A group of philosophers and scientists interested in cosmology have started a new project, funded by the Templeton Foundation, imaginatively titled the Rutgers Templeton Project on Philosophy of Cosmology. It’s a great group of people, led by David Albert and Barry Loewer, and I’m looking forward to interesting things from them. (Getting tiresome questions quickly out of the way: like the Foundational Questions Institute or the World Science Festival, I’m totally in favor of this project even though I’m not a big fan of the Templeton Foundation. This isn’t the place to talk about that, okay?)
They also have a blog, because blogs are awesome. It has a humble title: What There Is and Why There Is Anything. They have a new post up, by Eric Winsberg, that brings up the issue of whether the multiverse can help explain the arrow of time. The post is basically a pointer to this paper by Eric, which is a close analysis of the kind of scenario I’ve been pursuing since my 2004 paper with Jennifer Chen. If this kind of thing is your bag, consider going over there and commenting on Eric’s paper.
I am working on a real science paper about some of these issues myself, but going has been admittedly slow. Let me just lay out a couple of the major issues here. One is, naturally, the question of whether the Farhi-Guth process of baby-universe creation really happens. In 2004 I was pretty confident that it did, but now I’m less sure. Aguirre and Johnson have looked closely at these kinds of tunneling events and come back pessimistic; others have looked at similar processes from the perspective of the AdS/CFT correspondence with similarly unpromising results. I don’t think the issue is settled, however; for the moment I’m willing to take the possibility of spontaneous baby-universe creation as an allowed hypothesis, while continuing to search for more well-grounded alternatives.
The other issue, which I think it should be more possible to make progress on, is a problem of counting — comparing the likelihood of different occurrences. In fact there are two sub-problems here. One is what’s now called the measure problem in cosmology. Assuming that many things (like the appearance of people exactly like you) happen an infinite number of times, how can we compare appearances to calculate probabilities? In this context the question is how we can compare the number of observers who appear in a nice warm post-Big-Bang environment to the number who pop randomly out of the nothingness as thermal fluctuations. In a scenario like ours, you need thermal fluctuations to create new universes, so there is always some possibility of making observers as well. I think that our picture is much better than most versions of eternal inflation from this perspective, as it seems easier to make a baby universe than to make an observer — the magic of inflation is that a bubble ready to inflate can be almost arbitrarily tiny, while an observer needs space for its thinking apparatus. But it’s harder to actually calculate things in a well-defined measure once your spacetime becomes disconnected by the appearance of new universes, so it’s certainly a legitimate question.
The other sub-problem, more subtle, might be called the “genericity problem.” The most important point of my paper with Jennie was to argue that a dynamical origin of the arrow of time is possible if and only if the space of states is infinitely big — the universe can keep evolving forever without reaching an equilibrium or entering a recurrent cycle. Baby universes were just the means to that end. But if there are an infinite number of possible states, how do you pick a “generic” one?
We tried to argue that “almost any” initial state would robustly evolve to a condition where baby universes were produced and became the dominant channel for creating observers. Our strategy for doing so was to say that a low-energy de Sitter vacuum was the highest-entropy state you could be in where space was still connected, and that most conditions evolve toward such a state. (At least if we discount Minkowski space with exactly vanishing vacuum energy, maybe for anthropic reasons.) Of course we then immediately evolve to a state with more than one connected component via the nucleation of baby universes. So then you could ask why we didn’t start there. Our idea is that there is no maximum number of components (separate universes), so any finite number can still grow. So why don’t we have an infinite number?
It’s a legitimate question, but not a show-stopper. In toy models it’s certainly easy to construct examples where there is no equilibrium state (as we mentioned in the paper). It could be difficult in those cases to fix what counts as a generic initial condition, but it might not be impossible. That’s something worth further investigation.
Nobody ever said explaining what there is and why there is anything would be easy.