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Why does it seem to us that the universe is expanding where there is no gravity


SergUpstart
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Hawking wrote in his book "A Brief History of Time" (or in "M-Theory" ) that it is a big mystery that the universe expands where there is no gravity. That the expansion of the universe cannot be explained by a simple change in the scale of distances. After all, if all the sizes and distances, from the size of atoms, the radius of the Earth, the distance from the Earth to the Sun, the diameter of the galaxy and further to the distances to distant galaxies, would change in the same number of times, then we would simply not notice it.

This is indeed the case. Thus, the distance standard changes more strongly in intergalactic space than in galaxies where there is gravity.

And it's easy to explain it. The distance standard decreases in proportion to the gravitational potential. Let's assume that in intergalactic space the gravitational potential is equal to Phio , and we are inside the galaxy, where the total gravitational potential is equal to Phio+Phig, where Phig is the gravitational potential of the galaxy .
Now let's assume that after some time the gravitational potential of the Universe between the galaxies will decrease and become Phi1.
Then the total gravitational potential inside the galaxy will become Phi1+Phig.
It is easy to see that the gravitational potential between galaxies will decrease by a large relative amount, because

f1f0g.jpg.97aff855abea8ab4dd42d41ab737ad08.jpg

For example, if Phio is taken as 100%, Phig as 2%, and Pfi1 as 98%,
then the gravitational potential inside the galaxy will become 0.98039% of the initial one, and between galaxies it will become 98%.
The distance scales will also change accordingly, but we will not notice that our distance scale has become 0.98039% of the previous one, we will only notice that the distances between the
galaxies have increased by 0.00039% .
That's why it seems to us that the universe is expanding where there is no gravity.

 

 

Edited by SergUpstart
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2 hours ago, SergUpstart said:

Hawking wrote in his book "A Brief History of Time" (or in "M-Theory" ) that it is a big mystery that the universe expands where there is no gravity. That the expansion of the universe cannot be explained by a simple change in the scale of distances. After all, if all the sizes and distances, from the size of atoms, the radius of the Earth, the distance from the Earth to the Sun, the diameter of the galaxy and further to the distances to distant galaxies, would change in the same number of times, then we would simply not notice it.

This is indeed the case. Thus, the distance standard changes more strongly in intergalactic space than in galaxies where there is gravity.

And it's easy to explain it. The distance standard decreases in proportion to the gravitational potential. Let's assume that in intergalactic space the gravitational potential is equal to Phio , and we are inside the galaxy, where the total gravitational potential is equal to Phio+Phig, where Phig is the gravitational potential of the galaxy .
Now let's assume that after some time the gravitational potential of the Universe between the galaxies will decrease and become Phi1.
Then the total gravitational potential inside the galaxy will become Phi1+Phig.
It is easy to see that the gravitational potential between galaxies will decrease by a large relative amount, because

f1f0g.jpg.97aff855abea8ab4dd42d41ab737ad08.jpg

For example, if Phio is taken as 100%, Phig as 2%, and Pfi1 as 98%,
then the gravitational potential inside the galaxy will become 0.98039% of the initial one, and between galaxies it will become 98%.
The distance scales will also change accordingly, but we will not notice that our distance scale has become 0.98039% of the previous one, we will only notice that the distances between the
galaxies have increased by 0.00039% .
That's why it seems to us that the universe is expanding where there is no gravity.

 

 

You have it totally arse up. Gravity actually decouples regions of high mass/energy density from the overall expansion rate. We see the universe expanding over the larger scales, but our local group of galaxies, and even well beyond to our cluster and galactic wall, are decoupled from that expansion due to the higher mass/energy density.

Edited by beecee
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46 minutes ago, beecee said:

Gravity actually decouples regions of high mass/energy density from the overall expansion rate.

Isn't that what I'm talking about? Where there is gravity, more precisely, where gravity is stronger, the change in the distance scale is slower than where gravity is weaker. And we see only a relative change in scale.

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24 minutes ago, SergUpstart said:

Isn't that what I'm talking about? Where there is gravity, more precisely, where gravity is stronger, the change in the distance scale is slower than where gravity is weaker. And we see only a relative change in scale.

My apologies...Off with my head on on goes a pumpkin!!😬 The title threw me actually, "Why does it seem to us that the universe is expanding where there is no gravity"

The universe is expanding as opposed to "seems to us" The acceleration in the expansion rate is theorised to be caused by DE. The expansion itself, energy from the BB I suggest, although not positive on that score. 

Apologies again!

Edited by beecee
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25 minutes ago, beecee said:

My apologies...Off with my head on on goes a pumpkin!!😬 The title threw me actually, "Why does it seem to us that the universe is expanding where there is no gravity"

A correct remark, in the title it would be necessary to write " Why it seems to us that the Universe expands ONLY where there is no gravity"

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3 hours ago, SergUpstart said:

A correct remark, in the title it would be necessary to write " Why it seems to us that the Universe expands ONLY where there is no gravity"

But that’s not the case. Expansion occurs where gravity is too weak to stop it, but that’s not the same thing as no gravity, as you acknowledged earlier. 

Frankly, this does not seem to be a difficult concept; the concepts of motion and force it evokes are Newtonian. If gravity in a region is strong enough to prevent expansion, that’s what happens. 

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2 hours ago, swansont said:

Frankly, this does not seem to be a difficult concept; the concepts of motion and force it evokes are Newtonian. If gravity in a region is strong enough to prevent expansion, that’s what happens. 

Does this mean that there is an anti-gravitational force in the universe and is this force a manifestation of the fifth fundamental interaction ?????

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37 minutes ago, SergUpstart said:

Does this mean that there is an anti-gravitational force in the universe and is this force a manifestation of the fifth fundamental interaction ?????

What leads you to this conjecture? There is no fifth interaction in mainstream physics.

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45 minutes ago, swansont said:

What leads you to this conjecture? There is no fifth interaction in mainstream physics.

Then which of the four interactions is this force associated with??

If with gravity, then it is necessary to introduce a negative mass, because this force, the force of repulsion.

Edited by SergUpstart
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@SergUpstart, are you trying to talk about dark energy?  Here's a little snippet.....

 

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating.[1][2] Understanding the evolution of the universe requires knowledge of its starting conditions and its composition. Prior to these observations, it was thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research aimed at understanding the fundamental nature of dark energy...

I found this wiki passage particularly helpful,  in its description of a negative pressure throughout all spacetime.... 

 

The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation.[24] Dark energy is thought to be very homogeneous and not very dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive—roughly 10−27 kg/m3—it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.

Independently of its actual nature, dark energy would need to have a strong negative pressure (repulsive action), like radiation pressure in a metamaterial,[25] to explain the observed acceleration of the expansion of the universe. According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is the stress–energy tensor, which contains both the energy (or matter) density of a substance and its pressure and viscosity[dubious  discuss]. In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled "gravitational repulsion".

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1 hour ago, SergUpstart said:

Then which of the four interactions is this force associated with??

If with gravity, then it is necessary to introduce a negative mass, because this force, the force of repulsion.

What are you saying is the fifth force? The mainstream interpretation is that it’s speculative.

In physics, there are four observed fundamental interactions (also known as fundamental forces) that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories

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

 

Expansion of space is not a force. Matter feels no acceleration from space being added between it and other matter. i.e. it acts on space.

1 hour ago, TheVat said:

dark energy

Dark energy gives us acceleration of expansion, not the expansion itself.

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5 minutes ago, swansont said:

Expansion of space is not a force.

Yes, the expansion of space is not a force. The expansion of space is a change in the scale of distances. In the first post of this topic, I offered my explanation for the fact that we observe a change in scale only in intergalactic space.

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2 minutes ago, SergUpstart said:

Yes, the expansion of space is not a force. The expansion of space is a change in the scale of distances. In the first post of this topic, I offered my explanation for the fact that we observe a change in scale only in intergalactic space.

Good. Now please explain what you mean by the fifth force, and why you phrased it as if this were mainstream physics.

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16 minutes ago, swansont said:

 

Dark energy gives us acceleration of expansion, not the expansion itself.

Yes, as the last paragraph of my post alluded to.... 

 

In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled "gravitational repulsion".

It seemed to me that perhaps this vacuum energy was what Serg was driving at,  but at this point I really have no idea.   

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53 minutes ago, swansont said:

Good. Now please explain what you mean by the fifth force, and why you phrased it as if this were mainstream physics.

 

Here's what you wrote "Frankly, this does not seem to be a difficult concept; the concepts of motion and force it evokes are Newtonian. If gravity in a region is strong enough to prevent expansion, that’s what happens. " 

I just asked a counter question, the meaning of which is "Won't we need the fifth interaction to explain the nature of this Newtonian force ?"

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1 hour ago, SergUpstart said:

 

Here's what you wrote "Frankly, this does not seem to be a difficult concept; the concepts of motion and force it evokes are Newtonian. If gravity in a region is strong enough to prevent expansion, that’s what happens. " 

I just asked a counter question, the meaning of which is "Won't we need the fifth interaction to explain the nature of this Newtonian force ?"

You mentioned antigravity, and cited a fifth force as if it were an accepted thing.  You agree that expansion is not a force, so your question/comment makes no sense, because you refer to it as a force. You can’t have it both ways.

 

Expansion adds space between two objects, so they move apart, even if locally there is no motion. Space doesn’t exert a force on them.

They don’t move apart if they are gravitationally bound, because space doesn’t exert a force on them.

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On 9/5/2021 at 12:54 PM, SergUpstart said:

Hawking wrote in his book "A Brief History of Time" (or in "M-Theory" ) that it is a big mystery that the universe expands where there is no gravity.

I’m pretty sure he didn’t actually say this, since metric expansion is itself a gravitational phenomenon. Can you provide an exact reference, so we can see the context?

Metric expansion means simply that measurements of distances depend on when you make them - the results get bigger (on large enough scales) as you age into the future.

On 9/5/2021 at 12:54 PM, SergUpstart said:

The distance standard decreases in proportion to the gravitational potential.

There is no such thing as “gravitational potential” in general curved spacetimes - the concept only makes sense under some very special conditions, and certainly not for large regions of the universe. I seem to remember that this has been pointed out numerous times in past threads on here.

The other issue of course is that the strong and weak interactions (and gravity itself!) are not invariant under rescaling; hence, metric expansion and rescaling are physically very different things. You can’t rescale a region with matter in it, and expect physics to still work the same.

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10 hours ago, Markus Hanke said:

There is no such thing as “gravitational potential” in general curved spacetimes - the concept only makes sense under some very special conditions, and certainly not for large regions of the universe. I seem to remember that this has been pointed out numerous times in past threads on here.

The gravitational redshift can be expressed as

{\displaystyle z={\frac {\Delta \nu }{\nu _{1}}}=(1+\alpha ){\frac {\Delta U}{c^{2}}}}

where {\displaystyle \Delta \nu =\nu _{2}-\nu _{1}}is the gravitational redshift, {\displaystyle \nu _{1}} is the optical clock transition frequency, {\displaystyle \Delta U=\Delta U_{2}-\Delta U_{1}} is the difference in gravitational potential, and }\alpha denotes the violation from general relativity.

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

It is impossible to do without the gravitational potential.

If you look at the definition of the gravitational potential, then this is the energy that must be communicated to a resting body of a unit mass so that it flies to infinity. Probably, the fact that this energy is difficult to calculate from the GRT equation does not mean that this energy does not exist.

In addition, it follows from the definition of the gravitational potential that it is equal to the square of the escape velocity from a given point in space. Does the escape velocity exist?

10 hours ago, Markus Hanke said:

I’m pretty sure he didn’t actually say this, since metric expansion is itself a gravitational phenomenon. Can you provide an exact reference, so we can see the context?

Metric expansion means simply that measurements of distances depend on when you make them - the results get bigger (on large enough scales) as you age into the future.

 

I can't give you a link. Hawking's books do not contain formulas, so it is convenient to listen to them in audio format, which I did. But here I found the link http://kosmos-x.net.ru/news/kuda_rasshirjaetsja_vselennaja/2018-08-09-5403 It is in Russian, but now this is not a problem, since many browsers automatically translate the text. Here is a quote from there, Space expands only where the gravity of matter and energy are limited. Therefore, space does not expand inside galaxies or complex galactic groups, but only between galactic clusters and superclusters.

 

Edited by SergUpstart
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13 hours ago, SergUpstart said:

It is impossible to do without the gravitational potential

In general spacetimes, the concept of gravitational potential is replaced by the metric (you seem to have missed that on Wiki). In order to meaningfully define ‘gravitational potential’, the following conditions must hold for your spacetime:

1. It must be static

2. It must be asymptotically flat

3. It must be spherically symmetric

4. It must admit a time-like Killing vector field

These conditions are met only by a very few special spacetimes, and the cosmological FLRW spacetime is not one of them. An example of a spacetime where it does work is Schwarzschild.

13 hours ago, SergUpstart said:

If you look at the definition of the gravitational potential, then this is the energy that must be communicated to a resting body of a unit mass so that it flies to infinity.

Yes. This implies asymptotic flatness of the spacetime. It also implies path independence, ie the energy must be independent from the path taken to get to infinity (the value is a path integral), meaning it is a function of change in r alone. So this definition requires all four of the above conditions to be true, just like I said.

13 hours ago, SergUpstart said:

Does the escape velocity exist?

What is the escape velocity of the universe? Where are you going to escape to?

If asymptotic flatness does not hold, then no, there is no escape velocity at all, since there’s no meaningful notion of ‘escape’; gravity is non-negligible even at infinity. If it does hold, but any of (1)/(3)/(4) above are violated, then the escape velocity will explicitly depend on the precise trajectory and timing of the motion, in more or less complicated ways. Only if all four conditions are met is it a path-independent scalar, and thus related to a gravitational potential.

Note that ordinary Newtonian single-body gravity already presupposes these four conditions, which is why the potential can be defined so neatly there.

13 hours ago, SergUpstart said:

where {\displaystyle \Delta \nu =\nu _{2}-\nu _{1}}is the gravitational redshift, {\displaystyle \nu _{1}} is the optical clock transition frequency, {\displaystyle \Delta U=\Delta U_{2}-\Delta U_{1}} is the difference in gravitational potential,

Again, all this already assumes (1)-(4) to be true, because it only works if the differences are path-independent. Even something as simple as adding angular momentum will prevent path-independence already.

Cosmological FLRW spacetime is not asymptotically flat, not spherically symmetric, and does not admit a time-like Killing field, so the notion of ‘gravitational potential’ is simply meaningless there.

13 hours ago, SergUpstart said:

Space expands only where the gravity of matter and energy are limited. Therefore, space does not expand inside galaxies or complex galactic groups, but only between galactic clusters and superclusters.

Yes, this is more accurate than the original statement.

Edited by Markus Hanke
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15 hours ago, SergUpstart said:

Space expands only where the gravity of matter and energy are limited. Therefore, space does not expand inside galaxies or complex galactic groups, but only between galactic clusters and superclusters.

This is perfectly consistent with what you have been told, and does not say what you have claimed. "limited gravity" does not mean "no gravity"

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23 hours ago, Markus Hanke said:

In general spacetimes, the concept of gravitational potential is replaced by the metric

Does the metric reflect the Mach principle? The fact that the gravitational potential of the Universe is the sum of the gravitational potentials of all particles in the universe is essentially a mathematical formulation of the Mach principle.

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3 hours ago, SergUpstart said:

Does the metric reflect the Mach principle? The fact that the gravitational potential of the Universe is the sum of the gravitational potentials of all particles in the universe is essentially a mathematical formulation of the Mach principle.

It's not a fact. You've previously been told that it's wrong

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23 hours ago, SergUpstart said:

Does the metric reflect the Mach principle? The fact that the gravitational potential of the Universe is the sum of the gravitational potentials of all particles in the universe is essentially a mathematical formulation of the Mach principle.

As I already explained in my previous post, there is no such thing as ‘gravitational potential of the universe’; the concept is meaningless. It’s very frustrating when something is being explained at length, and then goes ignored. Besides, gravity is nonlinear, so even in cases where potentials are meaningful, you cannot just add them linearly.

And yes, GR reflects Mach’s principle explicitly, since in order to find solutions to the field equation, you must specify both local sources, as well as distant sources as boundary conditions. This has nothing to do with any potentials, it’s about initial and boundary conditions in a differential equation.

In fact, the aforementioned asymptotic flatness is an example of this.

 

Edited by Markus Hanke
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59 minutes ago, SergUpstart said:

I'm sorry?

Ok, no problem, no offence taken.

But I’m genuinely curious - why do you keep going on about the idea of replacing GR with a model based on a gravitational potential? It has been explained at length, in different threads, why such a thing cannot work; but you seem to keep pursuing it regardless.

The point is simply this: you need a certain amount of degrees of freedom to accurately capture all features of gravity - including gravitational radiation and its polarisation states. It can be formally shown (ref Misner/Thorne/Wheeler and others) that no scalar theory can do this, irrespective of its details; also no vector theory can do this. You need at the very least a rank-2 tensor theory, such as GR. That’s because gravitational radiation is quadrupole radiation with two polarisation states at 45 degree angle, and couples to the energy-momentum tensor as source. So nothing less than a rank-2 tensor will ever do (which corresponds to massless spin-2 radiation field).

Given this, why not just let the gravitational potential thing go? It won’t work because it can’t work. At best you’d get something that works as an approximation under special conditions, like Newton. GR is much more complete and general.

I just feel there’s no value in flogging a dead horse - you could be spending your time in more useful ways, wouldn’t you agree?

Edited by Markus Hanke
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