Does the Expansion of the Universe Affect Gravity?

[Chris Miller]

Does the universe’s ongoing expansion mean that some finite amount of space-time is stretching, or that there is simply more and more of the same (space-time) being produced? Given the obvious difference between the dense primordial universe and the present, I’m guessing the former, which, if so, might this not subtly impact things like gravity (i.e., its conduction of virtual particle-waves) ?

 

[phinds]

Does the universe’s ongoing expansion mean that some finite amount of space-time is stretching, or that there is simply more and more of the same (space-time) being produced? Given the obvious difference between the dense primordial universe and the present, I’m guessing the former, which, if so, might this not subtly impact things like gravity (i.e., its conduction of virtual particle-waves) ?

"stretching" of space is purely a pop-sci fantasy. Space is just geometry, not "stuff" that can stretch or bend. Things get farther apart but nothing is being produced. Google "metric expansion"

 

[Grinkle]

Space is just geometry, not "stuff" that can stretch or bend. Things get farther apart but nothing is being produced.

B thread, please go easy on me.

As space expands, if there are not more Higgs particles created to maintain a consistent Higgs field strength, whatever strength means in that context, wouldn't the Higgs field decay as time goes on?

I have no way to defend the implication that a fixed number of Higgs particles would somehow spread apart from each other as space expands if no new ones are created.

 

[PeterDonis]

Does the universe’s ongoing expansion mean that some finite amount of space-time is stretching, or that there is simply more and more of the same (space-time) being produced?

Neither. When we say the universe is "expanding", we are saying something about its 4-dimensional geometry. That geometry does not "change"; it just is.

 

[PeterDonis]

if there are not more Higgs particles created to maintain a consistent Higgs field strength

You don't need to create more Higgs particles to maintain a constant Higgs field strength.

wouldn't the Higgs field decay as time goes on?

No. The Higgs field (more precisely, the vacuum expectation value of that field) stays constant because it's in its lowest energy state. It can't decay because there is no state with lower energy for it to decay into.

 

[Chronos]

To quote Einstein, empty space has no ponderable properties. Lacking ponderable properties means to speak of it being stretched, torn, broken or otherwise defeormed in any way is virtually meaningless. The geometry of spcetime is an imaginary grid we overlay the vacuum with to compare the geometrical relationships between adjoining regions of spacetime. Geometry does have ponderable properties, which makes it useful for visualization purposes, but, should not be confused with spacetime, which does not..

 

[Chris Miller]

Wikiing up on a vacuum's ZPE (which seems to have "ponderable properties") I'm having trouble with the assertion here that "Space is just geometry," which was, to me, already putting the cart before the horse. Mathematics makes for exact and helpful metaphors that can be applied to, but <>, space.

 

[Grinkle]

empty space has no ponderable properties

The point I am confused on is whether current models and experimental evidence predict there is any such thing as empty space. Minimally, space contains the Higgs field, and maybe minimally it also contains other things that distinguish it from empty that a theory of quantum gravity could describe.

 

[PeterDonis]

The point I am confused on is whether current models and experimental evidence predict there is any such thing as empty space.

Current models don't "predict" that there is empty space; empty space (more precisely, spacetime) is an axiom on which the models are built. Some of the theories of quantum gravity that are being worked on do not have this axiom; they attempt to build "empty space" (spacetime) out of other more fundamental entities. But those theories are still speculative at this point.

Experimental evidence doesn't "predict" that there is empty space; it just tells us there is, by commonplace observation. That's why our current models can adopt this as an axiom without an issue.

 

[PeterDonis]

I'm having trouble with the assertion here that "Space is just geometry," which was, to me, already putting the cart before the horse.

Why?

 

[Chris Miller]

Why?

I just meant that geometry/math can be used to describe/model certain aspects of space, but not all. That space is probably more than geometry. E.g., if Feynman's and Wheeler's calculations suggesting "the zero-point radiation of the vacuum to be an order of magnitude greater than nuclear energy." Or if Paul Davies' "Many physicists believe that "the vacuum holds the key to a full understanding of nature" is true, then space is more than geometry.

 

[PeterDonis]

if Feynman's and Wheeler's calculations suggesting "the zero-point radiation of the vacuum to be an order of magnitude greater than nuclear energy." Or if Paul Davies' "Many physicists believe that "the vacuum holds the key to a full understanding of nature" is true, then space is more than geometry.

No, those things are not saying that space is more than geometry; they are saying that "vacuum" as a concept is not the same as "space" as a concept. (Or spacetime, if we are using relativity, as we should.)

 

[Chris Miller]

No, those things are not saying that space is more than geometry; they are saying that "vacuum" as a concept is not the same as "space" as a concept. (Or spacetime, if we are using relativity, as we should.)

Then getting back to my question, it would seem the universe's expansion would involve either creating ever more of this vacuum or changing the quality/nature of it, and I'm still thinking the latter.

 

[PeterDonis]

it would seem the universe's expansion would involve either creating ever more of this vacuum or changing the quality/nature of it

No, because the "vacuum" is not something that gets "created" or "stretches" as the universe "expands". The universe is a 4-dimensional geometry, and the "vacuum" is a 4-dimensional solution to a 4-dimensional field equation on that 4-dimensional geometry. It doesn't "change"; it just has particular values at each point of the 4-dimensional geometry. The solution describing the vacuum for our universe already contains all the information about the whole 4-dimensional geometry; nothing has to be "created" in one part of that geometry vs. another.

 

[Grinkle]

"vacuum" as a concept is not the same as "space" as a concept.

The universe is a 4-dimensional geometry, and the "vacuum" is a 4-dimensional solution to a 4-dimensional field equation on that 4-dimensional geometry.

Ah - thanks.

 

[nikkkom]

The point I am confused on is whether current models and experimental evidence predict there is any such thing as empty space. Minimally, space contains the Higgs field, and maybe minimally it also contains other things.

According to SM, every point in space contains all fields. There no "holes" in them. Mathematically speaking, "every point of spacetime is assigned a value" and that's what field is. (The value can be just a real number, a complex number, a matrix, a tensor etc).

The important point is a field can easily have zero value in a large area of space. If you have a cubic kilometer of space where photon field is everywhere zero, it's just a place which has no photons moving trough it. But it's not a cubic kilometer "without photon field".

 

[Grinkle]

The important point is a field can easily have zero value in a large area of space.

No issue with this.

Thinking a bit more, I'm probably just wanting someone to explain to me what the nature of dark energy is.

 

[PeterDonis]

I'm probably just wanting someone to explain to me what the nature of dark energy is.

We don't know at present. We just know that, from the standpoint of our cosmological models, dark energy looks like a small positive cosmological constant. But we don't know why the cosmological constant has the small positive value that it has.

 

[Chris Miller]

We don't know at present. We just know that, from the standpoint of our cosmological models, dark energy looks like a small positive cosmological constant. But we don't know why the cosmological constant has the small positive value that it has.

Seems widely theorized to uniformly fill otherwise empty space. If so, would expansion be introducing more of this energy or diluting the concentration of some fixed amount? In either case, expansion would seem to be a lot more than a geometric phenom.

 

[PeterDonis]

Seems widely theorized to uniformly fill otherwise empty space.

It uniformly fills spacetime. (That's true whether spacetime is "otherwise empty" or not. The dark energy--small positive cosmological constant--is everywhere, not just in "otherwise empty" regions.) That's what "small positive cosmological constant" means.

would expansion be introducing more of this energy or diluting the concentration of some fixed amount?

Neither of these is correct. I've already explained the correct viewpoint earlier in this thread.

 

[Chris Miller]

It uniformly fills spacetime. (That's true whether spacetime is "otherwise empty" or not. The dark energy--small positive cosmological constant--is everywhere, not just in "otherwise empty" regions.) That's what "small positive cosmological constant" means.
Neither of these is correct. I've already explained the correct viewpoint earlier in this thread.

Was quoting Wiki's "filling otherwise empty space." Which also says that this dark energy > all other energy sources, including matter. Wasn't sure whether the constant was for the universe as a whole (as per Newton's "energy cannot be created or destroyed") or constant over fixed regions (like Hubble expansion). Still am not.

 

[PeterDonis]

Was quoting Wiki's "filling otherwise empty space."

Wikipedia is not a good source. You need to look at actual textbooks or peer-reviewed papers.

Which also says that this dark energy > all other energy sources, including matter.

More precisely, in our current universe, the density of dark energy is larger than the density of any other energy sources, including matter. But that was not always true; in the early universe the density of dark energy was negligible compared to the energy density of matter and radiation.

Wasn't sure whether the constant was for the universe as a whole (as per Newton's "energy cannot be
created or destroyed") or constant over fixed regions (like Hubble expansion). Still am not.

The energy density of dark energy is constant everywhere in spacetime. At least, that's true according to our best current model. There are other proposed models that allow the density of dark energy to vary, but we have no evidence supporting any of those models over the simplest model where its density is constant.

 

[Chris Miller]

Wikipedia is not a good source. You need to look at actual textbooks or peer-reviewed papers.
More precisely, in our current universe, the density of dark energy is larger than the density of any other energy sources, including matter. But that was not always true; in the early universe the density of dark energy was negligible compared to the energy density of matter and radiation.
The energy density of dark energy is constant everywhere in spacetime. At least, that's true according to our best current model. There are other proposed models that allow the density of dark energy to vary, but we have no evidence supporting any of those models over the simplest model where its density is constant.

Thanks for your patience. I sort of assumed Wiki was quoting, paraphrasing and summarizing from such sources. From what you're saying, it sounds like the universe's expansion (addition of space) is adding dark energy. I thought, from what I read, that space's dark energy wasn't near as concentrated (dense?) as in matter, but because of how vastly more space there is in the universe now (e.g., in a 10 m radius scale model of our solar system, you'd need a microscope to see earth), it's cumulatively more. Wonder if it's dark energy then that's fueling its accelerating expansion.

 

[phinds]

for the universe as a whole (as per Newton's "energy cannot be created or destroyed")

Conservation of energy is a local phenomenon and not applicable on cosmological scales

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

 

[PeterDonis]

From what you're saying, it sounds like the universe's expansion (addition of space) is adding dark energy.

No. This is not correct. Please go back and read my previous posts again, carefully.

I thought, from what I read, that space's dark energy wasn't near as concentrated (dense?) as in matter

The density of dark energy is the same everywhere in spacetime. You are confusing yourself by thinking of "space" instead of "spacetime".

because of how vastly more space there is in the universe now

This is not a good way to look at it. Again, go back and read my previous posts again, carefully. You have not correctly understood the point I was making.

Wonder if it's dark energy then that's fueling its accelerating expansion.

Um, the reason dark energy is in our current cosmological models is because it is the only known cause for accelerating expansion.

 

[Chris Miller]

Conservation of energy is a local phenomenon and not applicable on cosmological scales

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Interesting article, accessibly written, too. Helpful to see,

Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10-10 joules per cubic metre.

I.e., more space = more dark energy. That expansion is adding more than scale. Also interesting to read,

It’s just as true for “radiation” — particles like photons that move at or near the speed of light. The thing about photons is that they redshift, losing energy as space expands. If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases.

Though it never goes so far as to suggest that they (radiant and dark energy) cancel each other out.

 

[PeterDonis]

more space = more dark energy

No, because "the universe is expanding" does not mean "there is more space at later times than at earlier times". The "amount of space" is infinite (because the universe is spatially infinite).

 

[Chris Miller]

No, because "the universe is expanding" does not mean "there is more space at later times than at earlier times". The "amount of space" is infinite (because the universe is spatially infinite).

Perhaps the article @phinds pointed me to was overly simplistic, because I gleaned nothing of this from it. Really, I can't even get my B-level head around it. The universe at t=0 was small. The universe "now" (in our more or less co-moving FOR) calculates "the diameter of the observable universe a sphere around 92 billion light-years" - https://www.space.com/24073-how-big-is-the-universe.html. A trillion years from now... The universe might be potentially spatially infinite, or not. But nothing can be measurably so. I feel I'm missing something crucial to what you are saying.

 

[PeterDonis]

The universe at t=0 was small.

There was no universe at $t = 0$. The "instant" $t = 0$ is not part of the model; it's a limit point that doesn't actually exist.

The universe "now" (in our more or less co-moving FOR) calculates "the diameter of the observable universe a sphere around 92 billion light-years"

The observable universe is not the same as the entire universe. All of the discussion in Carroll's article that you linked to refers to the entire universe, not just the observable universe. So the fact that the observable universe has a finite size is irrelevant here.

Another way of putting this is to say that the "observable universe" is not a closed system (matter and energy can cross its boundary), so one would not expect its total energy to be conserved. But the entire universe is a closed system, so "naively" one would expect its total energy to be conserved. That's why Carroll goes to the trouble to explain why that actually isn't the case for the entire universe--the closed system.

 

[PeterDonis]

The universe might be potentially spatially infinite, or not. But nothing can be measurably so.

This is irrelevant. We have a model of the entire universe which is spatially infinite. That's the only tool we have to even talk about concepts like "the total energy of the universe". If we're not allowed to use models that extrapolate beyond what we can directly observe, this whole discussion is pointless since it is based on nothing.

 

[Chris Miller]

This is irrelevant. We have a model of the entire universe which is spatially infinite. That's the only tool we have to even talk about concepts like "the total energy of the universe". If we're not allowed to use models that extrapolate beyond what we can directly observe, this whole discussion is pointless since it is based on nothing.

Been reading through this thread: https://www.physicsforums.com/threads/infinite-versus-finite-space.924230/ which has helped a little. Seems like what's called spacetime is a theoretical context for the big bang and the current known universe. By "known" I don't mean observable, but the universe that is expanding within this infinite hypothetical context. When I read that each cubic m of space contains 10^-10 joules of dark energy, it can't be referring to an infinite amount of space, can it? But only the space within the 90 billion ly's across of universe that is expanding.

 

[PeterDonis]

Seems like what's called spacetime is a theoretical context for the big bang and the current known universe.

"Spacetime" is a general term in relativity; it refers to any 4-dimensional geometry that is a solution of the Einstein Field Equation.

Our current cosmological model uses a particular solution of the EFE--i.e., one particular spacetime geometry out of an infinite number of possible ones--to describe our universe.

the universe that is expanding within this infinite hypothetical context

This is fine as long as you understand that the statement "the universe is expanding" means nothing more than "the particular spacetime geometry our current model uses to describe the universe has a particular 4-dimensional shape". In other words, most of the connotations of the word "expanding" do not apply. That's one reason why physicists use math to do physics, not ordinary language; ordinary language is too vague and misleading.

When I read that each cubic m of space contains 10^-10 joules of dark energy, it can't be referring to an infinite amount of space, can it?

Yes, it can. That's what our current cosmological model says.

 

[jbriggs444]

but the universe that is expanding within this infinite hypothetical context

The hypothetical infinite thing is the entire universe, observable and not. It can be thought of as a completed four-dimensional whole (a "block universe"). It is not a context within which something else expands.

Edit: @PeterDonis explained it better, it seems.

 

[Chris Miller]

There was no universe at $t = 0$. The "instant" $t = 0$ is not part of the model; it's a limit point that doesn't actually exist.

Of course, I meant approaching t=0.

The observable universe is not the same as the entire universe. All of the discussion in Carroll's article that you linked to refers to the entire universe, not just the observable universe. So the fact that the observable universe has a finite size is irrelevant here.

Isn't the size of the observable universe determined by the speed of light, while the entire universe is the size to which we believe it has expanded. This contextual infinite spacetime in which it's expanding is new to me. Thanks.

 

[phinds]

Isn't the size of the observable universe determined by the speed of light, while the entire universe is the size to which we believe it has expanded.

Only if the entire universe is finite. The current consensus seems to be that it is most likely infinite in extent. Personally, I have a hard time getting my head around the concept that the universe is infinite in extent (and therefor of course has ALWAYS been infinite in extent] but I also have a hard time getting my head around the concept of a finite universe because then you have to worry about what shape it is and other things. Basically, I have a hard time getting my head around any of this. I have a small head.