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Is the early universe, around the Big Bang, an example of high energy and low entropy state? Seems like we’ve been cooling off since, so we were hotter earlier

Is the early universe, around the Big Bang, an example of high energy and low entropy state? Seems like we’ve been cooling off since, so we were hotter earlier and negative temperatures would be hotter than non-negative temperatures.

toxik 5y ago
In the limit of the universe being a point, yes. Entropy must then have been zero since phase space is a point. But that’s just conjecture.
- roywiggins 5y ago
  The very early universe more or less must have had much lower entropy than we do today, you don't need to wind back all the way. A uniformly mixed goop of a universe is going to be very low entropy due to all the different ways it could collapse under the influence of gravity.
  - AnimalMuppet 5y ago
    Can you (or anyone) explain how entropy interacts with gravity? A uniformly mixed goop should be the highest entropy state, with stuff partitioned into clumps being a lower entropy state... apart from gravity. But if you add gravity to the mix, then... what?
    - roywiggins 5y ago
      My understanding is fully based on pop-sci, but: Eventually the influence of gravity starts generating black holes, which are really high entropy objects (in fact, for a given region of space, you can't actually fit any more entropy into it than a black hole does). So it seems to me that stars and planets and so on are intermediate steps between a hot, low-entropy universe at one end and a cold, high-entropy universe consisting of black holes. Eventually the black holes maybe evaporate leaving an extremely fine mist of radiation, but by that point space will have stretched out the the point that the undiffentiated radiation cannot collapse again. I found this from an actual physicist: "our gravitationally bound ball of gas has a negative specific heat! In other words, the less energy it has, the hotter it gets." https://math.ucr.edu/home/baez/entropy.html https://math.ucr.edu/home/baez/entropy.html
  - toxik 5y ago
    A uniformly mixed universe sounds like maximal entropy.
    - roywiggins 5y ago
      Any very slightly denser patches eventually start collapsing under gravity, and as they collapse they're doing work or could be made to do work so entropy begins to rise as the universe starts to sort itself into denser and less dense areas. Eventually some bits get so dense they collapse into black holes, which are maximally entropic regions of space (if you start trying to pack more entropy in, it just gets bigger).
roywiggins 5y ago
I think the low entropy in the early universe was mostly/entirely due to gravity- a uniformly mixed universe could collapse into denser regions in lots of ways, but (eg) one black hole at the other end of time is very high entropy. "According to the Big Bang theory, the Universe was initially very hot with energy distributed uniformly. For a system in which gravity is important, such as the universe, this is a low-entropy state (compared to a high-entropy state of having all matter collapsed into black holes, a state to which the system may eventually evolve)." https://en.wikipedia.org/wiki/Entropy_as_an_arrow_of_time#Overview https://en.wikipedia.org/wiki/Entropy_as_an_arrow_of_time#Ov...

toxik 5y ago

In the limit of the universe being a point, yes. Entropy must then have been zero since phase space is a point. But that’s just conjecture.

roywiggins 5y ago
The very early universe more or less must have had much lower entropy than we do today, you don't need to wind back all the way. A uniformly mixed goop of a universe is going to be very low entropy due to all the different ways it could collapse under the influence of gravity.
- AnimalMuppet 5y ago
  Can you (or anyone) explain how entropy interacts with gravity? A uniformly mixed goop should be the highest entropy state, with stuff partitioned into clumps being a lower entropy state... apart from gravity. But if you add gravity to the mix, then... what?
  - roywiggins 5y ago
    My understanding is fully based on pop-sci, but: Eventually the influence of gravity starts generating black holes, which are really high entropy objects (in fact, for a given region of space, you can't actually fit any more entropy into it than a black hole does). So it seems to me that stars and planets and so on are intermediate steps between a hot, low-entropy universe at one end and a cold, high-entropy universe consisting of black holes. Eventually the black holes maybe evaporate leaving an extremely fine mist of radiation, but by that point space will have stretched out the the point that the undiffentiated radiation cannot collapse again. I found this from an actual physicist: "our gravitationally bound ball of gas has a negative specific heat! In other words, the less energy it has, the hotter it gets." https://math.ucr.edu/home/baez/entropy.html https://math.ucr.edu/home/baez/entropy.html
- toxik 5y ago
  A uniformly mixed universe sounds like maximal entropy.
  - roywiggins 5y ago
    Any very slightly denser patches eventually start collapsing under gravity, and as they collapse they're doing work or could be made to do work so entropy begins to rise as the universe starts to sort itself into denser and less dense areas. Eventually some bits get so dense they collapse into black holes, which are maximally entropic regions of space (if you start trying to pack more entropy in, it just gets bigger).

roywiggins 5y ago

I think the low entropy in the early universe was mostly/entirely due to gravity- a uniformly mixed universe could collapse into denser regions in lots of ways, but (eg) one black hole at the other end of time is very high entropy. "According to the Big Bang theory, the Universe was initially very hot with energy distributed uniformly. For a system in which gravity is important, such as the universe, this is a low-entropy state (compared to a high-entropy state of having all matter collapsed into black holes, a state to which the system may eventually evolve)." https://en.wikipedia.org/wiki/Entropy_as_an_arrow_of_time#Overview https://en.wikipedia.org/wiki/Entropy_as_an_arrow_of_time#Ov...