Evolution of the Universe in reverse

[javisot20]

Would you also agree that in case a star collapses to a black hole it doesn't make sense to run this back in time?

And I guess you'll say now, "So I don't understand anything, how can it be that for 2 and 3 to happen, 1 must happen and that doesn't imply retrocausality?"

Well, for there to be 3 (hot and dense state), there has to be 2 (contraction), and that doesn't imply retrocausality, right?

 

[PeterDonis]

Ask if there is a relationship in some sense that does not imply retro-causality between those three facts

Go read my post #20.

You are not even formulating a well-defined question. You need to think very carefully about what question you actually want to ask, and then formulate it in a way that is well-defined--if you can. I suspect you can't--that when you actually think carefully, you will find that the question you think you want to ask doesn't even make sense.

 

[PeterDonis]

for there to be 3 (hot and dense state), there has to be 2 (contraction)

No. The causal ordering is not changed by running the film in reverse. The causal ordering is that for there to be expansion, there has to be a hot, dense state. Running the film backwards and calling the expansion "contraction" instead does not mean the contraction is now causing the hot, dense state.

This point has already been made earlier in this thread, but you don't seem to be taking it into account. In fact, you don't seem to be taking into account any of the points people have tried to make to show you why the question you are asking doesn't make sense as you are asking it.

 

[timmdeeg]

And I guess you'll say now, "So I don't understand anything, how can it be that for 2 and 3 to happen, 1 must happen and that doesn't imply retrocausality?"

Well, for there to be 3 (hot and dense state), there has to be 2 (contraction), and that doesn't imply retrocausality, right?

Retrocausality means backwards causation. This can't be applied for 1 and 2 and 3.

For black holes you seem to agree. But e.g. "the universe contracts" can't be applied, because according to the Friedmann Equations the universe expands with the present energy densities.

 

[PeterDonis]

according to the Friedmann Equations the universe expands with the present energy densities.

With present energy densities and velocities. If you had a state with the same energy densities but all the velocities reversed (not a "run the film backwards" view but a different state in the forward direction), the universe would be contracting, not expanding. But that is a different solution of the Friedmann equations from the one we actually live in.

 

[javisot20]

No. The causal ordering is not changed by running the film in reverse. The causal ordering is that for there to be expansion, there has to be a hot, dense state. Running the film backwards and calling the expansion "contraction" instead does not mean the contraction is now causing the hot, dense state.

And I agree, but here we are mixing the fact that there is no retrocausality (we all agree on this) with the fact that we can "analyze" the universe in the opposite direction, and that in this analysis, even if there is no retrocausality, there must be some analytical relationship between 1, 2 and 3.

That's the assumption that gives rise to my question, I suppose.

 

[PeterDonis]

here we are mixing the fact that there is no retrocausality (we all agree on this) with the fact that we can "analyze" the universe in the opposite direction

No, we aren't. You are failing to understand that there is no such thing as "analyze the universe in the opposite direction" in the sense you are trying to use the term.

in this analysis, even if there is no retrocausality, there must be some analytical relationship between 1, 2 and 3

No. This is wrong. You have been told repeatedly that it is wrong.

That's the assumption that gives rise to my question, I suppose.

And that assumption is wrong. You have been told that repeatedly. That is not going to change.

And with that, this thread is now closed.