that the spherical region gets smaller as we go back in time. Since that spherical region of space has a center, you think the universe must have a center.

But in fact that is not what our cosmological models say. What our best current cosmological models say is that our universe is an infinite spatial expanse. An infinite expanse obviously has no center. But even if we adopt an alternate model (which is highly unlikely on our current data but not absolutely ruled out) in which the universe is spatially finite (but very large), it still has no center. That is because, in this model, the universe is spatially the "hypersurface" of a 3-sphere. Such a hypersurface has no center, just as the surface of an ordinary 2-sphere has no center. (The interior of an ordinary 2-sphere has a center, but its surface does not; and the spatial geometry of the universe in the model I'm talking about is analogous to the surface only.)

This idea is not easy to grasp because we have difficulty imagining how a 3-dimensional space could have the geometry of a 3-sphere. The best way I know of to imagine it is to imagine setting out in your rocket from Earth and heading out in the same direction indefinitely. In the 3-sphere geometry, you will eventually return to Earth, even though you never change direction at all. (This is analogous to circumnavigating the surface of a 2-sphere like the Earth.) Such a geometry has no center.

 

[PeterDonis]

I think the correct answer is that all of the matter was not compressed into a small area. Rather the density was super high.

All of the matter in our observable universe was compressed into a very small volume (not area). And its density was very high. But in our best current model, the universe as a whole is spatially infinite, so yes, all of the matter in the entire universe in this model did not occupy a very small volume.

(In the alternate model I mentioned in my previous post, where the universe is spatially a 3-sphere, the entire universe did occupy a much smaller volume in the far past--though still much larger than the volume occupied at that time by the matter in our observable universe.)

 

[chasmanian]

thank you so much for your excellent reply.
will read carefully and reply back.

 

[QuantumQuest]

if you rewind a movie of an expanding Universe back to the Big Bang (or 13 billion years, let's say), how is there possibly not a center?

I think that this misconception stems from the fact that you imagine rewinding the expanding universe as you observe it outside this. But the case according to the balloon analogy - or any relevant analogy for that matter, is that we are on the surface of this, while there is no "inside'' and no "outside". This is expanding to all directions, so there can't be a center.

 

[phinds]

I think cosmologists use a poor choice of words ...

We have such discussion here from time to time about how one term/phrase or another is not a good choice. At the end of the day, all such discussions are totally pointless since no one is going to change the accepted terminology no matter how cogent an argument (not that you have presented such) can be made against the accepted term.

 

[Gopher77]

That's interesting, you're right we have no idea how big the universe is. We could just be in a bubble in the universe that appears to be expanding while the rest of the universe is collapsing. Besides if the universe is expanding as most people would conceive, what is it expanding into?

 

[chasmanian]

thank you to everyone who replied.

Peter, thank you for both of your replies.
I have carefully read them both.
they are very good.
they are a great help for me, in this question that I have had for years.
I follow what you are saying.
one obvious logical thought that springs to my mind is:
if the density of all the matter in the observable universe was very high 13 billion years ago, then that means that all the matter in the early observable universe was very close together, and that implies to me that there was a center.
but, I think that your answer to me would be to learn about the hypersurface of a 3 sphere.
I just read a little.
it appears to be far beyond my grasp.
really unintelligible.
I get so far, and then, forget it.
it is a geometry thing, about which I am thinking it would take me years of study to grasp.
how very non intuitive.
I am so into learning about Relativity (Special and General).
I am used to having to re frame things, to go from Newtonian gravitational force to Einsteinian General Relativity geometrical description of gravity.
but the hypersurface of a 3 sphere...well, I'm lost.
thank you very much again for your excellent replies.
thank you all of you.

 

[phinds]

That's interesting, you're right we have no idea how big the universe is. We could just be in a bubble in the universe that appears to be expanding while the rest of the universe is collapsing. Besides if the universe is expanding as most people would conceive, what is it expanding into?

You would be well advised to read some basic cosmology.

 

[guywithdoubts]

thank you to everyone who replied.

Peter, thank you for both of your replies.
I have carefully read them both.
they are very good.
they are a great help for me, in this question that I have had for years.
I follow what you are saying.
one obvious logical thought that springs to my mind is:
if the density of all the matter in the observable universe was very high 13 billion years ago, then that means that all the matter in the early observable universe was very close together, and that implies to me that there was a center.
but, I think that your answer to me would be to learn about the hypersurface of a 3 sphere.
I just read a little.
it appears to be far beyond my grasp.
really unintelligible.
I get so far, and then, forget it.
it is a geometry thing, about which I am thinking it would take me years of study to grasp.
how very non intuitive.
I am so into learning about Relativity (Special and General).
I am used to having to re frame things, to go from Newtonian gravitational force to Einsteinian General Relativity geometrical description of gravity.
but the hypersurface of a 3 sphere...well, I'm lost.
thank you very much again for your excellent replies.
thank you all of you.

There is a center of the observable universe and that's wherever you are. Everything would get closer and closer to you if you rewound. You'd see the same thing if you were anywhere else. Then that the universe (not the observable universe) is finite or infinite is a different question.

Now to everybody else, did I get this right already?

 

[PeterDonis]

I think that your answer to me would be to learn about the hypersurface of a 3 sphere.

Yes. It might take some time for you to understand, but once you understand it, you will see that it falsifies your intuition that there must be a center because things were closer together in the past.

 

[PeterDonis]

There is a center of the observable universe and that's wherever you are.

There is a sense in which this is true, but this sense of "center" is not the one I think chasmanian is asking about. This "center" is different for each observer. There is no "center" that is a center for all observers; that is, there is no "center" that is picked out by the physics as having different properties from other points in space.

 

[chasmanian]

wow Peter.
awesome replies.
thank you very much.
feeling very very challenged.
but also very heartened.
fascinating how you have to find the right questions to ask, before you can get correct answers.

 

[Flatland]

thank you so much for your reply.
the thought that immediately comes to mind, though is:
if the density was high, then doesn't that mean that all the matter and energy was in a small area?
I must be missing some critical concept.
if you rewind a movie of an expanding Universe back to the Big Bang (or 13 billion years, let's say), how is there possibly not a center?

If the Universe is spatially infinite then rewinding it back 13.7 billion years it would still be spatially infinite and thus no center. If the Universe is finite and unbounded it means that if you travel in any direction far enough you will eventually end back where you started. Now think about what would happen if you rewind the Universe under that scenario.

 

[chasmanian]

thank you for your post.
well, I guess, it would mean you would have to travel a much shorter distance before you "end back where you started".

 

[phinds]

well, I guess, it would mean you would have to travel a much shorter distance before you "end back where you started".

If you mean that a smaller finite universe would have that result then yes it would. Just as an aside, it's a good idea to quote the post that you are responding to so that there is not ambiguity about what you are responding to.

 

[Flatland]

thank you for your post.
well, I guess, it would mean you would have to travel a much shorter distance before you "end back where you started".

Precisely. If you keep rewinding this Universe the distance before you "end back where you started" approaches zero.

 

[phinds]

Precisely. If you keep rewinding this Universe the distance before you "end back where you started" approaches zero.

Exactly, and I always find it interesting to note that the opposite is not true. That is, if a finite universe continues to expand, the amount of time it would take you to get back to your starting point does not approach infinity, it simply becomes impossible at a point in time (long since passed in the universe we actually live in) where the speed of light limit on travel means you could never get back to your starting point.

 

[bahamagreen]

The finite universe seems to result in some peculiar geometry.

Assign a point to represent your location. Now project a ray (a geometric ray, not a light ray) in any direction from there, indefinitely.

I'm going to be careful and not ask what you would observe along that line (because of relativity) but ask what you would expect to be there. At a far enough distance you would expect to find your starting point (still not worrying about rigid rods or light rays - we are not travelling, just imagining the geometric situation).
So your point maps to all points on the surface of an encircling sphere (again, not that you could observe it, but imagine a sufficient resolution and magnification for your magical telescope that would allow you to ultimately see the back of your head, so to speak, and see the back of your head in every direction you looked - if you could).

That geometric ray from your point to the surface of the sphere comprises one lap (out and in) of pathway around the universe, but if you extend the ray it will pass through an infinite number of spherical concentric projections of the same original point.
Now we have a peculiar topology that has these characteristics:

- every arbitrary point maps to each point (actually IS THE SAME POINT) on the surface of an encircling sphere
- half the radial distance corresponds to the "size" of the universe (maximal distance)
- for every point there is an infinite series of concentric spheres each a multiple of the radius of the first, to which the point maps to every point on the surfaces of all of the spheres (your original point is actually THE SAME THING as the surfaces of the encircling spheres)

This all before even considering time (expansion). It looks like the topology for even a finite universe requires an infinite room for the geometry.

 

[PeterDonis]

The finite universe seems to result in some peculiar geometry.

The spatial geometry is a homogeneous 3-sphere with a 3-volume that increases with time. There's nothing peculiar about it; it works just like a 2-sphere with one additional dimension.

Now project a ray (a geometric ray, not a light ray) in any direction from there, indefinitely.

This is unclear. There is no such thing as a "geometric ray" without qualification. You have to specify what kind of ray (timelike, spacelike, or null--a light ray would be a null ray), and if the ray is timelike or spacelike, you have to give something that picks out one from the infinite number of possible rays that point in the same direction.

At a far enough distance you would expect to find your starting point

This seems to indicate that by "ray" you mean the 3-sphere equivalent of a great circle on a 2-sphere. Is that what you're thinking of? If so, this would be a spacelike geodesic lying in a surface of constant FRW coordinate time.

So your point maps to all points on the surface of an encircling sphere

I'm not sure what you mean by "maps to", but it doesn't seem like any standard mathematical concept.

Now we have a peculiar topology that has these characteristics:

This topology is not at all peculiar. It is the perfectly standard topology of a 3-sphere, ##S^3##. You might want to look up the properties of this topological space.

every arbitrary point maps to each point (actually IS THE SAME POINT)

This is incorrect; the topology of a 3-sphere does not work this way. (Nor does any topology, for that matter; there is no concept of "maps to" in topology that works like this.)

half the radial distance corresponds to the "size" of the universe (maximal distance)

This isn't a topological property. Topology has no metric or concept of "distance".

 

[bahamagreen]

I'm not trained in the jargon so my words probably did hurt your ears. I looked up 3-sphere and think I recognized the idea I was referring to... in topological construction - gluing:

"A 3-sphere can be constructed topologically by "gluing" together the boundaries of a pair of 3-balls. The boundary of a 3-ball is a 2-sphere, and these two 2-spheres are to be identified. That is, imagine a pair of 3-balls of the same size, then superpose them so that their 2-spherical boundaries match, and let matching pairs of points on the pair of 2-spheres be identically equivalent to each other. In analogy with the case of the 2-sphere (see below), the gluing surface is called an equatorial sphere.
Note that the interiors of the 3-balls are not glued to each other."

 

[PeterDonis]

I looked up 3-sphere and think I recognized the idea I was referring to... in topological construction - gluing:

Yes, I see, you were talking about how the 3-sphere can be defined using a quotient space (but see below for limitations of that). This is mathematically true, but the fact plays no role at all in the physical models used in cosmology.

Also, note that the construction you refer to does not make the entire 3-sphere a quotient space; as the quote you gave shows, only the "equatorial sphere" is a "gluing surface" (i.e., a quotient space). This is similar to the way that a 2-sphere can be constructed by "gluing" the boundaries of two 2-disks together; the "gluing curve" is a circle which corresponds to the "equator" of the 2-sphere. But the rest of the 2-sphere is not a quotient space; only the "equator" is.

 

[Flatland]

Exactly, and I always find it interesting to note that the opposite is not true. That is, if a finite universe continues to expand, the amount of time it would take you to get back to your starting point does not approach infinity, it simply becomes impossible at a point in time (long since passed in the universe we actually live in) where the speed of light limit on travel means you could never get back to your starting point.

This actually brings up another interesting question. Suppose the Universe stopped expanding or even started contracting and it became possible to circumnavigate the Universe. What would happen if you did this at relativistic speeds? Suppose you set off at very close to the speed of light and arrived back at your starting point on Earth. What kind of time dilation (if any) would be experienced since no symmetry was broken?

 

[phinds]

This actually brings up another interesting question. Suppose the Universe stopped expanding or even started contracting and it became possible to circumnavigate the Universe. What would happen if you did this at relativistic speeds? Suppose you set off at very close to the speed of light and arrived back at your starting point on Earth. What kind of time dilation (if any) would be experienced since no symmetry was broken?

You would have aged considerably less than the objects at your starting point, when you returned, due to differential aging.

 

[Flatland]

You would have aged considerably less than the objects at your starting point, when you returned, due to differential aging.

Yeah but you didn't turn your spaceship around. Can't you just say that you were stationary and that it was Earth that circumnavigated the Universe? It's unlike the twin paradox where one party had to turn their spaceship around.

Or does the kinetic energy of your spaceship blasting off come into play here?

 

[phinds]

Yeah but you didn't turn your spaceship around. Can't you just say that you were stationary and that it was Earth that circumnavigated the Universe? It's unlike the twin paradox where one party had to turn their spaceship around.

How did you leave Earth if you never accelerated?