Which seems to me to basically say that this anomaly is about as bad as it could possibly be.

Let us say instead that the anomaly is exactly as bad as it could possibly be: the large-scale entropy of the universe at the Big Bang is zero. This is a very strong claim, and (like all extreme statements) it’s simple too. It’s too strong, compared to the evidence, to justify stating it without qualification, but as a hypothesis I like it.

While I’m here, I’ll recommend Huw Price’s 1996 book Time’s Arrow and Archimedes’ Point on the arrow of time, although I don’t agree with Price about everything.

And I agree that the silence about this, the idea that the second law is fully understood, is a scandal.

Hard to tell. It is very speculative, and Penrose is reaching the end of his career, and some people are clearly humouring him… But there is some weak evidence supporting it, and it does appear to be testable.

And the mathematics appear correct – in the absence of the Ricci tensor, the slow, infinite, cold end of the universe is indistinguishable from the fast burning expansion of the big bang…

I’m not endorsing this simplified model at all, btw. Just pointing out that reasoning by analogy to a more familiar situation can lead to apparently nonsensical results.

Sean, the “past hypothesis” seems to me more of restatement of our failure than an actual solution. And I’d be more comfortable accepting it if I saw some formal calculations showing what it implied; the hand-waving makes me nervous.

The low-entropy-past hypothesis is neither a full solution nor a restatement of our failure; it is a partial explanation. It is like starting with the question “Why is this room hot?” and getting the answer “because there is a thermostat connected to a heating device, which produces heat until the room is brought to a certain temperature”. Sure, this explanation prompts the question “why is the thermostat set so high?”, but that doesn’t mean the explanation is just a useless restatement of the question.

In comparing the low-entropy-past hypothesis to the fictional “life appeared even though it seems unlikely” law, you ignore the substantial predictive power of the former.

[1]Unless you have, for example, the definitive explanation on whether the universe is open or closed, etc. You are confusing local, gravity-free cases with global, non-gravity free cases. Iow, don’t extrapolate from a gas enclosed inside a jar.

Phil, I said statistical mechanics was bad at predicting the past, not thermodynamics; it gets teakettles wrong too.

Scent, we know how to calculate the future, so calculating the past would be just as easy if the same approach applied.

James, Scent is right about records.

mjgeddes, tim is right; high entropy states can be very simply indicated.