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Entropee
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If the expansion of the universe DID increase entropy, it would explain a lot of things but also leave me with a lot more questions.
also this paper says otherwise... http://adsabs.harvard.edu/full/1991Ap&SS.186..157U
and so do many other sites.
Entirely, because it neglects dark energy.Entropee said:So do you disagree with what apeiron posted here?
Those aren't the only two possibilities. And a big rip is exceedingly unlikely.apeiron said:I deliberately left out dark energy because that is a further complication indeed. We have to agree whether it is even a constant cosmological constant or a big rip accelerating acceleration for a start.
No. I think I sufficiently explained myself in my previous post, and this is not it. The argument for how entropy is related to expansion in the presence of dark energy doesn't pay any attention to the complexity of the universe. It just shows that a universe with matter is lower entropy than one without.apeiron said:But presuming the constant case, what are you actually saying about dark energy? That it over-cools the universe by moving things even further apart than would have been the case if it was only the kinetics of the big bang "inertially" dispersing itself, swapping heat for sink?
Chalnoth said:No. I think I sufficiently explained myself in my previous post, and this is not it. The argument for how entropy is related to expansion in the presence of dark energy doesn't pay any attention to the complexity of the universe. It just shows that a universe with matter is lower entropy than one without.
It's a pretty well-known result, but here's a recent paper that goes a bit into these calculations of entropy:apeiron said:Can you please supply references to this argument. It is new to me and I'm not following its logic at all.
They can be measured in the same way one measures the microstates of a black hole. And no, by the way, you're misunderstanding entropy by assuming that if everything is the same, that is a form of order. The exact opposite is true: in an equilibrium state, everything is the same. It is in low entropy configurations that you have interesting things happening, not high entropy ones.apeiron said:For example, in a heat death void at near absolute zero, where is the microstate variety that can scramble macro distinctions? If everything falls to a common lowest mode, it all starts to look orderly again.
Except if you take any individual system that we know of and isolate it, its entropy will increase with time (or, if it has reached equilibrium, stay the same). In fact, I would say this fact is guaranteed by the definition we use for entropy: an overall decrease would require a system to spontaneously transition from a highly-probable configuration to a very improbable one.Entropee said:Well first off, according to the 2nd law of thermodynamics, entropy SHOULD increase, being that the universe is finite in extent. However if there is to be a 0 net increase in entropy you would need to take into account EVERYTHING that can possibly decrease or eliminate entropy. And some of which we don't know very much about, like dark energy.
The nice thing about black holes, however, is that their properties actually make it so that we can calculate their entropy exactly, even without knowing all of the details of the underlying quantum mechanics.Entropee said:Also what happens to the entropy of matter that falls into a black hole? This problem involves quantum mechanics AND gravity, not something were very good at explaining at the moment.
Pretty much, yes. Basically when things collapse, the probability that their angular momentum is zero is vanishingly small. So basically things would start to spin around one another at high velocity instead of collapsing together.Entropee said:And does entropy prevent the big crunch?
Chalnoth said:Except if you take any individual system that we know of and isolate it, its entropy will increase with time (or, if it has reached equilibrium, stay the same). In fact, I would say this fact is guaranteed by the definition we use for entropy: an overall decrease would require a system to spontaneously transition from a highly-probable configuration to a very improbable one.
No. Low entropy = less probable. High entropy = more probable. This is fundamentally why total entropy tends to increase (and large numbers of the constituent particles ensure that "tends to increase" equals "always increases" for all intents and purposes).PrometteusBR said:Well, a decrease of entropy is the oposite of said, isn´t?
I mean a smaller entropy means a system more probable, so a decrease of entropy require a system transition from a high improbable to a very probable with less internal configurations.
What you think?
Chalnoth said:by the way, you're misunderstanding entropy by assuming that if everything is the same, that is a form of order. The exact opposite is true: in an equilibrium state, everything is the same. It is in low entropy configurations that you have interesting things happening, not high entropy ones.
That may effectively apply to electromagnetic systems, but it doesn't appear to apply to gravitational ones, in particularly not de Sitter space.apeiron said:So you're not a big fan of the third law of thermodynamics I take it.
We've known for a long time now that the purely empirical laws of thermodynamics can be derived in full from statistical mechanics: by understanding the behavior of the constituents of the system, we can derive the system's thermodynamic behavior. And when we apply this to quantum mechanics, we find that entropy is proportional to the logarithm of the number of microstates that can replicate a given macrostate.apeiron said:So yes, we can see what cosmological mileage we can get out of simple Boltzmann entropy modelling. Not against that at all. But you mis-represent the certainty of that modelling. It makes necessary assumptions - assumptions that in thermo literature it would be normal to challenge.
It's given by the area of the horizon, just as with a black hole. This paper goes into a fair amount of detail on the entropy of de Sitter space: http://www.iop.org/EJ/article/0264-9381/5/10/013/cqv5i10p1349.pdfapeiron said:So putting aside black holes or any other matter, what is the entropy of actually empty space in a de sitter universe?
Chalnoth said:No. Low entropy = less probable. High entropy = more probable.
Entropee said:Teach me master :P
The thing is, you are using this quote as support for your argument:apeiron said:Gotta laugh. First I missed the point as I didn't presume dark energy. Now I miss the point because I do. Trouble is you just won't stick to the point and only seek to score points.
The entropy density s of a radiation field of temperature T is s ~ T3. The entropy S in a given comoving volume V is S = s V. Since the comoving volume V increases as the universe expands, we have V ~ R3. And since the temperature of the microwave background goes down as the universe expands: T ~ 1/R, we have the result that the entropy of a given comoving volume of space S ~ R-3 * R3 = constant. Thus the expansion of the universe by itself is not responsible for any entropy increase. There is no heat exchange between different parts of the universe. The expansion is adiabatic and isentropic: dSexpansion = 0.