Space Expanding v Things Receding in Space

[dogjones]

This thought was thanks to the "Space Expanding? Einstein Proof?" title, although the actual thread appears to be barking mad.

As far as I know, we can tell that something is moving away from us by measuring the red shift. However, from that how do we infer that space itself is expanding? Why can't the space be there already but the objects within it are sort of diffusing into it?

Put simply, how can we tell the difference between an object's recession due to the expansion of space, and its recession because it's just moving away from us within space? Is there in fact any difference here?

Now, assuming there is a difference. I have heard the analogy of an expanding balloon, which allows all objects on the surface to get further away from each other. If this analogy holds, than suppose a dot on the surface represents the earth. The expansion of the balloon stretches out this dot. So is the earth expanding very slightly to accommodate the expansion of the space it occupies? Am I expanding, despite my avoidance of cream cakes? Are the cream cakes expanding? Are individual atoms expanding?

Can anyone expand on this?

 

[leafman91]

This thought was thanks to the "Space Expanding? Einstein Proof?" title, although the actual thread appears to be barking mad.

As far as I know, we can tell that something is moving away from us by measuring the red shift. However, from that how do we infer that space itself is expanding? Why can't the space be there already but the objects within it are sort of diffusing into it?

Put simply, how can we tell the difference between an object's recession due to the expansion of space, and its recession because it's just moving away from us within space? Is there in fact any difference here?

Now, assuming there is a difference. I have heard the analogy of an expanding balloon, which allows all objects on the surface to get further away from each other. If this analogy holds, than suppose a dot on the surface represents the earth. The expansion of the balloon stretches out this dot. So is the earth expanding very slightly to accommodate the expansion of the space it occupies? Am I expanding, despite my avoidance of cream cakes? Are the cream cakes expanding? Are individual atoms expanding?

Can anyone expand on this?

I'll give it a shot. The balloon analogy only works up to a certain point, which you have crossed.It isn't the atoms that are expanding, it's the what isn't the atoms that's expanding. If that makes any sense at all. The planets, galaxies, are all getting shoved outwards, by the proliferation of...space. In reality, it's probably the proliferation of dark matter that is shoving everything places, but there isn't any proof for that. I wouldn't go as far to say that you are expanding, nor the cream cakes. Small things follow different physics to big things. Keep in mind that I have no idea what you refer to by 'cream cakes' I'm guessing you can pick them up at a bakery.

 

[dogjones]

I'll give it a shot. The balloon analogy only works up to a certain point, which you have crossed.It isn't the atoms that are expanding, it's the what isn't the atoms that's expanding. If that makes any sense at all. The planets, galaxies, are all getting shoved outwards, by the proliferation of...space. In reality, it's probably the proliferation of dark matter that is shoving everything places, but there isn't any proof for that. I wouldn't go as far to say that you are expanding, nor the cream cakes. Small things follow different physics to big things. Keep in mind that I have no idea what you refer to by 'cream cakes' I'm guessing you can pick them up at a bakery.

So if I get you right - the word 'space' is probably the wrong word to use when we refer to the 'expansion' of same? In which case, space isn't really expanding - it's just that the (large) objects within it are receding from each other within it, for whatever reason (possibly dark matter?)

ETA: Yes you can pick them up at a bakery

 

[sol invictus]

Put simply, how can we tell the difference between an object's recession due to the expansion of space, and its recession because it's just moving away from us within space?

It's not easy. But consider the following: assume space is homogeneous (i.e. there's nothing special about the location of the earth - we'd see more or less the same thing if we were somewhere else in the universe). But what we observe (slightly oversimplified) is that all astrophysical objects are moving directly away from the earth, with a speed more or less proportional to their distance. Since we've assumed space is homogeneous, we'd see that same thing if we were located somewhere else. Now, try to explain that by recession velocity alone... (hint - it's actually possible, but only in a special case that isn't supported by data).

Is there in fact any difference here?

Yes, as you'll see if you think about the above question.

Now, assuming there is a difference. I have heard the analogy of an expanding balloon, which allows all objects on the surface to get further away from each other. If this analogy holds, than suppose a dot on the surface represents the earth. The expansion of the balloon stretches out this dot.

A better analogy are pennies glued to the balloon. Bound objects, like the earth, solar system, and Milky Way, do not stretch as the space expands, because they are bound by forces stronger than the force due to the expansion - just as the penny is bound by forces much stronger than the force of the stretching rubber.

 

[dogjones]

It's not easy. But consider the following: assume space is homogeneous (i.e. there's nothing special about the location of the earth - we'd see more or less the same thing if we were somewhere else in the universe). But what we observe (slightly oversimplified) is that all astrophysical objects are moving directly away from the earth, with a speed more or less proportional to their distance. Since we've assumed space is homogeneous, we'd see that same thing if we were located somewhere else. Now, try to explain that by recession velocity alone... (hint - it's actually possible, but only in a special case that isn't supported by data).

The only alternative I can think of is the way I used to think of the big bang before reading stuff on this forum. Namely, imagining an infinite space with an insanely dense, but finite point of matter somewhere within it, at a kind of arbitrary 'centre'. The point explodes. From any point within that explosion you should be able to see recession of matter in all directions. But I guess that ignores homogeneousness?

How about if we have both infinite space and infinite matter, but the matter was compressed into an infinite singularity, with the larger infinity of space surrounding it. Again the point explodes, and again you should be able to see recession of matter in all directions from any point within said explosion - but AGAIN is this ignoring homogeneousness?

A better analogy are pennies glued to the balloon. Bound objects, like the earth, solar system, and Milky Way, do not stretch as the space expands, because they are bound by forces stronger than the force due to the expansion - just as the penny is bound by forces much stronger than the force of the stretching rubber.

OK. This may be "stretching" the analogy too far... but in this scenario, we have pennies glued to the balloon. The glue stops space expanding within the penny, but allows it to expand outside the penny. Does this put any "stress" on space? Like having a rumpled, unexpanded bit of space within a star, but expansion at the surface?

Would a better analogy would be pennies resting lightly on the surface of the balloon, allowing space to expand 'through' them, but at the same time still affecting the distance from each other?? Weird. It's like space is weakly interacting with the bound objects. I always thought of space in a sort of "second order" way, if you know what I mean. Like "existence" or something. It's necessary for an object but doesn't directly affect the behaviour of an object. If that makes sense.

 

[dogjones]

Also, how to tell the difference between recession of matter because of velocity and recession due to expansion. Am I right in thinking that non-expansion related movement would always have a gravitational cause of some kind, whereas 'unexplainable' movement would by default be expansion related?

 

[sol invictus]

Namely, imagining an infinite space with an insanely dense, but finite point of matter somewhere within it, at a kind of arbitrary 'centre'. The point explodes. From any point within that explosion you should be able to see recession of matter in all directions. But I guess that ignores homogeneousness?

Yes - that has one unique center. If the earth were at or near that center, we'd see something roughly like what we see. But if it were off-center, we wouldn't. So unless you think we're incredibly special, that theory is a bit hard to swallow.

And in fact one can even find observations that test that possibility, because not all the signals we can observe come along straight, radial lines.

How about if we have both infinite space and infinite matter, but the matter was compressed into an infinite singularity, with the larger infinity of space surrounding it. Again the point explodes, and again you should be able to see recession of matter in all directions from any point within said explosion - but AGAIN is this ignoring homogeneousness?

I'm not sure how this differs from the above.

OK. This may be "stretching" the analogy too far... but in this scenario, we have pennies glued to the balloon. The glue stops space expanding within the penny, but allows it to expand outside the penny. Does this put any "stress" on space?

In a sense, yes. Pretty much anything larger than a planet is gravitationally bound, and for those systems it's gravity which holds them together - their local gravity is stronger than the larger-scale gravitational forces that would otherwise make them expand.

Would a better analogy would be pennies resting lightly on the surface of the balloon, allowing space to expand 'through' them, but at the same time still affecting the distance from each other??

No, not really, because the space really doesn't expand inside a galaxy. But the space between them - or really between galaxy clusters - does, because the local mass density is too low to stop it.

The whole thing is a little dicey because space cannot be separated from time in a unique way. When you talk about space stretching as time passes, you've made such a separation. If the universe were perfectly smooth (no pennies, just balloon) there'd be a completely natural way to do that - and therefore a natural way to separate recession velocities from space expansion. But once you start throwing in galaxies, it's no longer so clear. Still, I think the simplest way to think about it is that the space only expands where the mass density is really low - like in the voids between galaxy clusters - and not within galaxies themselves.

 

[sol invictus]

Also, how to tell the difference between recession of matter because of velocity and recession due to expansion.

Well again, it's not easy, partly because in general relativity there isn't a truly well-defined distinction.

Let me give an example which is closely related mathematically to your question. Consider two lines which appear to be diverging from each other. You can only see part of each line, not including their intersection. You could measure the angle between them by extrapolating them (using a straightedge) along a flat plane to their point of intersection and then measuring the angle there - that's like accounting for the divergence in the lines by a recession velocity in flat space. Or you could account for the divergence by supposing the lines are on a curved space (think of two longitude lines on the earth, which intersect, diverge, and converge depending on where you are) - that's like explaining the divergence with expansion.

So if you know the lines are on a sphere, what's the "true" explanation for the fact that they are diverging? Is it the angle between them, or the curvature of the sphere, or where you are along the line? It's all the above, really.

 

[Pixie of key]

Atoms expanding

Atoms expanding, i mean atoms really exploding all a time.

So, 13 billion years ago, all energy inside stars was very density and time was very slowly. Thats why old light it is redshifting! Space dsont expanding at all!

www.onesimplerprinciple.com

.

 

[dogjones]

Yes - that has one unique center. If the earth were at or near that center, we'd see something roughly like what we see. But if it were off-center, we wouldn't. So unless you think we're incredibly special, that theory is a bit hard to swallow.

Ah. And if we were 'at or near that center' - this means that our galaxy cluster would be the center, not the earth itself, yes? Is extrapolating that our galaxy cluster is the center of the universe just as arrogant/unlikely as assuming the earth is the center of the galaxy? I guess probably so, huh?

Anyway, I give up. What was the possibility that assumes homogeneousness and also explains objects moving away from us with a speed roughly proportionate to distance by recession velocity alone but isn't supported by data?

And in fact one can even find observations that test that possibility, because not all the signals we can observe come along straight, radial lines.

Cool. Such as?

I'm not sure how this differs from the above.

Neither am I!

In a sense, yes. Pretty much anything larger than a planet is gravitationally bound, and for those systems it's gravity which holds them together - their local gravity is stronger than the larger-scale gravitational forces that would otherwise make them expand.

Wait, so space itself is affected by gravity?

No, not really, because the space really doesn't expand inside a galaxy. But the space between them - or really between galaxy clusters - does, because the local mass density is too low to stop it.

Again, space is affected by gravity?

The whole thing is a little dicey because space cannot be separated from time in a unique way.

Is this part of the reason that 'space' appears to be affected by gravity?

When you talk about space stretching as time passes, you've made such a separation. If the universe were perfectly smooth (no pennies, just balloon) there'd be a completely natural way to do that - and therefore a natural way to separate recession velocities from space expansion. But once you start throwing in galaxies, it's no longer so clear.
Still, I think the simplest way to think about it is that the space only expands where the mass density is really low - like in the voids between galaxy clusters - and not within galaxies themselves.

So is it the case in GR that:

"space stretching" = "time passing"?

In which case, how would a perfectly smooth universe facilitate a natural separation of this? (I really am out of my depth here). I mean, it seems to me that the fact that we have space not expanding within galaxy clusters, but expanding between them, implies that there is a separation. If the universe were smooth then there would be no difference, so no way to separate them?

 

[dogjones]

Well again, it's not easy, partly because in general relativity there isn't a truly well-defined distinction.

Let me give an example which is closely related mathematically to your question. Consider two lines which appear to be diverging from each other. You can only see part of each line, not including their intersection. You could measure the angle between them by extrapolating them (using a straightedge) along a flat plane to their point of intersection and then measuring the angle there - that's like accounting for the divergence in the lines by a recession velocity in flat space. Or you could account for the divergence by supposing the lines are on a curved space (think of two longitude lines on the earth, which intersect, diverge, and converge depending on where you are) - that's like explaining the divergence with expansion.

So if you know the lines are on a sphere, what's the "true" explanation for the fact that they are diverging? Is it the angle between them, or the curvature of the sphere, or where you are along the line? It's all the above, really.

I think I see what you mean. Is 'knowing the lines are on a sphere' akin to assuming homogeneousness?

 

[Rocko]

Well again, it's not easy, partly because in general relativity there isn't a truly well-defined distinction. <snip>

Not remotely related to the OP's question, but I just wanted to observe that you have a real gift for explaining very complicated concepts in a way that even I can grasp. Your contributions to these threads are always excellent, and I just wanted to say thanks.

 

[sol invictus]

Ah. And if we were 'at or near that center' - this means that our galaxy cluster would be the center, not the earth itself, yes? Is extrapolating that our galaxy cluster is the center of the universe just as arrogant/unlikely as assuming the earth is the center of the galaxy? I guess probably so, huh?

Yes, because there are lots and lots of galaxy clusters in the part of the universe we can observe directly. You could try to quantify how near we'd have to be to the center, but I'm not sure what the answer is.

What was the possibility that assumes homogeneousness and also explains objects moving away from us with a speed roughly proportionate to distance by recession velocity alone but isn't supported by data?

Infinite space full of matter, moving with a velocity that satisfies the simplest form of the Hubble law: v = H x, where v is velocity and x is a vector pointing from an origin. It turns out that if you're not at the origin but rather at some other x, and you're moving with that velocity, things will look just the same way (i.e. it will still look as though you're at the center). But that simple form of the Hubble law isn't the one we observe.

Cool. Such as?

Well, when structures form via gravitational collapse, the way they form depends on what's around them. Then later we can look at them, and perhaps they're spinning and we look after they've rotated around half way (so we see what was the far side). So if the stuff on what was originally the far side is different from the near side, and you look at enough structures, you'll see the difference. That's not exactly what's done, but if you think about it you'll see there are lots of possibilities like that.

Wait, so space itself is affected by gravity?

Yeah - space, or really spacetime curvature, IS gravity.

So is it the case in GR that:

"space stretching" = "time passing"?

Well, space can stretch as time passes, but they're not the same thing.

In which case, how would a perfectly smooth universe facilitate a natural separation of this?

The problem is that in relativity there's no uniqueness of simultaneity - observers in relative motion will disagree when when "now" is for distant events. In a smooth universe, you can use the density - which for some special set of observers (the ones at rest with respect to all that smooth stuff) is equal everywhere in space, but decreases with time - as a clock. Equal density means equal time.

I think I see what you mean. Is 'knowing the lines are on a sphere' akin to assuming homogeneousness?

Yeah - a sphere as opposed to a potato or something.

 

[Soapy Sam]

There is a tide in the affairs of universes.
Sometimes it ebbs, sometimes it flows.

This is my theory of universes. Book tour to follow.

 

[Dancing David]

I'll give it a shot. The balloon analogy only works up to a certain point, which you have crossed.It isn't the atoms that are expanding, it's the what isn't the atoms that's expanding. If that makes any sense at all. The planets, galaxies, are all getting shoved outwards, by the proliferation of...space. In reality, it's probably the proliferation of dark matter that is shoving everything places, but there isn't any proof for that. I wouldn't go as far to say that you are expanding, nor the cream cakes. Small things follow different physics to big things. Keep in mind that I have no idea what you refer to by 'cream cakes' I'm guessing you can pick them up at a bakery.

Um, the thing that is called 'dark energy' is hypothesized for the expanding driver.

Dark matter is like nutrinos, but different.

ETA: Thanks Sol!

It is nice to read about this without the high level distractions that occur in other threads.

 

[dogjones]

Many thanks Sol. You are a great resource. You should write science books for the layman, perhaps.