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A Paper on Sean Carroll’s Multiverse model for explaining the low entropy initial condition

March 1, 2012

Hi everyone,

In his introductory post, Barry listed several questions that are central to this blog.   The first three revolve around the question of whether anything explains the low entropy condition of the Universe at the Big Bang; and they are the subject of a paper I just published in Entropy.    In the paper, “Bumps on the Road to Here (from Eternity)”  (It’s open access!) I discuss Sean Carroll’s proposal which says that the world consists of an ever-growing set of universes that cleave off from each other in Big-Bang-like states.   Like Barry, I’m uncertain if the proposal works even if the fundamental laws do the work that Sean hopes they do.      I’m hoping to attract some comments on the paper here and to get the blog ball rolling!

Abstract: In his recent book, From Eternity to Here, and in other more technical papers, Sean Carroll (partly in collaboration with Jennifer Chen) has put forward an intriguing new way to think about the origin of the Universe. His approach, in a nutshell, is to raise certain worries about a standard Boltzmannian picture of statistical mechanics, and to present certain commitments that he thinks we ought to hold—commitments that the standard picture doesn’t share. He then proposes a cosmological model—one that purports to give us insight into what sort of process brought about the “initial state” of the universe—that can uniquely accommodate those commitments. The conclusion of Carroll’s argument is that statistical mechanical reasoning provides grounds for provisionally accepting that cosmological model. My goal in this paper is to reconstruct and critically assess this proposal. I argue that “statistical cosmology” requires a careful balance of philosophical intuitions and commitments against technical, scientific considerations; how much stock we ought to place in these intuitions and commitments should depend on where they lead us—those that lead us astray scientifically might well be in need of philosophical re‑examination.

“Bumps on the Road to Here (from Eternity)”

13 Comments leave one →
  1. March 2, 2012 11:10 am

    I quite enjoyed the paper. But I am not convinced by this: “[N]o amount of evidence, and no number of experiments (or a[t] least not records of experiments–which is all we ever have) … can ever show that I am not a Boltzmann Brain. Only a … desire to avoid insanity can argue against the PBB scenario once we go down the statistical mechanical road.”

    I think there is a gap in the argument. It does not follow from
    1. There can be no evidence that I am not a BB
    2. There can be no evidence that the PBB scenario is false.

    For the PBB scenario makes other predictions besides the prediction that I am probably a BB. For instance, the PBB scenario seems to predict that I’m probably not going to be a very sophisticated BB. On PBB, the vast majority of the BBs are going to be more like a mouse brain than a human brain. And even among brains capable of human-like conceptual thought (I don’t know if we’re allowed to restrict our consideration to those), the vast majority of those won’t have occurrent rich and complex sensory experiences (or memories of such). So, the kind of BB that PBB predicts me to be differs from what I in fact observe. Hence, probably, the PBB scenario is false.

    I am not entirely convinced, but at least this shows more needs to be said about the move from 1 to 2.

    • ericwinsberg permalink
      March 2, 2012 12:09 pm

      Hi Alexander,

      Thanks for the comment! Here is how I understand the PBB: I’m sitting here at my computer, happily typing away on a blog, and it suddenly occurs to me that I believe in a Past Hypothesis, a “Low entropy probability postulate”, and the laws of evolution that Carroll postulates. I ask myself: “what is most likely true?” That I am having the experience of seeing a computer screen in front of me is given! The question is what else can I infer? That I am a brain-in-a-vat that is incapable of human-like conceptual thought is rule out to start. That I dont have rich and complex sensory experiences is ruled out. Hence, I dont consider those possibilities to be on the table as _part of the PBB problem_. The problem is that, given what I have assumed, it is far more likely that I am a disembodied brain that _is_ capable of rich and complex human sensory experience that has spontaneously appeared than that I am a real human with veridical sensory experiences and memories. And THAT possibility, the possibility that I am a PBB, a brain-in-a-vat capable of all those things, though it is far more likely than its less troubling alternative, can’t be ruled out by any set of evidence. And so its THAT possibility that either has to be eliminated by hand (with the NPH) or else finessed out of existence in the way Sean hopes to do.



      • March 2, 2012 12:48 pm

        It seems to me that the fact that I have rich and complex sensory experiences is genuine evidence, even if it is old evidence.

        I have nothing smart to say about how to technically handle old evidence in confirmation. But it’s still very plausible that old evidence is evidence. 🙂

        Suppose I am sitting here at my computer, happily typing away on a comment, and it suddenly occurs to me to entertain the specific hypothesis that this is all a hallucination induced by chemical C. But I recall that C only very rarely induces hallucinations of sitting at a computer. In the vast majority of cases, C induces hallucinations of seven-legged monsters. That I am having the experience of seeing a computer in front of me is as you say given, and is old evidence, but it is still significant evidence against the hypothesis that I took C. Certainly, if the question is “Did I take C, which tends to induce hallucinations of monsters, or did I take D, which tends to induce hallucinations of computers?” I should conclude that my experience of sitting at the computer provides strong evidence against the hypothesis that I took C over the hypothesis that I took D. And likewise it should provide strong evidence against the hypothesis that I took C over the hypothesis that things are roughly as they seem.

        • ericwinsberg permalink
          March 2, 2012 1:02 pm

          I agree with everything you say, but I don’t think your case is an apt analogy. Remember that, unlike your C hypothesis, which is just a wild speculation, the PH+LEPP+DL hypothesis is posited as the best explanation of what I am seeing and remembering: namely a bunch of memories of reading physics books that talk about dynamics and statistical mechanics and Boltzmann, and all that good stuff. So, I am taking the existing evidence of PH+LEPP+DL to bery strong.

          The question on the table here is: GIVEN that I am entertaining PH+LEPP+DL for whatever reason, does this consequence of it RULE IT OUT–does it unequivocally show it to be false. And the answer is no. I can simply add more to it to protect it. So, in your case, if, for whatever reason, I am committed to C, I can easily make it coherent by adding to it the additional hypothesis, that, say, the drug is having an unusual effect this time, or that I took a lot of vitamin C with it. 🙂

  2. March 2, 2012 2:01 pm

    1. I took your claim in the paper to be that there can’t be significant evidence against the PBB scenario. But if the claim I quoted is simply that there can’t be evidence completely ruling out the PBB scenario (i.e., entailing its denial??), then I wasn’t arguing against that. I agree that one can make the observations fit with the hypothesis that I took C by adding a further assumption, but that increases the cost of C.

    2. I can make the C hypothesis be non-speculative. I may know I took a pill, but not remember if it was C or aspirin.

    3. “I am taking the existing evidence of PH+LEPP+DL to be [ve]ry strong.” Here’s something that puzzles me, and I don’t know what to say. Suppose PBB is true and I am a very short-lived BB. For definiteness, suppose I live for three seconds. A lot of the evidence for PH+LEPP+DL depends on empirical data gathered by a lot of people, with a fair amount of mathematical work to show that this data is well-explained by PH+LEPP+DL. Even if I were a really good physicist, I wouldn’t be able to pack much of this evidence into occurrent thoughts over three seconds. At best, I’d be able to pack in something like: “I remember that there is a lot of evidence for PH+LEPP+DL” and then a very, very sketchy thumbnail sketch of one or two more striking apparent pieces of evidence. But a thought of the form “I remember there is a lot of evidence for H”, and a very, very sketchy thumbnail sketch of one or two more striking apparent pieces of evidence, is not very strong evidence for H. So if there’s going to be a lot of evidence for PH+LEPP+DL, it’s going to have to include non-occurrent beliefs about the past. But is it legitimate to use those non-occurrent beliefs as evidence, given that once we start taking BB hypotheses seriously, it becomes an open question whether we have these non-occurrent beliefs at all, since most BBs are going to have only the occurrent thoughts without a rich fundament of underlying non-occurrent thoughts?

    • ericwinsberg permalink
      March 2, 2012 2:16 pm

      On 1 and 2: The claims of Carroll’s I am responding to are : “this scenario seems to be ruled out experimentally” and “it’s a direct disagreement between theory and observation.” That its evidence against the scenario is right. Its strong enough evidence against it that I add a new piece to the package in order to vastly weaken the strength of the evidence. But Sean’s worry was that it rules out the possibility of the package. That’s the claim I’m worrying about.

      On 3: yeah, I agree. In fact, I started thinking about this point as I was writing the paper but decided not to pursue it because I wasn’t sure what to say about it. I think maybe the problem can be finessed, but I’m not sure. Maybe Barry will chime in on that point when he’s feeling better.

  3. alison2142 permalink
    March 4, 2012 8:15 pm

    Thanks, I enjoyed that. Just wondering what you thought of the transcendental grounds for the PH and the NPH as an actual source of their justification? My rough and vague intuition is that showing Boltzmannian dynamics plus the probabilistic postulate without the PH is epistemically unstable, for example, is not a justification of the PH. The justification is rather that the package of the dynamics plus the low-entropy-probabilistic postulate plus the PH (and perhaps the NPH) has been empirically successful.

    As you’ve noted, this does require taking something as given—evidence for past successes. But I’m guessing we don’t expect justification to come ex nihilo here. And while it’s true that these successes are in some sense bound up with the combination being epistemically stable (we wouldn’t have any evidence for or empirical success trying work with an epistemically unstable position), this doesn’t seem to imply that merely because a piece changes an epistemically unstable position to a stable one, the piece itself is justified.

    Could you say some more about this?

    • ericwinsberg permalink
      March 4, 2012 8:33 pm

      Thanks for the comment, Alison.

      This is a tricky question, I think. The worry is that DL+LEPP is just as empirically successful as DL+LEPP+PH (and, as you say, perhaps NPH). The former explains everything I see IF WHAT I SEE IS VERIDICAL, but then tells me its extremely unlikely that what is see is veridical–ooops!. The latter basically adds the claim AND WHAT YOU SEE IS VERIDICAL. Does that make the latter more empirically successful over-and-above the removal of epistemic instability? I’d like to say yes, but I’m not seeing how. Remember that the PH just says something like: whatever the past state of the universe looks like it was like using ordinary inferences: that was it was like.

      What do you think? Can you say more about the worry that we are looking for justification ex-nihilo? I’m not sure I get the thrust of that particular remark. I confess I probably havent thought about that particular issue as much as I should have. My main commitments in the paper were that a)there can be decent grounds for positing a PH, and whatever grounds one can offer for PH, one can also offer for NPH. Does that much seem right?


      • alison2142 permalink
        March 4, 2012 9:58 pm

        Thanks, that’s helpful. I guess the thought is this. In an epistemically unstable position, you simply don’t get evidence of empirical success: what you get is a conflict between what the theory predicts and what evidence would confirm it. By it’s own lights, it’s empirically adequate, sure, but nor is there any evidence of its empirical success. So perhaps I don’t get the requirement for a theory to be more empirically successful than it’s unstable rival, beyond the removal of the instability, since the unstable rival doesn’t have, by its own lights, any reason to take itself to be empirically successful.

        And I think what I had in mind about justification coming ‘ex nihilo’ might have been the following. It might seem like we want to be able to evaluate the two theories without ever taking on the commitments of either theory, to determine which is more justified. And given two theories where only one is empirically stable, the stable one is more justified than the other, and if they differ only in one component, that component is thereby justified. But that’s nuts, right?

        It seems what we want to be able to do is provisionally accept evidence and play around with theories until we come up with what seems like a fair package that explains a chunk, explains away another perhaps smaller chunk, and explains how we managed to do all this. And the justification for the theory is going to come by pointing at evidence the theory accepts, including evidence of its empirical success. But once we’re playing this game, there’s no need to be worried about an unstable rival, because what we’re comparing is package deals, and the unstable rival just doesn’t have a package deal to offer that includes evidence of empirical success. Or at least I think that’s what I might have been thinking 🙂

        As for the main commitments of the paper… I think I’m fine with them, at least for now 🙂

        • ericwinsberg permalink
          March 4, 2012 11:35 pm

          Hi Alison,
          Yes, I think I agree with all that. Especially the your explanation of the ex-nihilo remark. And I think its a more clear and more apt way to put it than i did in the paper. So, thanks for that. And thanks for reading the paper!


  4. March 29, 2012 6:25 pm


    As described in your paper, Sean Carroll proposes multiple low-entropy universes that bud off an equilibrium universe as a way of avoiding the need to invoke PH, and your critique of the utility of his approach is well taken. However, if we sympathize with his goal, if not his solution, one might ask if there are other ways that our universe, in which PH appears to hold, can arise from an underlying chaotic state through what can be viewed as a physical process other than a random fluctuation from equilibrium, which would seem to inevitably invite the PBB argument. Your paper can be viewed as a challenge to discover such a process that can better withstand a critique such as you present here. Was this the intent of your article?


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