Objects in motion continue in motion. So without energy being created or destroyed, why would you expect a contraction instead of continuing expansion?
Real closed loop systems are purely theoretical and the only one we know of is the Universe, so your statements about them are circular. One big unidirectional aspect of the universe is entropy - it’s a non-reversible state. Going to a previous state requires more energy than what you started with.
> Objects in motion continue in motion. So without energy being created or destroyed, why would you expect a contraction instead of continuing expansion?
I think space is a little like an insanely low-friction version of water, i.e. a quantum foam [1] (or similar fluctuations caused by the structure of space-time). It may not be perceivable but it would mean that space could exert a drag in the same way water does. You could imagine it pulling out energy from a photon and red-shifting it, until eventually it just absorbed it. If that was true, it would also add a distance-based offset for all observations of the Universe and affects things like the Hubble constant.
That would be just one way in which a contraction may occur - nothing exciting, just all the things travelling further away worn down by the substance they travel in. Then there would be several ways to bring it all back together into a big bang.
> Real closed loop systems are purely theoretical and the only one we know of is the Universe, so your statements about them are circular.
We have simulated closed-loop systems of many kinds, I think we can make educated guesses about how they may behave. As long as all state transitions undergo a reversible function, it should eventually return to its original state?
> One big unidirectional aspect of the universe is entropy - it’s a non-reversible state.
I don't think that's entirely true. Going by the big bang, the Universe started as one large point in space-time that exploded. Not long after things started to clump together into larger and larger particles/objects, there is currently nothing to make me believe that it will not create larger and larger clumps of matter, i.e. super massive black holes.
> Going to a previous state requires more energy than what you started with.
Then where did that energy go?
Anyway, I understand this is highly theoretical and I'm not convincing anybody here, but it is nice to share some ideas.
> I think space is a little like an insanely low-friction version of water, i.e. a quantum foam [1] (or similar fluctuations caused by the structure of space-time). It may not be perceivable but it would mean that space could exert a drag in the same way water does. You could imagine it pulling out energy from a photon and red-shifting it, until eventually it just absorbed it. If that was true, it would also add a distance-based offset for all observations of the Universe and affects things like the Hubble constant
As I understand it, expansion of the universe is expansion of the fabric of spacetime. It's not about objects just travelling away from each other. Thus the quantum foam friction analogy (even if true) wouldn't really apply.
> We have simulated closed-loop systems of many kinds, I think we can make educated guesses about how they may behave.
Don't mistake the incompleteness of our models for the truth. They are approximations for testing out ideas about the universe but that doesn't mean the approximations themselves exist.
> As long as all state transitions undergo a reversible function, it should eventually return to its original state?
But we know that non-reversible functions exist, both in terms of entropy and time irreversibility, & these functions happen all the time in our universe.
> Then where did that energy go?
Waste heat that can't be recovered to reverse entropy because it's already diffused throughout the universe & thus there's no local maximum to exploit. That's what the heat death of the universe refers to.
Real closed loop systems are purely theoretical and the only one we know of is the Universe, so your statements about them are circular. One big unidirectional aspect of the universe is entropy - it’s a non-reversible state. Going to a previous state requires more energy than what you started with.