Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Does Dark Energy drive up Entropy?

  1. Mar 7, 2017 #1
    So say I smash a glass plate on a chess board much larger than the plate. Simplistically, say entropy is the number of ways of rearranging the glass pieces across the squares of the board. Over time, it's likely that entropy increases since the glass would spread out, meaning each configuration would be just one combination. But in theory, it is still possible that the entropy can decrease (just an infintestimally low chance).

    However if the chess board expands in size until the glass pieces are no longer in causal contact with each other, then there is no probability( not even in theory) that the entropy would decrease.

    So macroscopically, we see that entropy does get driven up by the expansion of space. Is this true?

    But what about microscopically? Could the expansion of space at the level of atoms slightly affect the trajectories of particles so that the probabilty of re entering the original configuration with each small increment of time is slightly lower?
    Last edited: Mar 7, 2017
  2. jcsd
  3. Mar 7, 2017 #2


    User Avatar
    Science Advisor

    In the simple, classical analysis, yes, dark energy increases entropy, and it does so pretty dramatically when it is the dominant energy density.

    Microscopically, I think we can safely say that the value of the dark energy is much too small to have any noticeable impact.
  4. Mar 7, 2017 #3
    got it. but can we conjecture that dark energy and whatever drives up the entropy locally has a common origin?
  5. Mar 7, 2017 #4


    User Avatar
    Science Advisor

    In the far future, if the dark energy behaves like a cosmological constant, then the universe will eventually reach a state where the entropy density is a constant. Overall entropy increases because of the expansion of space (same entropy density + larger volume = more total entropy).

    It's not so easy to infer from this how the dark energy impacts the entropy of local, compact systems (such as stars and galaxies). Remember that a star has much higher entropy than a diffuse gas cloud, while a black hole has higher entropy still. Spread apart doesn't necessarily mean higher entropy. I honestly don't know what dark energy does to the entropy of matter.
  6. Mar 7, 2017 #5
    That makes sense. If the groups of galaxies keep moving further and further apart, the groups would be so small compared to the spacing in between that it would look like equilibrium is established.

    I think locally, things are becoming more compact(e.g with the merger of milky and and andromeda and all), so if dark energy has no local effect, then all of the seperate local groups should eventually contract under their own gravity such that they become seperate black holes meaning the entropy would be maximized locally. So maybe the curving/contraction of space(gravity) and the expansion of space(dark energy) are just flip sides of the same coin that leads higher entropy. That's only assuming that dark energy will never have a local effect.

    So no theories have been developed as to the local effects of dark energy? Does dark energy still expand space at the local level though, even if minutely?
    Last edited: Mar 7, 2017
  7. Mar 8, 2017 #6
    My understanding is most of a galaxy's constituents will eventually be ejected from the system, and everything left will join the central SMBH (see Wikipedia's article: Future of an expanding universe).

    I read an analogy on here the other day for the local effects of dark energy being like an ant pushing against a house; there is a transfer of energy, but not enough to do anything relative to the fundamental forces at work.
  8. Mar 8, 2017 #7


    User Avatar
    Science Advisor

    Right. But I'm pretty sure that this is just due to momentum transfers between particles due to close orbital interactions, not due to dark energy.

    It's similar in concept to when our space probes use "gravity assists" to gain speed: by carefully maneuvering the craft towards a planet or moon, they can get the craft to pick up kinetic energy from the planet or moon. This kind of thing happens periodically in galaxies as well, when stars make close passes of one another: one of them tends to lose kinetic energy, while the other gains it. This takes a very large amount of time, however, due to the vast distances involved.
  9. Mar 8, 2017 #8
    Agreed. My post was in response to the previous one I should have quoted for better context.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted