Does the expansion of the universe also cause an increase in overall entropy?

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Discussion Overview

The discussion centers on the relationship between the expansion of the universe and the increase in overall entropy. Participants explore theoretical implications, interpretations of entropy in cosmology, and the role of dark energy, while addressing various models and perspectives on the topic.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that if the expansion of the universe increased entropy, it would clarify many phenomena, but they also acknowledge that this raises further questions.
  • Charlie Lineweaver's perspective is presented, arguing that the expansion itself does not increase entropy, as the entropy density remains constant in a comoving volume due to the relationship between temperature and volume during expansion.
  • One participant emphasizes the duality of heat and space in the universe, proposing that the balance between localized heat and expanded coolness complicates the understanding of entropy in the context of expansion.
  • Another participant asserts that the entropy of the universe as a whole is increasing, referencing black holes and their relationship to entropy, while challenging the notion that expansion alone accounts for entropy changes.
  • Some participants express confusion about the second law of thermodynamics and its application in cosmological contexts, indicating a need for deeper understanding.
  • There are discussions about the implications of dark energy, with some arguing that it complicates the relationship between expansion and entropy, while others suggest it may lead to a higher entropy state.
  • One participant raises questions about the nature of entropy measurement and the implications of reaching a heat death state, where microstate variety may diminish.
  • Another participant mentions that classical formulas for entropy suggest that a larger universe, being colder, should lead to an increase in entropy, although this is noted as a classical rather than quantum perspective.

Areas of Agreement / Disagreement

Participants express multiple competing views on the relationship between universe expansion and entropy, with no consensus reached. Some support the idea that expansion does not inherently increase entropy, while others argue that the overall entropy of the universe is indeed increasing.

Contextual Notes

Participants note the complexity of the universe's entropy, particularly in relation to dark energy and black holes, and highlight the unresolved nature of various calculations and definitions surrounding entropy in cosmological contexts.

  • #31
Wow that actually helps a lot. Have you taken lots of physics courses? or do you just read a lot.
 
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  • #32
My training was in biology and neuroscience. Which led to systems science and second law/open systems models.
 
  • #33
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 thing is, you are using this quote as support for your argument:

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.

The problem with the statement that the expansion causes no increase in entropy when given by this argument is that the argument above depends critically upon the contents of the universe. While it is true that this is the case for radiation domination, it won't work for other cases. Radiation is nice because it doesn't clump, so gravitational entropy never comes into account. But matter does clump, a process that changes in entropy quite significantly with time, and the way in which matter clumps depends critically upon the expansion. So I would still disagree with the statement that the expansion alone does not cause an increase in entropy: it doesn't necessarily cause an increase, as it depends upon the contents of the universe, as well as the future fate (e.g. if there is any positive cosmological constant at all, then even a radiation-dominated universe will have increasing entropy as the horizon scale grows with time).
 
  • #34
Well... 2nd law applies, as we now, to equilibrium state. A dynamic system is usually seen as a succession of equilibrium states, but it is not really true. There are studies about how thermodynamic principles applies to chaotic states, as a pan of water on fire. So, I do not know about total entropy, but local one might be impacted by expansion, I suppose.
 
  • #35
So if a system settles into its state of maximum entropy, wouldn't that dismiss the need for a big crunch?
 
  • #36
Entropee said:
So if a system settles into its state of maximum entropy, wouldn't that dismiss the need for a big crunch?
I don't understand, why would there ever be a need for a big crunch?
 
  • #37
Well expansion speed does not exceed escape velocity, so wouldn't mutual gravitational attraction of matter cause it to contract at some point?
I read that somewhere, not sure if it's true.
 
  • #38
Entropee said:
Well expansion speed does not exceed escape velocity, so wouldn't mutual gravitational attraction of matter cause it to contract at some point?
I read that somewhere, not sure if it's true.
This sort of idea leads to what is known as the "Jean's Length". Below the Jean's length, the typical speeds of particles are indeed lower than the escape velocity, and so stuff collapses, leading to things like galaxies, galaxy clusters, etc. Above the Jean's length, this isn't the case, and stuff continues to move apart.

Remember that due to the falloff of gravity, the escape velocity depends upon how far away from the center of mass you are.
 
  • #39
Oh that's a good point, so how would the escape velocity change as universe expands?
 
  • #40
Entropee said:
Oh that's a good point, so how would the escape velocity change as universe expands?
Well, escape velocity is more a local effect. So it isn't really changed by the expansion. The expansion basically just sets the initial conditions.
 

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