Exploring the Expansion of the Universe: The Role of Gravity

[Albrecht]

No, it doesn't. That is pop science, not real science. Real cosmology tells us that comoving objects in our universe are moving apart. That is not the same as "space expands".

Sorry but I have learned that differently. Wrong? In my knowledge Lemaitre has used Einstein's general concept of space to describe the development of the universe. He has related the expansion to space. - Not true? And in the discussions about Dark Energy, the argument (also in univerity discussions) is always that the "extension of space" goes on in an accelerated way. Stars are only measured to proof this.

 

[Albrecht]

No, it isn't. It has nothing to do with length contraction in SR.

The rest of your post just builds on these misconceptions.

It was my understandig of Einstein that he has developed a generel concept about space. Also in his later papers about, what he called "new ether", he discussed general properties of the space and did not differentiate between the space in SRT, in GRT, or in cosmology.

 

[rede96]

Expanding universe just means there is always room for comoving bodies to continue moving away from each other, not that space is pushing them, or that two 'positions' are moving apart (except in particular coordinates).

Comoving bodies are moving away from each other but the two ' positions' are not moving apart. Could you explain what you mean by this?

 

[PeroK]

Comoving bodies are moving away from each other but the two ' positions' are not moving apart. Could you explain what you mean by this?

Try this:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[rede96]

Try this:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

Thanks for the link. If we imagine galaxies plotted on some come cosmic size coordinate system then yes I understand that the galaxies are at rest wrt that coordinate system (comoving galaxies) and the points on the coordinate grid are moving further apart (expansion).

But what I was interested in is what is powering the expansion of the grid? I understood dark energy to be a sort of negative 'pressure' that caused expansion but it seems most tend to model expansion as objects "free falling" like in a gravitational field, just away from each other.

What confused me is that in Leonard Susskind's lectures on cosmology he does describe dark energy as a kind of negative pressure and not as a type of anti-gravity. In fact he describes all bound systems being very slightly and equally effected by expansion in that the the point of equilibrium the systems end up in is very slightly more expanded then it would be without dark energy. (excuse my poor terminology!)

Hence my confusion to this:

not that space is pushing them

as that's exactly how I understood it. That dark energy was applying a negative pressure to matter / energy and causing matter in non bound systems to move apart.

 

[PeroK]

Thanks for the link. If we imagine galaxies plotted on some come cosmic size coordinate system then yes I understand that the galaxies are at rest wrt that coordinate system (comoving galaxies) and the points on the coordinate grid are moving further apart (expansion).

But what I was interested in is what is powering the expansion of the grid? I understood dark energy to be a sort of negative 'pressure' that caused expansion but it seems most tend to model expansion as objects "free falling" like in a gravitational field, just away from each other.

What confused me is that in Leonard Susskind's lectures on cosmology he does describe dark energy as a kind of negative pressure and not as a type of anti-gravity. In fact he describes all bound systems being very slightly and equally effected by expansion in that the the point of equilibrium the systems end up in is very slightly more expanded then it would be without dark energy. (excuse my poor terminology!)

Hence my confusion to this: as that's exactly how I understood it. That dark energy was applying a negative pressure to matter / energy and causing matter in non bound systems to move apart.

That would be normal positive pressure. A model where a gas is expanding and pushing things with it. That's not what's happening here.

Negative pressure means that energy is required to expand the vacuum - rather than energy required to compress it.

Note that vacuum energy is positive: if a vacuum expands and its energy density remains constant - as it must, assuming vacuum is vacuum and you can't dilute a vacuum - then that implies more total energy.

The other energies (matter and radiation) have the obvious property that as space expands, their energy densities must decrease. Vacuum energy has what seems the extraordinary property that this does not happen and, as space expands, the total vacuum energy goes up and up.

As I understand it, that is where we reach the mystery of dark energy.

 

[PAllen]

Comoving bodies are moving away from each other but the two ' positions' are not moving apart. Could you explain what you mean by this?

Simple. Comoving bodies, by any reasonable definition, are moving away from each. Meanwhile, position has no meaning beyond coordinate choice in relativity. Consider just SR: every distinct frame entails a different definition of what constant position means, and they are all equally valid. Constant position cannot possibly have more meaning in GR than it does in SR. Expansion is the feature of spacetime geometry that makes it possible for comoving bodies to continue moving apart. Using my simple 1x1 analogy, if the universe is cone shaped, you are able to have comoving bodies moving apart forever. A cylinder, or a cone whose sides bent to asymptote a cylinder would not allow this.

Specifically, it is trivial to define constant position such that positions don't move apart. Pick a particular galaxy. Define that world lines that show no redshift as seen by this galaxy are lines of constant position. Just as valid as any other choice. No matter how far away some other comoving galaxy is, if a light source in that galaxy is moving towards us with a speed relative to that galaxy (this relative velocity, being local, is well defined) corresponding to the galactic redshift interpreted as SR Doppler, then light from that source will show no red shift. Define constant position lines by such light sources rather than comoving galaxies. Even with Dark Energy, this is still possible, but such light sources will need to move more and more non-inertially to maintain absence of redshift as observed by us, the more Dark Energy there is. But as long as we can see a comoving object, there is colocated non-superliminal velocity trajectory which would have no redshift as observed by us.

Note, this is also a justification why I consider the phrase "superluminal recession velocities" misleading.

 

[PAllen]

Hence my confusion to this: as that's exactly how I understood it. That dark energy was applying a negative pressure to matter / energy and causing matter in non bound systems to move apart.

Note, I stated the same as Susskind, that with Dark energy there is tiny shift of equilibria even for bound systems. The difference in how we phrase interpretation is a matter of which side of an equation we are focusing on. Assuming you put the cosmological constant on the right hand side of the GR field equation (e.g. to allow the possibility for it to be a dynamical field rather than a constant), then as a field contributing to stress energy it has a natural interpretation as negative pressure. However, possibly paradoxically, the effect of negative pressure on the metric corresponding the Einstein tensor (the left hand side of the equation) is to produce a homogeneous tendency of geodesics to diverge (and positive pressure produces the opposite gravitational effect). To me, this is just as much gravity as the tendency of geodesics in Schwarzschild spacetime to converge in two directions while diverging in the third. Gravity is effect of the metric tensor on inertial stucture. Negative pressure produces a certain form of gravity.

 

[PeterDonis]

space would contract in motion

"Space" doesn't contract in SR. Objects appear to have shorter lengths to observers moving relative to them.

but fields would not follow this

Once again: I agree that electromagnetism is consistent with SR. That's not the issue. The issue is that you are using the word "contraction" to refer to how electromagnetic fields transform under a Lorentz transformation. That word is not appropriate because fields do not have a "length" or a "size", so they can't contract. So there's no point in you continuing to say that SR has to be valid; nobody is disputing that. You need to show what property of fields can be properly said to be a "length" and to "contract".

Imagine a light clock in a gravitational field. The time indication of such clock has to show the dilation which happens there. This can only work correctly if (1) the speed of light is reduced as given by GRT and (2) the distance of the mirrors is reduced.

You are confused. Spacetime is curved in the presence of gravity. You are trying to use intuitions from SR, that only work in flat spacetime, in the presence of gravity. That doesn't work.

Also, the "speed of light" you refer to is a coordinate speed, not a physical speed. You don't appear to understand the distinction; it's crucial.

I have learned that differently.

From where? Can you give an actual textbook or peer-reviewed paper as a reference?

It was my understandig of Einstein that he has developed a generel concept about space.

Where are you getting this understanding from? Again, can you give an actual textbook or peer-reviewed paper as a reference?

Einstein developed GR as a theory of spacetime, not space.

in his later papers about, what he called "new ether", he discussed general properties of the space and did not differentiate between the space in SRT, in GRT, or in cosmology.

Are these actual peer-reviewed papers, or popular articles? Please give specific references.

 

[Albrecht]

Once again: I agree that electromagnetism is consistent with SR. That's not the issue. The issue is that you are using the word "contraction" to refer to how electromagnetic fields transform under a Lorentz transformation. That word is not appropriate because fields do not have a "length" or a "size", so they can't contract. So there's no point in you continuing to say that SR has to be valid; nobody is disputing that. You need to show what property of fields can be properly said to be a "length" and to "contract".

Fields cannot contract? How can you say this?
Example: There may be a field E of a charge q described by the equation: E = q/r². Then assume that conditions change so that now there is E = q/(2r). So, if one looks to the spatial distribution of the field, the field (i.e. its shape) is contracted by a factor of 2. What is the problem here?

Historically Oliver Heaviside has in 1888 deduced from the Maxwell equations that an electrical field contracts for a moving charge by the equation E --> E' = E / gamma, where gamma = sqrt(1/(1-v²/c²)). This was a well known result and an important step towards relativity.

This fact of a "contraction of a field" is well known and was never questioned by the physical community to my knowledge.

 

[Albrecht]

You are confused. Spacetime is curved in the presence of gravity. You are trying to use intuitions from SR, that only work in flat spacetime, in the presence of gravity. That doesn't work.

Also, the "speed of light" you refer to is a coordinate speed, not a physical speed. You don't appear to understand the distinction; it's crucial.

Of course I know the distinction. All this is the view of an observer from outside the field. For the observer in the field there is no difference visible. This is demanded by the principle of relativity.

First about time: There is time dilation in a gravitational field. An observer can put a clock into the field and after a time take it back again. The clock in the field compared to a clock which stayed outside is retarded according to the dilation formula for gravity. And now the case of contraction; here again as said for SRT: the contraction can only be determined indirectly. I have proposed the case of a light clock. This light clock has to represent the dilation in gravity. And this dilation can only be explained if contraction is assumed and a reduction of the speed of light. Where speed of light means of course the coordinate value. And that is appropriate because all this is viewed by an observer outside the field.

You say correctly that space-time is curved. This curvature includes of course the contraction of space in the radial direction for a radial field (like for a star). In case of the light clock the calculation depends on the orientation of the clock.

 

[Albrecht]

Where are you getting this understanding from? Again, can you give an actual textbook or peer-reviewed paper as a reference?

Einstein developed GR as a theory of spacetime, not space.

That is in general correct. But as said before: in a radial field or in general in a radial situation curvature means contraction.
And in the case of the expanding universe: this is also a radial situation and so the change of the space means expansion.

 

[Albrecht]

Are these actual peer-reviewed papers, or popular articles? Please give specific references.

Einstein died in 1955. So there are no actual papers from him.

In the years 1925 until 1955 he developed his understanding of space which means his understanding of those properties of the space which he called the "new ether". He published a series of papers and also books about this topic. I do not know which of his papers were peer-reviewed. But I know that Einstein did not like this peer-reviewing. Because he said "I am Einstein and any peer-reviewing is inappropriate".

I do not have a list of his papers and books about this topic at hand. But university libraries should have them.

 

[PeterDonis]

Fields cannot contract? How can you say this?

Because, as I have said several times and you have apparently not grasped, "fields contract" is not the same as "fields transform according to Lorentz transformations". You keep giving arguments for the latter. Those are not arguments for the former.

Oliver Heaviside has in 1888 deduced from the Maxwell equations that an electrical field contracts for a moving charge by the equation E --> E' = E / gamma

Which does not describe "contraction" unless E is a length. Which it isn't. It describes the relativistic transformation of the field strength when you change frames. Field strength is not length.

 

[PeterDonis]

There is time dilation in a gravitational field. An observer can put a clock into the field and after a time take it back again. The clock in the field compared to a clock which stayed outside is retarded according to the dilation formula for gravity.

Yes.

Where speed of light means of course the coordinate value. And that is appropriate because all this is viewed by an observer outside the field.

No, it's not appropriate because it's just a coordinate effect. No physics can depend on how you choose your coordinates.

This curvature includes of course the contraction of space in the radial direction

No, it doesn't. The coordinate effect you are describing is not "contraction of space". It's just a coordinate effect. No physics can depend on how you choose your coordinates.

If you actually look at the "space" in question, and compare it to a Euclidean space enclosed by a 2-sphere with the same surface area, you will find that it is more appropriate to describe the "space" around a massive body as "expanded" rather than "contracted"; the 3-volume enclosed by a 2-sphere of a given surface area is larger if there is a gravitating mass inside the 2-sphere than if there is only vacuum there.

in a radial field or in general in a radial situation curvature means contraction.

No, it doesn't. See above.

in the case of the expanding universe: this is also a radial situation and so the change of the space means expansion.

I'm not sure what you mean by "a radial situation". The universe is homogeneous; spacetime around a gravitating mass is not. They're not the same.

Also, as I have already said, the expansion of the universe is not properly described as a "change of space".

 

[PeterDonis]

Einstein died in 1955. So there are no actual papers from him.

Um, you do realize that Einstein published scientific papers for at least 50 years before he died, right? Apparently you do, since you go on to say:

In the years 1925 until 1955 he developed his understanding of space which means his understanding of those properties of the space which he called the "new ether". He published a series of papers and also books about this topic.

Again, please give specific references.

I know that Einstein did not like this peer-reviewing. Because he said "I am Einstein and any peer-reviewing is inappropriate".

Please give a reference for where you got this quote from.

I do not have a list of his papers and books about this topic at hand.

Then how can you so confidently assert what Einstein said or didn't say? Either you have references to support your claims, or you don't. Either give specific references or stop making the claims.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

The thread has run its course and will remain closed.