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Author Topic: how does entropy relate to dark energy?  (Read 8539 times)

Offline chiralSPO

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how does entropy relate to dark energy?
« on: 22/04/2015 14:01:59 »
If we imagine the universe as a quantity of ideal gas, expanding into a pre-existing void (not including dark energy expansion yet), this expansion is easily explained as being "driven" by entropy: w, and therefore log(w), increases as the gas particles are distributed over more space.

If instead we view the gas as already expanded to fill some container, but then the container begins to expand (only dark energy expansion now), presumably the larger volume allows for greater log(w) in the same way. It seems to me that there shouldn't be any difference between expanding a container by standard means or by spatial expansion. But, if there is no difference, the entropy increase of either such system should be a function of the amount of gas in the system.

Does the entropic contribution to the free energy required for expansion affect the rate of expansion on any local basis? (is it more favorable for hot and/or dense regions of the universe to expand than for cold and sparse regions? is there any observation of different rates of expansion in different parts of the universe?)


 

Offline jccc

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Re: how does entropy relate to dark energy?
« Reply #1 on: 22/04/2015 15:18:50 »
after the 2 monks banged the truth out of the big bang

dark matter vanished into quantum field

dark energy turned over

dark age is over

over


 

Offline PmbPhy

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Re: how does entropy relate to dark energy?
« Reply #2 on: 22/04/2015 16:39:36 »
Quote from: chiralSPO
If we imagine the universe as a quantity of ideal gas, expanding into a pre-existing void (not including dark energy expansion yet), this expansion is easily explained as being "driven" by entropy: w, and therefore log(w), increases as the gas particles are distributed over more space.
I don't understand how you see the gas expanding.

Quote from: chiralSPO
If instead we view the gas as already expanded to fill some container, but then the container begins to expand (only dark energy expansion now), presumably the larger volume allows for greater log(w) in the same way. It seems to me that there shouldn't be any difference between expanding a container by standard means or by spatial expansion. But, if there is no difference, the entropy increase of either such system should be a function of the amount of gas in the system.

Does the entropic contribution to the free energy required for expansion affect the rate of expansion on any local basis? (is it more favorable for hot and/or dense regions of the universe to expand than for cold and sparse regions? is there any observation of different rates of expansion in different parts of the universe?)
If the expansion of the gas is an isentropic process then the entropy remains constant. An isentropic process is a process in which entropy remains constant. Such processes occur during adiabatic processes. If you can find an argument which concludes that such a process as you've described is an isentropic one then the entropy will remain unchanged. Unfortunately I don't know the answer to such a question. Sorry.
« Last Edit: 23/04/2015 04:45:05 by PmbPhy »
 

Offline chiralSPO

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Re: how does entropy relate to dark energy?
« Reply #3 on: 22/04/2015 18:28:50 »
Quote from: chiralSPO
If we imagine the universe as a quantity of ideal gas, expanding into a pre-existing void (not including dark energy expansion yet), this expansion is easily explained as being "driven" by entropy: w, and therefore log(w), increases as the gas particles are distributed over more space.
I don't understand how you see the gas expanding.

I'm not sure I understand your point of confusion. I am trying to think of the matter in the universe as an ideal gas, and rationalize the expansion of the universe on this basis.

The expansion of an ideal gas into a vacuum does not change the enthalpy of the system (as long as no work is done), so entropic considerations must be dominant.

If the expansion of the gas is an isentropic process then the entropy remains constant. An isentropic process is a process in which entropy remains constant. Such processes occur during adiabatic processes. So if you can find an argument which concludes that such a process as you've described is an adiabatic one then the entropy will remain unchanged. Unfortunately I don't know the answer to such a question. Sorry.

Expansion of gas is not isentropic--the degrees of freedom of each gas particle increases as the gas has more volume to sample, so entropy increases unless there is some other simultaneous counterbalancing phenomenon.

I don't think whether a process is adiabatic or not has anything to do with the change in entropy associated with that process. Entropy is a state function, so it shouldn't matter how the process occurred, only that it did.
 

Offline PmbPhy

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Re: how does entropy relate to dark energy?
« Reply #4 on: 22/04/2015 20:15:28 »
Quote from: chiralSPO
... so it shouldn't matter how the process occurred, only that it did.
Not true at all. However to explain why would require an in-depth discussion of thermodynamics and I hate thermodynamics. Lol!  Plus it's been a long long time since I've thought about it so I've forgotten a great deal of it. However this is one thing that I do remember.
 

Offline chiralSPO

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Re: how does entropy relate to dark energy?
« Reply #5 on: 22/04/2015 20:51:31 »
If a process is isentropic and reversible it must be adiabatic. But it does not follow that adiabatic proceses are isentropic, or that isentropic proceses are adiabatic.

And if you are saying that entropy is not a state function: here is one link to a proof showing it is https://www.khanacademy.org/science/physics/thermodynamics/v/proof-s-or-entropy-is-a-valid-state-variable.
 

Offline yor_on

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Re: how does entropy relate to dark energy?
« Reply #6 on: 22/04/2015 21:52:08 »
Chiral? I'm not sure I follow it correctly here, but are you thinking of it as being equal (energy), acting as a gas containing kinetic energy. pushing/pressuring 'boundaries', to another model in where a gas is 'inert', but expanding/adapting to boundaries that 'move'?
=

The first one is a real headache, if it is correct. As I don't see that 'energy' in any measurements except inferred from a accelerating expanding universe. If we then assume this energy to also be dynamically produced inside a container one has to define from where. And if we then use the way you defined it before, as happening 'in each point', which seem the logical way to look at it to me too, it becomes even trickier, from a container model that is. There are alternatives, as a holographic universe etc, in where one might be able to define it (energy) differently possibly?

so yes, I think I see why you took up the other model, but in that one you still assume a container of some sort, expanding but now also diluting its 'energy' as I presume a defined amount to exist.

Maybe it is possible to think of it as if each patch of SpaceTime I measure on belongs to the same principles rules etc. In that case, for it to exist and be found to expand, it 'lends' from what's outside a arrow. Energy is after all a coin of exchange, and if a arrow is a construction, and limits, then what's unmeasurable (inside our universe, and local arrow) can have as much 'energy' as you like. Thinking that way conservation laws is a artifact belonging to a arrow. then again, so would we be :)

What it means would then be that first came principles, rules etc, they setting limits 'c'. Or better expressed, 'c' is your local arrow, casualty, and the reason we are able to define physics. It is this last one I like the most, because you get away from the container, although it still will exist in our measurements. Getting away from it allow 'energy' to be in a equilibrium, inside what we measure in time on. And even thought the universe then can be defined as 'infinite' it also will be able to inflate as well as expand. There is no container in the usual mean, if one use that one, instead we find conservation laws, constants and limits. And we need them, to exist.

(I'm not referring to 'virtual particles' btw. I don't like that interpretation, although it's highly usable it constricts you. A way to define it would be from the Big Bang initiation. Even before that first light 'moving', you then must have those limits, rules principles etc secured. Which makes a lot of sense to me, assuming 'c' to only be a speed doesn't, though :)
« Last Edit: 22/04/2015 23:09:43 by yor_on »
 

Offline PmbPhy

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Re: how does entropy relate to dark energy?
« Reply #7 on: 23/04/2015 04:46:19 »
Quote from: chiralSPO
If a process is isentropic and reversible it must be adiabatic. But it does not follow that adiabatic proceses are isentropic, or that isentropic proceses are adiabatic.
Yes. I know that all too well. I simply made a mistake when I wrote that post. I've now corrected it.

Quote from: chiralSPO
And if you are saying that entropy is not a state function: ...
Why would you think I said or implied such a thing?
 

Offline chiralSPO

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Re: how does entropy relate to dark energy?
« Reply #8 on: 24/04/2015 15:56:55 »
Quote from: chiralSPO
If a process is isentropic and reversible it must be adiabatic. But it does not follow that adiabatic proceses are isentropic, or that isentropic proceses are adiabatic.
Yes. I know that all too well. I simply made a mistake when I wrote that post. I've now corrected it.
Ok, I figured it was just a mistake, but I wasn't sure my point hadn't been misunderstood.

Quote from: chiralSPO
And if you are saying that entropy is not a state function: ...
Why would you think I said or implied such a thing?
[/quote]

I thought you said or implied this in your quote below, arguing against my (complete) statement:
I don't think whether a process is adiabatic or not has anything to do with the change in entropy associated with that process. Entropy is a state function, so it shouldn't matter how the process occurred, only that it did.
Quote from: chiralSPO
... so it shouldn't matter how the process occurred, only that it did.

Not true at all. However to explain why would require an in-depth discussion of thermodynamics and I hate thermodynamics. Lol!  Plus it's been a long long time since I've thought about it so I've forgotten a great deal of it. However this is one thing that I do remember.

Nothing to get ourselves into a tizzy over. I understand that you don't enjoy thermo (not many people do).
 

Offline chiralSPO

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Re: how does entropy relate to dark energy?
« Reply #9 on: 24/04/2015 16:19:50 »
Chiral? I'm not sure I follow it correctly here, but are you thinking of it as being equal (energy), acting as a gas containing kinetic energy. pushing/pressuring 'boundaries', to another model in where a gas is 'inert', but expanding/adapting to boundaries that 'move'?
=

The first one is a real headache, if it is correct. As I don't see that 'energy' in any measurements except inferred from a accelerating expanding universe. If we then assume this energy to also be dynamically produced inside a container one has to define from where. And if we then use the way you defined it before, as happening 'in each point', which seem the logical way to look at it to me too, it becomes even trickier, from a container model that is. There are alternatives, as a holographic universe etc, in where one might be able to define it (energy) differently possibly?

so yes, I think I see why you took up the other model, but in that one you still assume a container of some sort, expanding but now also diluting its 'energy' as I presume a defined amount to exist.

Maybe it is possible to think of it as if each patch of SpaceTime I measure on belongs to the same principles rules etc. In that case, for it to exist and be found to expand, it 'lends' from what's outside a arrow. Energy is after all a coin of exchange, and if a arrow is a construction, and limits, then what's unmeasurable (inside our universe, and local arrow) can have as much 'energy' as you like. Thinking that way conservation laws is a artifact belonging to a arrow. then again, so would we be :)

What it means would then be that first came principles, rules etc, they setting limits 'c'. Or better expressed, 'c' is your local arrow, casualty, and the reason we are able to define physics. It is this last one I like the most, because you get away from the container, although it still will exist in our measurements. Getting away from it allow 'energy' to be in a equilibrium, inside what we measure in time on. And even thought the universe then can be defined as 'infinite' it also will be able to inflate as well as expand. There is no container in the usual mean, if one use that one, instead we find conservation laws, constants and limits. And we need them, to exist.

(I'm not referring to 'virtual particles' btw. I don't like that interpretation, although it's highly usable it constricts you. A way to define it would be from the Big Bang initiation. Even before that first light 'moving', you then must have those limits, rules principles etc secured. Which makes a lot of sense to me, assuming 'c' to only be a speed doesn't, though :)

I guess this model requires a "container" universe, but I do not think it needs to be finite, for the model to be valid (I'll have to ponder this one a little longer...) Dilution of energy and matter should increase entropy on any scale unless, somehow the expanding universe doesn't actually increase the number of states the system could adopt.

Another thought experiment: you have eight boxes arranged in a 2x2x2 cube. 2 boxes are red, and the rest are blue. There are 8!(40320) different ways the boxes can be assembled into the cube (only three distinct arrangements though). If each of the boxes grows by a factor of 8 (doubles in each dimension), there is no difference in the number of ways this larger cube can be assembled. However, if each of the new larger boxes is cut into 8 boxes of the original size, there are now 64! (1.27x1089) different ways the boxes can be assembled into a cube (and somewhere between 6561 and 1.1x1011 unique arrangements--I'm too lazy to actually figure it out right now, so lower and upper limits will have to suffice).

So if the expansion of the universe is just a dilation that increases the scale of the coordinates we use, then I don't think it would increase the overall entropy. But, if the expansion is actually increasing the amount of information needed to describe the state of the universe, then entropy should increase with the expansion. (don't let the example of the boxes confuse you--I'm not suggesting that there are more particles in a larger universe, only that the resolution of the universe might be increasing)
 

Offline Atomic-S

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Re: how does entropy relate to dark energy?
« Reply #10 on: 25/04/2015 05:47:48 »
Quote
So if the expansion of the universe is just a dilation that increases the scale of the coordinates we use, then I don't think it would increase the overall entropy. But, if the expansion is actually increasing the amount of information needed to describe the state of the universe, then entropy should increase with the expansion
That is an interesting point: inasmuch as no one really knows what dark matter or dark energy are, how do we even know that they have entropies?
 

Offline PmbPhy

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Re: how does entropy relate to dark energy?
« Reply #11 on: 25/04/2015 13:15:59 »
Quote from: chiralSPO
I'm not sure I understand your point of confusion.
I mean normally a gas is in a container and when the container expands (e.g. a piston of a combustion chamber expands) the gas does work on the walls. But in interstellar space there's no wall to exert any forces on. For that reason I'm a bit lost as to how to treat this problem. Not to mention that it's been a decade since I even thought about thermodynamics. Lol!
 

Offline yor_on

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Re: how does entropy relate to dark energy?
« Reply #12 on: 25/04/2015 19:35:10 »
Chiral, I enjoy your thinking. Although, isn't there some presumptions you make ? As "however, if each of the new larger boxes is cut into 8 boxes of the original size, there are now 64! (1.27x1089) different ways the boxes can be assembled into a cube (and somewhere between 6561 and 1.1x1011 unique arrangements-" When you define it that way you define a finite volume, don't you? If space isn't a volume, except as defined from a inside, must that hold true?
==

Think of observer dependencies, then instead define it locally. The idea behind mathematics is that it covers a 'whole universe', but if you think of whom is doing those experiments, proving those mathematics, then they all are done locally.
« Last Edit: 25/04/2015 19:40:22 by yor_on »
 

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Re: how does entropy relate to dark energy?
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