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.
I think I’m claiming something weaker than what you want. You seem to want to have a way to compare the relative probabilities of completely different initial states, for example of being in AdS5xS5 vs being in 11 dimensional flat space, so that you can put a completely unconditioned probability distribution on the “space of all states” of whatever unknown completion of string theory is supposed to have all those things unified together. Personally I have no idea if concepts like phase space and Hamiltonian evolution make sense in such a general context.
What Bousso etc. are saying is that there is a well-defined arrow of time problem that doesn’t require going so far beyond what we know. Consider a theory of stable dS space (which probably doesn’t exist but let’s ignore that). There is an arrow of time problem – even if you start in an initial state where there are observers who see entropy production for a while, eventually the thing equilibrates and crazy fluctuations happen, so most observers are Boltzmann. The claim is that this is not true once decays to terminal vacua are included. It seems that any state which leads to eternal inflation in the “future ”, with future being an arbitrary choice of the direction to evolve in, can lead predominantly to observers who see entropy production and are not Boltzmann. Since this is NOT true without terminals, I think something has been gained, even though it might not be a sufficiently broad set of initial states to make you happy.
I know you said this wasn’t the place, but still: http://whyevolutionistrue.wordpress.com/2012/03/07/yet-another-reason-not-to-take-templeton-money/ .
This has got to be the most cryptic Carroll blog ever. It doesn’t even seem to be aimed at public outreach. Why such concern with observers, with or without Boltzmann brains?
I always wanted to be a cosmologist. When you do solid state or surface physics, you’re pretty well constrained by reality. In cosmology, you start at bat-guano crazy and extrapolate from there.
Sean writes:
This idea has been known and explored since the 60s by very-well prepared people. It gives to fundamental difficulties. That is the reason for which, the Brussels School (leaded by the last Nobel laureate Ilya Prigogine) moved beyond the Hilbert space. Unfortunately, there are technical difficulties with such abandon (the precise mathematical nature of the space needed to deal with an infinite number of degrees of freedom is not known). Moreover, its can be shown that the assumption of an infinite number of degrees of freedom does not introduce any arrow of time (it is not a surprise that the Brussels School obtains *two* semigroups).
There are many more reasons for the which your preprint do not hold up on close inspection. At first I would recommend you a look to the section 8 in the paper Non-redundant and natural variables definition of heat valid for open systems where several misunderstandings of the so-named “Generalized Second Law” (you allude to in your paper with Chen) and other mistaken topics in the gravitational thermodynamics literature are corrected.
Templeton? Tell me it ain’t so Sean. Tell me it ain’t so!
Maybe a quote (Aristotle ?) could be a guide to us : Asking the right question is to know half
the answer. Our present observation is that the universe is expanding with increasing acceleration.
Beyond that we could be just speculating with or without maths.
Similar questions were posed 2500 years ago, for example “Is the universe finite or infinite?”
The unequivocal answer was : It is a profitless question.