B What happens at the end of entropy?

The most widely accepted idea is that the Universe will continue to expand until every particle is too far away from other particles for any interactions to occur.
https://en.wikipedia.org/wiki/Heat_death_of_the_universe
That is the maximum entropy state, thermodynamic equilibrium.
If you somehow could push the bits back together so that interactions can occur again, the act of doing so would reduce the entropy of the Universe.
It no longer is at maximum.
That can't be done of course because it would require something external to the Universe to accomplish the feat.

 

The big bang theory does not attempt to describe an initial state of the Universe.
Whatever it was though, it was 'singular', there is only one possible state that it could exist in.
It gets worse, there could be NO possible states.
When math produces conclusions like that, it means our theories need working on.

 

yes

yes

no

no

no

 

Since the universe is expanding, anything that is not gravitationally bound will be separated. The largest gravitationally bound objects are superclusters. As far as I know, anything gravitationally bound will decay by gravitational radiation until the whole cluster collapses into a big big black hole. This takes a very long time.

Also, black holes will eventually evaporate by Hawking radiation. Currently, the cosmic microwave background is hotter than macroscopic black holes, so black holes do not evaporate. But as the universe expands, the CMB temperature continues to decrease, so eventually the temperature will be low enough for black holes to evaporate (mostly into photons). Evaporation also takes a very long time, and I have no idea if it is faster than orbit decay. If it is faster, then I suppose systems that were gravitationally bound could become not gravitationally bound*, as the black holes holding them together evaporate. Maybe you will end up with some separated stable macroscopic objects which are too small to form black holes.

So, maybe eventually everything will be an occasional stray photon and a dilute background of gravitational waves. Or maybe you will have macroscopic objects like dead stars which cannot decay any further.