Arrow of time and the low-entropy past

3 problems

• A coarse-grained entropy for self-gravitating matter

  Open

  For an ordinary gas, spreading out raises entropy, but under gravity clumping is the likely direction, so the usual formula fails. A gravitational entropy that grows as stars and galaxies form would make the second law consistent from the Big Bang onward.

• Why the early universe had low entropy

  Open

  Entropy (a count of how many microscopic arrangements look the same) has been rising since the Big Bang, so it was extremely low at the start. Physics has no accepted explanation for that starting condition.

• Does the thermodynamic arrow reverse in a recollapsing universe

  Open

  If a universe stopped expanding and fell back together, would entropy keep rising until the crunch, or would the direction of time flip? The answer shows whether the arrow of time is tied to cosmic expansion or only to the initial state.