Does the Expansion of the Universe Affect Gravity?

[Chris Miller]

@Chris Miller You only need to change one thing in your visualization. The universe was denser in the past than it is now. Where you are picturing a tiny point, instead picture an infintie universe that is as dense as possible, more dense than we have models or theories to describe. Then picture that universe becoming less dense - this is the big bang / expansion etc. Its not very different from your picture, and to me at least, it makes a lot more sense than your picture. I was also carrying the picture you describe in my head for a long time - and replacing the point with a dense infinite-expanse universe was a big light-bulb moment for me - it resolved my confusion / wondering what the small point was expanding into if it was already everything.

It left me with the problem of needing to grapple with a universe that is infinite in extent somehow becoming larger, but for no good reason that I can articulate that doesn't bother me as much as the expanding point visualization did.

I don't think you need to be having any crisis in faith - just tweak your mental model a bit!

Thanks, Grinkle. Maybe "crisis" was too strong a word. More frustration, or maybe confusion, mixed with interest. For me, your tussling with the theory is of greater consolation (thanks again) than your resolution's model. While I understand (mathematically) how infinite sets may be contained by "larger" ones, I cannot at all picture an infinite physical universe of nigh infinite density, which would describe infinite mass/energy. It's much easier to picture the infinitesimally small, nigh infinitely dense expanding universe (of unknown context/origin) that's been taken away from me here in this thread. Q: Does this new, improved universe contain infinite mass? Infinite galaxies? Or are these finite within infinite space? Or, somehow, neither?

Also, could it just now be so large that our tiny observable segment only appears flat (i.e., is immeasurably curved)?

 

[jbriggs444]

"The singularity does not "exist"... It is something... a boundary... Open sets do not contain their own boundaries."

You see my confusion?

It does not exist as an entity within the model. It is a feature that we refer to when we talk about the model. Like the graph of $f(x) = \frac{1}{x^2}$ . There is a pole at x=0. There is no point on the graph where x=0. That point does not exist on the graph. Yet we can talk about a "pole at x=0" when talking about the graph.

 

[Chris Miller]

It does not exist as an entity within the model. It is a feature that we refer to when we talk about the model. Like the graph of $f(x) = \frac{1}{x^2}$ . There is a pole at x=0. There is no point on the graph where x=0. That point does not exist on the graph. Yet we can talk about a "pole at x=0" when talking about the graph.

Thanks, yes, I see. Where $y = \frac{1}{x^2}$ both the x-axis and positive y-axis are poles, in that two y-symmetric curves approach but never touch either. Translating this into a model/description/explanation of the physical, material universe though still evades me.

 

[jbriggs444]

Thanks, yes, I see. Where $y = \frac{1}{x^2}$ both the x-axis and positive y-axis are poles, in that two y-symmetric curves approach but never touch either. Translating this into a model/description/explanation of the physical, material universe though still evades me.

It is an example of something which does not "exist" within a model but which nonetheless has a name. We can say "look, a pole" even though there is no point on the graph that is a "pole".

We can take the FLRW model and say "look, a singularity" even though there is no point in the model that is a "singularity".

 

[plillies]

No. The quote you gave from me already explains why.

And when the light ray was emitted, 13.8 billion years ago, the point it was emitted from, then, was much closer than 13.8 billion light years to the point where the Earth would have been if it had existed then

Now why would the point it has been emitted from be much closer than 13.8 billion light years. Remember we are talking about the image of the surface of last scattering here. The light was emitted; that surface disappeared. The light traveled travelled for 13.8 billion years. So the point where it was emitted was 13.8 billion light years away.

 

[Bandersnatch]

Now why would the point it has been emitted from be much closer than 13.8 billion light years. Remember we are talking about the image of the surface of last scattering here. The light was emitted; that surface disappeared. The light traveled travelled for 13.8 billion years. So the point where it was emitted was 13.8 billion light years away.

Visualise expanding space as a band of rubber that is being stretched. On this rubber band, an ant is walking from point A to point B. The ant represents a light signal sent from the emitter at point A to the observer at point B.
Let's say the initial distance is 100 cm, the ant walks at the constant speed of 1 cm/s, and the rubber band stretches by 1% every 1 second (we're assuming the rate is constant, for simplicity).
So, after 1 second, the ant will have traveled 1 cm of the 100 cm, but the expansion of space will have stretched the remaining 99 cm by 1% to 99.99 cm. During the same 1 second, the original distance from A to B will also have increased by 1% to 101 cm.
After another second, the ant goes another 1 cm onwards, but the expansion pushes it back by 1% of the distance to 99.9799 cm. Point A will have receded to 102.01 cm.

Can you see how:
1 - by the time the ant arrives at B, more than 100 seconds will have elapsed?
2 - by the time the ant arrives at B, point A will be much farther than 100 cm?
3 - the time the ant has traveled times its speed is neither the original distance between A and B, nor the final one?

 

[plillies]

Visualise expanding space as a band of rubber that is being stretched. On this rubber band, an ant is walking from point A to point B. The ant represents a light signal sent from the emitter at point A to the observer at point B.
Let's say the initial distance is 100 cm, the ant walks at the constant speed of 1 cm/s, and the rubber band stretches by 1% every 1 second (we're assuming the rate is constant, for simplicity).
So, after 1 second, the ant will have traveled 1 cm of the 100 cm, but the expansion of space will have stretched the remaining 99 cm by 1% to 99.99 cm. During the same 1 second, the original distance from A to B will also have increased by 1% to 101 cm.
After another second, the ant goes another 1 cm onwards, but the expansion pushes it back by 1% of the distance to 99.9799 cm. Point A will have receded to 102.01 cm.

Can you see how:
1 - by the time the ant arrives at B, more than 100 seconds will have elapsed?
2 - by the time the ant arrives at B, point A will be much farther than 100 cm?
3 - the time the ant has traveled times its speed is neither the original distance between A and B, nor the final one?

You seem to be suggesting that a photon (the ant) would be affected by the movement of space? My understanding is that objectifying space is a very Newtonian way of thinking. According to relativity, light always travel at the same speed except in the presence of a strong gravitational field, no? And then the speed only changes for an outside observer. To my way of thinking the CMB was at a temperature of 2.7 K 13.8 billion years ago, and that is why we perceive it at that temperature when the light reaches us today. Why not?

 

[Drakkith]

You seem to be suggesting that a photon (the ant) would be affected by the movement of space? My understanding is that objectifying space is a very Newtonian way of thinking. According to relativity, light always travel at the same speed except in the presence of a strong gravitational field, no? And then the speed only changes for an outside observer.

This has nothing to do with the speed of light changing, it has to do with the fact that the two observers (the sender and receiver) are moving apart. Even in SR a light signal would take far longer to reach an observer who is traveling at high velocity away from the sender than one who is not (time and velocity measured relative to the sender). Expansion adds another complication. Not only are the two observers moving apart, but the rate at which they move apart, the recession velocity, is increasing over time.

To my way of thinking the CMB was at a temperature of 2.7 K 13.8 billion years ago, and that is why we perceive it at that temperature when the light reaches us today. Why not?

Because the CMB only makes sense in the context of the big bang theory, which incorporates expanding space. There are simply no plausible mechanisms by which the CMB could have been emitted at 2.7K.

 

[stoomart]

You seem to be suggesting that a photon (the ant) would be affected by the movement of space?

The light wave (ant or CMB) doesn't change speed during its trip, but it's path is affected by the changing geometry of spacetime, both from expansion and gravitational lensing.

 

[plillies]

Because the CMB only makes sense in the context of the big bang theory, which incorporates expanding space. There are simply no plausible mechanisms by which the CMB could have been emitted at 2.7K.

Well, we wouldn't want to deny the big bang theory. I am just having trouble understanding how radiation from the CMB could be at 2.7 K when it reaches us unless it was at that temperature to begin with, allowing, of course, for any relativistic doppler effect that would have added to the apparent cooling. As to how the CMB could have cooled down to 2.7 K before the radiation was emitted, I guess it would have taken some time, enough time for the baryons in the CMB to displace themselves. If temperature is proportional to area, then they would have had to displace themselves quite a bit. If recombination is at 3000 k, then we can calculate a linear displacement of SQRT(3000/2.7) = 33.3.

The light wave (ant or CMB) doesn't change speed during its trip, but it's path is affected by the changing geometry of spacetime, both from expansion and gravitational lensing.

Not sure about the changing geometry of spacetime. I thought that only gravity could change spacetime geometry and in intergalactic space, gravity is pretty weak.

 

[rootone]

... relativistic doppler effect ...

That's how I understand it

 

[stoomart]

Not sure about the changing geometry of spacetime. I thought that only gravity could change spacetime geometry and in intergalactic space, gravity is pretty weak.

Expansion is continually altering the geometry of spacetime, whereas gravity is more of a rearrangement.

Edit: Think of expansion like urban sprawl, only instead of adding more lots, the existing lots get bigger while the homes stay the same size.

 

[plillies]

That's how I understand it

Trouble is if we assume that the apparent cooling (from 3000 K to 2.7 K) is due to relativistic doppler effect alone, then we need to attribute a recession velocity close to the speed of light to the surface last scattering that is presenting itself to us.So if that surface is both now 13.8 billion lt-yrs away and was traveling near the speed of light when the radiation was emitted, then we have a universe that is about twice as old as the generally accepted age, or alternatively, that the radiation from the CMB is arriving at us from half as far (6.9 billion lt-yrs instead of 13.8).

 

[stoomart]

Trouble is if we assume that the apparent cooling (from 3000 K to 2.7 K) is due to relativistic doppler effect alone, then we need to attribute a recession velocity close to the speed of light to the surface last scattering that is presenting itself to us.So if that surface is both now 13.8 billion lt-yrs away and was traveling near the speed of light when the radiation was emitted, then we have a universe that is about twice as old as the generally accepted age, or alternatively, that the radiation from the CMB is arriving at us from half as far (6.9 billion lt-yrs instead of 13.8).

There's a ton of science that goes into analyzing the CMB and determining the age of the observable universe, I suggest starting with the insights written by @bapowell.

https://www.physicsforums.com/insights/poor-mans-cmb-primer-part-0-orientation/

 

[PeterDonis]

Expansion is continually altering the geometry of spacetime, whereas gravity is more of a rearrangement.

This is not correct. First, the geometry of spacetime isn't being "altered" by expansion; "expansion" just means the 4-dimensional geometry of spacetime, which doesn't "change", it just is, has a certain shape.

Second, gravity doesn't "rearrange" the geometry of spacetime; as above, that geometry just is, it doesn't change. "Gravity" is just one way of describing certain effects of the geometry of spacetime.

 

[PeterDonis]

I am just having trouble understanding how radiation from the CMB could be at 2.7 K when it reaches us unless it was at that temperature to begin with

Because of the geometry of spacetime.

allowing, of course, for any relativistic doppler effect that would have added to the apparent cooling

Relativistic doppler is not a good way of thinking about the effect of curved spacetime geometry on light rays. Relativistic doppler is really only a workable model for the case of flat spacetime and a light source and receiver that are in relative motion.

As to how the CMB could have cooled down to 2.7 K before the radiation was emitted

It didn't.

Also, please review the PF rules on personal speculation, which is what the rest of your post after the above quote is.

 

[PeterDonis]

Trouble is if we assume that the apparent cooling (from 3000 K to 2.7 K) is due to relativistic doppler effect alone, then we need to attribute a recession velocity close to the speed of light to the surface last scattering that is presenting itself to us.So if that surface is both now 13.8 billion lt-yrs away and was traveling near the speed of light when the radiation was emitted, then we have a universe that is about twice as old as the generally accepted age, or alternatively, that the radiation from the CMB is arriving at us from half as far (6.9 billion lt-yrs instead of 13.8).

None of this is correct. You need to take the time to learn the correct model from a textbook; a detailed explanation of the model is well beyond the scope of a "B" level thread.

 

[PeterDonis]

This thread is degenerating into speculation and is now closed.