Expanding universe and thermodynamics

[Paul Howard A]

As I understand it, space continues to expand everywhere. But bound systems such as atoms and solar systems stay the same size due to local interactions. If this be true, should not energy be continually released by this "falling in" of matter within every atom, molecule, and planetary system?

If so, has this been accounted for in the energy budget of the universe?Paul Allen

 

[phinds]

As I understand it, space continues to expand everywhere. But bound systems such as atoms and solar systems stay the same size due to local interactions. If this be true, should not energy be continually released by this "falling in" of matter within every atom, molecule, and planetary system?

If so, has this been accounted for in the energy budget of the universe?Paul Allen

What "falling in" ? I can't understand what you are talking about. Can you state it differently?

Nothing happens to the items you describe due to the expansion of the universe. Why do you think it does? Again, what does "falling in" mean to you?

 

[Paul Howard A]

What "falling in" ? I can't understand what you are talking about. Can you state it differently?

Nothing happens to the items you describe due to the expansion of the universe. Why do you think it does? Again, what does "falling in" mean to you?

Perhaps that is where I misunderstand. But there is space between the components of an atom, and certainly between the bodies in a solar system. That space is expanding. In order for the masses to remain bound, should there not be a conversion of potential to kinetic energy?

 

[Paul Howard A]

Do not the forces of expansion apply everywhere? Even inside atoms?

 

[phinds]

Yes that is definitely where you misunderstand. The expansion of the universe takes place only on the scale ABOVE galactic clusters. Galactic clusters, galaxies, stars, planets, you, me, atoms, and so forth are not affected.

 

[Paul Howard A]

Thank you, "phinds".

I believe I do understand the reality that objects are moving apart only at large scales, but I thought I also understood that space itself was expanding subsequent to the Big Bang. If space itself is expanding, then there should be small scale implications as well. And since we do not observe bound objects to be moving apart, how does one explain this? My initial query was directed to the thought that small scale bonding enforces small scale structure.

As the space inside a small scale structure expands, the components must simultaneously "collapse" to maintain their original relationships. And then there should be a conversion of potential to kinetic energy. Something we apparently do not observe. So I must have something wrong.

Is not space expanding everywhere? and, if not, what force or phenomenon could act only on scales larger than galactic clusters?

 

[phinds]

The force of "dark energy" is so staggeringly tiny and it has absolutely no effect on bound objects, as I have already said. It's like an ant pushing on a house. It's not that the ant's pushing has a really tiny effect, it's that it has absolutely no effect at all because the ant can't shift the house off of its foundation even a little. Space does not expand in bound objects.

Even out in intergalactic space, where it DOES have an effect, Dark Energy has such an infinitesimal effect on local scales that it's just about not there. My favorite way of remember this is that if you could magically paint parking place stripes in intergalactic space, you'd have to wait something like 20 BILLION years before the lines got far enough apart to park a second car.

On cosmological scales, Dark Energy has a huge effect, and objects at the edge of our observable universe are receding from us at about 3c.

 

[Orodruin]

Let me add to phind's answer that your concepts of potential and kinetic energies get completely roughed up in general relativity. In classical mechanics and special relativity, global energy conservation is based on invariance under time translations, which in general no longer holds in GR.

 

[Drakkith]

Even if expansion is taking place on our scale, it manifests as a very, very small outward force. To put it in perspective, expansion would mean that the radius of Earth's orbit would be about 1 nanometer (or some other absurdly small number) greater than it would be without expansion. This orbit would be stable and would not "collapse" back to its original size. To do so would require the removal of the force, aka expansion would need to stop happening.

 

[Paul Howard A]

Thank you all for your considered responses. This is helpful and I'll try not to belabor the point.

phinds: I get it that dark energy &/or the momentum of expansion have miniscule/undetectable effects at small distances. But is the result of an ant pushing on a house really absolutely nothing? The base of the house may not shift, but the structure would deform the tiniest amount and, in the end, there would be the tiniest transfer of energy manifest as heat, warming of the house. And any dark energy applied to a gravitationally bound system in space will certainly effect the orbits of it's components, even if the effects are so small as to be undetectable.

Orodruin: Why do we need to invoke relativistic properties? In the small systems that we are discussing, small speeds and weak gravitation are operational.

Drakkith: You seem to be acknowledging that space does or can expand in bound systems. This would necessarily mean that either closed systems do expand or that my contention has merit: ie, energy is released in the process of maintaining spatial relationships. This would apply to every atom in every body inside of every galactic cluster in the universe. Although the effects on an atom may be miniscule, there are a lot of atoms...

Thanks for your patience. I'm only being argumentative in pursuit of clarity.

 

[Drakkith]

Drakkith: You seem to be acknowledging that space does or can expand in bound systems.

I make no such acknowledgment. It's actually a complicated issue. The model that describes how the universe expands has to assume (for calculation purposes) that the universe is homogeneous. Obviously it isn't, as we have stars, planets, and other compact objects instead of just a diffuse gas spread throughout the universe. The models that deal with spacetime closer to our scale don't/can't take into account the effect of expansion. (Remember that expansion isn't caused by dark energy. Dark energy causes the acceleration of the expansion)

So we're kind of stuck trying to fit two different models together into one description of the universe. We actually don't know if expansion is just overwhelmed by attractive forces in bound objects, or if it literally doesn't even occur.

This would necessarily mean that either closed systems do expand or that my contention has merit: ie, energy is released in the process of maintaining spatial relationships. This would apply to every atom in every body inside of every galactic cluster in the universe. Although the effects on an atom may be miniscule, there are a lot of atoms...

If expansion affects us then spatial relationships aren't maintained. The force of the expansion on a steel bar would be just that. A force. It would pull on the bar with a very, very weak force, far weaker than required to pull the bar apart, and negligible to the point of dismissal in almost every way, but still there. The bar would be very slightly larger than it otherwise would be in the absence of expansion.

 

[Orodruin]

Orodruin: Why do we need to invoke relativistic properties? In the small systems that we are discussing, small speeds and weak gravitation are operational.

We are talking about the expansion of space. This is a pure GR effect. As you say it is small and as Drakkith told you, its effects would be small even if it was happening at our scales, but it still is a GR effect and if you want to consider it at all you are doing relativity whether you want to or not.

I guess you could try something similar ad-hoc in classical mechanics. The result would still be loss of time translational invariance and thus loss of energy conservation.

 

[Paul Howard A]

You have all been very helpful. Thank you.

I sense that part of my confusion may be semantic. I was thinking of space as kind of a pervasive fabric, but perhaps it is better defined by measurement or distance relationships. In that sense, space really does not expand inside bound systems as phinds was trying to tell me from the get go.

 

[phinds]

You have all been very helpful. Thank you.

I sense that part of my confusion may be semantic. I was thinking of space as kind of a pervasive fabric, but perhaps it is better defined by measurement or distance relationships. In that sense, space really does not expand inside bound systems as phinds was trying to tell me from the get go.

Right. It is most helpful to think of space as just a framework. There is a LOT of discussion on this forum as to whether or not "space" is a "thing" but the consensus is clearly on the side of it just being a framework inside which things move. The expansion of the universe is therefore described as "Metric expansion" (worth a Google if you haven't already)

 

[Paul Howard A]

Thank you, the links to metric expansion speak directly to the source of my confusion. And I see the link to general relativity.

Now, all I need is a primer in differential geometry, metric tensors, and comoving coordinates.

I find the whole framework deeply disturbing.

 

[Drakkith]

I find the whole framework deeply disturbing.

Why's that?

 

[Paul Howard A]

Mostly, I'm frustrated that I lack the mathematical tools to deeply understand the concepts.

I also sense a situation analogous to the ancient's model of celestial spheres. As refined observations were made, the great thinkers and mathematicians of the time created more and more overlapping shells upon which to embed the moving lights they saw in the sky. The model became progressively unwieldy, yet many were so invested they were loath to discard it.

Dark matter and dark energy seem much like shells added to an already teeteringly complex model.

 

[phinds]

Dark matter and dark energy seem much like shells added to an already teeteringly complex model.

I completely disagree. These two separate and unrelated issues are STRONGLY supported by empirical evidence and the placeholder names of "dark ..." just mean "OK, SOMETHING is happening here and we can describe what it's doing but we don't know what is doing it". There is no attempt to add layers of complication.

 

[Paul Howard A]

OK, there is evidence that "something" is/has happened, But those labels of dark energy and dark matter do nothing to further our understanding. And as a consequence of those labels we are actively looking for hidden 'matter' which may not exist and mysterious 'energy' which may not exist. There may be other explanations for the binding of galaxies and for
accelerated expansion.

The observations may be solid, but the concepts to explain them may be shells...?
The ancients made new solid observations and then modified their models just as we are. Eventually the model became unwieldy and was discarded.

 

[Drakkith]

Dark matter and dark energy seem much like shells added to an already teeteringly complex model.

Teeteringly complex model? As far as I understand, the model is actually very, very simple. One major idea, that the universe is expanding, appears to explain almost everything we see in the universe in regards to its structure and formation. Similarly, the idea that the Sun was the center of the solar system, not the Earth, appeared to explain nearly everything that astronomy had seen up to that time. There were of course many other questions to ask and to answer, but one key idea greatly simplified their early model of the universe.

Dark matter and dark energy were only added after decades of observations showed that something was amiss. They have so far proven to be the simplest and most accurate ways of explaining our observations. When it comes to dark matter, the idea isn't even that big of a leap. Physicists discovered what could be considered a type of dark matter back in the 50's. Neutrinos. I see no reason there can't be some other particles with similar properties out there that we simply haven't been able to find/create in the lab.

 

[Paul Howard A]

I like thermodynamics. I thought I could trust thermodynamics. Intuitive, statistical,...solid. Expansion seems like a simple idea. But I don't know how to marry it to thermodynamic budgetry.

There is abundant evidence for the big bang, but where did the energy come from? Then there was inflation, but where did the energy come from? And now there is accelerated expansion. But where is the dark energy?

It doesn't seem that simple to me...

We're making solid, good observations but stretching credulity to patch up the existing theory.

 

[Drakkith]

There are either answers to your questions or very good reasons why we don't yet have the answers. For starters, if you are thinking of the big bang as some kind of single event that created everything, then you are already starting off with a misunderstanding of what the big bang is. At no point in our model do we ever have something created from nothing.

 

[Paul Howard A]

There are either answers to your questions or very good reasons why we don't yet have the answers. For starters, if you are thinking of the big bang as some kind of single event that created everything, then you are already starting off with a misunderstanding of what the big bang is. At no point in our model do we ever have something created from nothing.

You took no issue when I described the expansion as possessing momentum. Does it not take energy to initiate a change in momentum? And what kind of energy or energy field could be so pervasive as to cause uniform expansion between the far flung galactic clusters and yet have NO effect on the orbits of bodies within those clusters?

Simple?

A sufficiently sophisticated set of ideas may seem indistinguishable from magic to the ignorant. This seems like magic to me.

 

[Drakkith]

You took no issue when I described the expansion as possessing momentum. Does it not take energy to initiate a change in momentum?

I don't know the math of GR well enough to talk about this. Cosmological expansion and energy/momentum are tricky to discuss unless both sides are up to speed on the math.

And what kind of energy or energy field could be so pervasive as to cause uniform expansion between the far flung galactic clusters and yet have NO effect on the orbits of bodies within those clusters?

I don't think there needs to be a field to drive "normal" (non-inflationary) expansion. I know there are some theories where gravity actually becomes repulsive at great densities, which can lead to a "bounce" scenario and drive expansion without requiring some sort of field

As for expansion not occurring within galaxy clusters, I've read two explanations:

1. The expansion manifests as a sort of "counter-force", pulling everything apart. The strength of this force is proportional to the rate of expansion and the distance between the objects. The current rate of expansion is so slow that this repulsive force doesn't overpower the attractive force of gravity until the objects are hundreds of millions of light-years apart, give or take some depending on the exact strength of gravity between them. To rip apart something like a galaxy cluster or a galaxy the rate of expansion would need to increase drastically.

2. Galaxies are expanding now because they were all flung away from each other in the past by some sort of repulsive force (maybe inflation, maybe something else) and have been drifting under inertia every since. In this case gravity simply attracts things and if the strength is high enough then the objects slow and get bound together. Expansion doesn't affect them because gravity has already stopped the expansion between them.

Simple?

A sufficiently sophisticated set of ideas may seem indistinguishable from magic to the ignorant. This seems like magic to me.

Yes, the overall idea is quite simple. I'm sorry this seems like "magic" to you. The model is well developed, supported by both theory and observation, and is overwhelmingly accepted by professional scientists around the world.

 

[Drakkith]

Odd, I didn't realize this was still in the Classical Physics section. I've moved it to the Cosmology section where our members who are more knowledgeable in this area usually set up camp.

 

[Adrian B]

Space does not expand in bound objects.

Doesn't the "Big Rip" hypothesis depend on space expanding within bound objects?

 

[Chalnoth]

As I understand it, space continues to expand everywhere. But bound systems such as atoms and solar systems stay the same size due to local interactions. If this be true, should not energy be continually released by this "falling in" of matter within every atom, molecule, and planetary system?

If so, has this been accounted for in the energy budget of the universe?Paul Allen

Expansion is an effect due to the action of gravity on matter. That exact same theory of gravity that describes the expansion of space also says that the Earth will remain in orbit around the sun pretty much indefinitely*.

Basically, on average space is expanding. But local systems that are gravitationally-bound, such as solar systems, galaxies, and galaxy clusters do not have space expanding within them. This actually drops right out of the equations (though the math is a bit difficult to work out).

* Not actually indefinitely, because the Sun will go red giant eventually.

 

[Chalnoth]

Doesn't the "Big Rip" hypothesis depend on space expanding within bound objects?

The way the big rip hypothesis works is it postulates a type of energy that fills the entire universe whose density increases with time. This energy that fills the universe adds an additional repulsive component to gravity. And since in the big rip hypothesis, its energy density increases over time, this additional repulsive component pushes apart matter at smaller and smaller scales as time goes on.

 

[phinds]

Doesn't the "Big Rip" hypothesis depend on space expanding within bound objects?

Yes, and the big rip hypothesis is now considered dead, I believe.

 

[PeterDonis]

I thought I could trust thermodynamics. Intuitive, statistical,...solid. Expansion seems like a simple idea. But I don't know how to marry it to thermodynamic budgetry.

That's because the thermodynamic budgetry you are used to is based on an underlying assumption of time translation invariance (which someone mentioned in a previous post). The expansion of the universe breaks time translation invariance; that means the thermodynamic budgetry you learned no longer holds in the form you learned it. There are ways of constructing a modified budgetry, but it still has limitations.

This article by Sean Carroll is worth reading in this connection:

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

 

[Paul Howard A]

That's because the thermodynamic budgetry you are used to is based on an underlying assumption of time translation invariance (which someone mentioned in a previous post). The expansion of the universe breaks time translation invariance; that means the thermodynamic budgetry you learned no longer holds in the form you learned it. There are ways of constructing a modified budgetry, but it still has limitations.

This article by Sean Carroll is worth reading in this connection:

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

Excellent link, PeterDonis. Directly addresses the question.