How does the expanding universe affect matter/energy

[michael879]

The OP would have to answer this, but I would say the problem is that it's frame-dependent. Potential energy, in spacetimes that admit such a concept (stationary spacetimes) is not frame-dependent, and the OP's question was about potential energy.

Agreed, but potential energy in an expanding universe is kindof paradoxical to begin with

 

[PeterDonis]

potential energy in an expanding universe is kindof paradoxical to begin with

I've been trying to convince the OP of exactly that point...

 

[TheScienceOrca]

Um, what? Either they're moving relative to each other or they're not.
Huh? There weren't any forces on them before. They were both moving inertially.

You do understand that in our expanding universe, all the galaxies, which are moving apart because of the expansion, are moving inertially, feeling zero force, right? The expansion is not a force pushing on the galaxies to move them apart. It just is the galaxies moving apart.
That depends on the spacetime. See below.
According to GR, I am not dropping an apple on my head; I am releasing it and letting it float freely, while the Earth pushes me upward into it. I am the one feeling a force, not the apple, and the force I feel is not "gravity", it's the Earth pushing up on me.
When I say the concept of "potential energy" doesn't apply to the expanding universe, I'm not talking about terminology or labels; I'm talking about physics. I'm saying that the physics that's going on in an expanding universe is not the same as the physics going on when an apple falls. I'm saying that the physics you are trying to apply the the expanding universe is the wrong physics; you are trying to apply the physics that works in a stationary spacetime, where the concept of "potential energy" is meaningful, to a non-stationary spacetime (the expanding universe), where it isn't. The same underlying theory (GR) can be used to describe both, but the specific models used--the specific solutions to the Einstein Field Equation--are different, and have different properties. The solution that applies to the apple falling has "potential energy" as one of its properties; the solution that applies to the expanding universe does not.

Ahhh I know why you weren't getting the understanding now ok.

So this is a big point for the explanation.Two masses that are moving towards each other and the distance between them can increase if the expansion rate between the masses is great enough.

I would like to call this point or radius the gravitational expansion radius for a simple understandable term to be used in posts throughout this thread.Let me know if you have any questions about this, I think you will catch yourself once you put this point together.

 

[TheScienceOrca]

Anyone posting on this thread saying potential energy is not relevant to GR or gravity is not a force is completely ignorant and fails to read full posts.I have made it very clear what I am saying when I say potential energy and I have explained it doesn't matter if gravity is a force or not!

PLEASE read my earlier posts fully it will save you time from posting the same question over and over.

I am not accusing all members as some have acknowledged my corrections in earlier posts.

If you have a question or counter not already stated, please post!

Thank you

 

[TheScienceOrca]

If energy were to be conserved in GR, there would indeed have to be a contribution from "gravitational field energy", which is, I believe the point of your somewhat long example. Unfortunately. if you consult an actual GR textbook such as "General Relativity", Robert M Wald, 1984, (see section $11.2 pg 286)

[quote =Wald ]
...not surprising since $T_{ab}$ represents only the energy content of matter, whereas "gravitational field energy" should make a contribution to total energy and thus should appear in any conservation law. However, as already mentioned in chapter 4, there is no known meaningful notion of the energy density of the gravitational field in general relativity.

So most of your post is wishful thinking based on assuming your conclusion :(. I'd summarize your "argument" as follows. "Energy is conserved in Newtonian theory, so I want it to be conserved in GR too. In Newtonian theory we can break total energy into two components, kinetic energy and potential energy, whose sum is a conserved quantity, so we'll assume that we can do the same in GR. We will ignore the textbooks, papers, and FAQ's that tell us that we can't do this because we don't want to believe them or even take the time to check out contrary references.[/QUOTE]all energy has mass, and all mass acts gravitationally

 

[PeterDonis]

Two masses that are moving towards each other and the distance between them can increase if the expansion rate between the masses is great enough.

This doesn't even make sense. If the distance between them is increasing, they are not moving towards each other. Either your thinking is confused or your terminology is very bad (or quite possibly both).

I would like to call this point or radius the gravitational expansion radius for a simple understandable term to be used in posts throughout this thread.

Sorry, labeling this with a term doesn't help since the concept doesn't make sense.

Let me know if you have any questions about this

Consider the above as letting you know.

Anyone posting on this thread saying potential energy is not relevant to GR or gravity is not a force is completely ignorant and fails to read full posts.

I'm reading your posts just fine. I just think they're wrong. Repeating the same assertions won't change that.

I have made it very clear what I am saying when I say potential energy and I have explained it doesn't matter if gravity is a force or not!

And your explanations don't help matters, because they still end up saying things which are wrong. As I said in an earlier post, this isn't about labels, it's about physics. The problem is that you are in error about the physics; complaining that we don't like your labels won't change that.

 

[PeterDonis]

all energy has mass, and all mass acts gravitationally

Nobody has denied this. The problem is that you have an incorrect conception of what "all mass acts gravitationally" means.

 

[TheScienceOrca]

This doesn't even make sense. If the distance between them is increasing, they are not moving towards each other. Either your thinking is confused or your terminology is very bad (or quite possibly both).Or you don't have an understanding of EXPANSION which based on this comment is what it seems. This is the difference between expansion and explosion.

1) First let's understand what velocity is, I will quote a reputable source; "Velocity is the rate of change of the position of an object,"So I am sure you know what that means.

2) To help you better understand expansion, let's imagine two masses in a universe where only those are the two masses separated by a large distance

Lets say this masses have a velocity of 0. There POSITION is not changing, ALTHOUGH the distance between them is increasing due to the expansion. This is the big difference between expansion and explosion. If all energy/mass exploded from an origin then the vector quantities would all be different directions thus the mass/energy would be physically MOVING through the universe.

Expansion != Motion if it was then how could the universe expand faster than the speed of light?

The space between the objects is literally expanding.

This is a key point to the concept and one I made earlier in the thread, but I will elaborate it.

Actually;
I would further go into depth, but let's just keep it simple for now let me know if you have any questions about this post or if there are any flaws in it.

 

[TheScienceOrca]

Nobody has denied this. The problem is that you have an incorrect conception of what "all mass acts gravitationally" means.

Actually I know exactly what it means and its one the fundamental points of my concept.

All masses affect each other gravitationally at all times.

 

[TheScienceOrca]

I appreciate and responded to your constructive replies that had information and counter arguments the others involving emotion I ignored.

 

[TheScienceOrca]

This doesn't even make sense. If the distance between them is increasing, they are not moving towards each other. Either your thinking is confused or your terminology is very bad (or quite possibly both).
Sorry, labeling this with a term doesn't help since the concept doesn't make sense.
Consider the above as letting you know.
I'm reading your posts just fine. I just think they're wrong. Repeating the same assertions won't change that.
And your explanations don't help matters, because they still end up saying things which are wrong. As I said in an earlier post, this isn't about labels, it's about physics. The problem is that you are in error about the physics; complaining that we don't like your labels won't change that.

Only because you can't grasp that two objects can have velocity vectors towards each other greater than 0 and the distance STILL increase between them OR stay the same OR decrease less than suggested by the vector due to the distance.

Distance != Position. I have thought of a simple analogy;Lets say I have object A at a stop light and down the Road I have object B at another stop light.

We will call these position L1 and L2.
The objects are at rest and never change position from (L1 or L2).
The distance between L1 and L2 at the start of the experiment is 1 million light years. Due to expansion the DISTANCE between L1 and L2 and thus object A and B is becoming greater now due to the change in position but due to the fundamental expansion between L1 and L2.

 

[PeterDonis]

"Velocity is the rate of change of the position of an object,"

More precisely, this is a definition of coordinate velocity, since it is frame-dependent. See below.

Lets say this masses have a velocity of 0. There POSITION is not changing, ALTHOUGH the distance between them is increasing due to the expansion.

More precisely, their position in your chosen coordinates is not changing, but the distance between them in your chosen coordinates is increasing. That's not to say there's anything wrong with your chosen coordinates as such: they're the standard coordinates used in cosmology, after all. But they're still just coordinates--arbitrary labels for events. (Your simple analogy with the stop lights has the same problem.) All the actual physics has to be contained in invariants--things that are independent of the choice of coordinates. So instead of continuing to repeat the same coordinate-dependent things, can you give any invariants that justify your statement that "the space between them is expanding"?

I know exactly what it means and its one the fundamental points of my concept.

All masses affect each other gravitationally at all times.

Since you know exactly what this means, can you give any invariants that correspond to this statement?

 

[michael879]

Two masses that are moving towards each other and the distance between them can increase if the expansion rate between the masses is great enough.

Ok here is a huge problem, that Peter did point out but I don't think he saw where your confusion was. Your problem with this scenario is that you are completely throwing out relativity and using an absolute reference frame. To say that two masses are moving towards each other even though every conceivable measurement says they are moving away from each other violates relativity in a pretty major way, as you are assuming that they are moving with respect to a preferred background which sees them both having velocities towards each other

 

[michael879]

Also, reading through ur most recent posts I see a number of statements that repeat this problem. I don't think you fully understand the concept of relativity, or are intentionally ignoring it. Either way, you can't have a discussion about the expansion of the universe without at least taking special relativity into account (many people here would argue you need general relativity too, but I can imagine informal discussions without the need for it)

If the universe is expanding, an object is ONLY fixed in its own personal reference frame. In every other frame, its position IS moving. The second you move to 2 or more masses, none of them will ever see the others as fixed (unless you put some very complicated force on them). Yes, they all see themselves as fixed but it is a completely invalid statement to say they are all fixed because there is no frame of reference where this is true

 

[michael879]

One more point: it is impossible to keep two objects at a fixed distance without putting a force on one of both of them. That is, you cant send them off at some constant velocity and have them stay at a fixed distance.

Think about it like this: if at any point they are at rest relative to each other, then the space between them will continue to expand, forcing them further apart at which point they are no longer at rest relative to each other

 

[PeterDonis]

Your problem with this scenario is that you are completely throwing out relativity and using an absolute reference frame.

This is what I was trying to get at by pointing out that the "position" and "distance" he was using was coordinate-dependent; but I think your way of putting it is better because it emphasizes that he is treating those coordinates as an absolute reference frame--you're right that I didn't really pick up on that.

 

[Dale]

Closed. ScienceOrca, please learn a little GR before rejecting the experts information.