Is our universe natural?

By Sean Carroll | December 14, 2005 9:27 pm

Hey, has anyone heard about this string theory landscape business, and the anthropic principle, and some sort of controversy? Hmm, I guess they have. Perhaps enough that whatever needs to be said has already been thoroughly hashed out.

But, hey! It’s a blog, right? Hashing stuff out is what we like to do. So I’ll modestly point to my own recent contribution to the cacophony: Is Our Universe Natural?, a short review for Nature. To give you an idea of the gist:

If any system should be natural, it’s the universe. Nevertheless, according to the criteria just described, the universe we observe seems dramatically unnatural. The entropy of the universe isn’t nearly as large as it could be, although it is at least increasing; for some reason, the early universe was in a state of incredibly low entropy. And our fundamental theories of physics involve huge hierarchies between the energy scales characteristic of gravitation (the reduced Planck scale, 1027 electron volts), particle physics (the Fermi scale of the weak interactions, 1011 eV, and the scale of quantum chromodynamics, 108 eV), and the recently-discovered vacuum energy (10-3 eV). Of course, it may simply be that the universe is what it is, and these are brute facts we have to live with. More optimistically, however, these apparently delicately-tuned features of our universe may be clues that can help guide us to a deeper understanding of the laws of nature.

The article is not strictly about the anthropic principle, but about the broader question of what kinds of explanations might account for seemingly “unnatural” features of the universe. The one thing I do that isn’t common in these discussions is to simultaneously contemplate both the dynamical laws that govern the physics we observe, and the specific state in which we find the universe. This lets me tie together the landscape picture with my favorite ideas about spontaneous inflation and the arrow of time. In each case, selection effects within a multiverse dramatically change our naive expectation about what might constitute a natural situation.

About the anthropic principle itself (or, as I much prefer, “environmental selection”), I don’t say much that I haven’t said before. I’m not terribly fond of the idea, but it might be right, and if so we have to deal with it. Or it might not be right. The one thing that I hammer on a little is that we do not already have any sort of “prediction” from the multiverse, even Weinberg’s celebrated calculation of the cosmological constant. These purported successes rely on certain crucial simplifying assumptions that we have every reason to believe are wildly untrue. In particular, if you believe in eternal inflation (which you have to, to get the whole program off the ground), the spacetime volume in any given vacuum state is likely to be either zero or infinite, and typical anthropic predictions implicitly assume that all such volumes are equal. Even if string theorists could straightforwardly catalogue the properties of every possible compactification down to four dimensions, an awful lot of cosmological input would be necessary before we could properly account for the prior distribution contributed by inflation. (If indeed the notion makes any sense at all.)

I was asked to make the paper speculative and provocative, so hopefully I succeeded. The real problem is that draconian length constraints prevented me from making arguments in any depth — there are a lot of contentious statements that are simply thrown out there without proper amplification. But hopefully the main points come through clearly: calculating probabilities within an ensemble of vacua may some day be an important part of how we explain the state of our observed universe, but we certainly aren’t there yet.

Here’s the conclusion:

The scenarios discussed in this paper involve the invocation of multiple inaccessible domains within an ultra-large-scale multiverse. For good reason, the reliance on the properties of unobservable regions and the difficulty in falsifying such ideas make scientists reluctant to grant them an explanatory role. Of course, the idea that the properties of our observable domain can be uniquely extended beyond the cosmological horizon is an equally untestable assumption. The multiverse is not a theory; it is a consequence of certain theories (of quantum gravity and cosmology), and the hope is that these theories eventually prove to be testable in other ways. Every theory makes untestable predictions, but theories should be judged on the basis of the testable ones. The ultimate goal is undoubtedly ambitious: to construct a theory that has definite consequences for the structure of the multiverse, such that this structure provides an explanation for how the observed features of our local domain can arise naturally, and that the same theory makes predictions that can be directly tested through laboratory experiments and astrophysical observations. Only further investigation will allow us to tell whether such a program represents laudable aspiration or misguided hubris.

  • LambchopofGod

    Sean says:
    “if you believe in eternal inflation…..the spacetime volume in any given vacuum state is likely to be either zero or infinite…”
    I’ve never really understood this claim, and I’d be very grateful indeed for an explanation, however finite!

  • LambchopofGod

    By the way, it would also have been good to hear about efforts to find a selection mechanism for the landscape. For example etc

  • Count Iblis

    There is an article by Vilenkin about eternal inflation. The conclusion is that Elvis is alive and Al Gore was elected president (somewhere), because all possible quantum states the observable part of the universe can be in are realized somewhere. :)

  • Elliot

    Al Gore was elected president in our universe. He was appointed (annoited?) president by the SCOTUS. Probably a bad example.


  • Arun

    Does at least one corner of the landscape exhibit the huge hierarchies of energy scales?

  • Science

    If you want a directly testable theory, try

    Of course, you want much more than you claim. You want it put on and in Physical Review Letters first by someone famous or at least respectable in the physics community. It is fashionable to sell science as being open to all to contribute to, and not a closed shop of paranoid people who are more obsessed with attacking ‘dangerous crackpots’ than finding things out. This is why no advances are likely to come from outsiders, not because they are all 100% ignorant laymen, but because they don’t need crackpot labels and paper rejections on the basis of a lack of affiliation. 😉

  • Count Iblis

    Science, that’s not true. Not lack of affiliation, but lack of scientific merit leads to rejection of articles.

    I challenge you to produce a referee report on your article that proves me wrong.

  • Mark

    Science. I don’t mean any disrespect, but would like to ask you to stop posting about your personal, unpublished theory here. It appears in many posts and we are not running the blog for that purpose. As you have pointed out many times, you have your own page for that. Best wishes.

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  • Science

    Count Iblis: see

    Mark: it is not personal, its not ‘mine’, its a proof based on facts of many people, and it is published in peer-reviewed article in a lesser journal. Why do you think ‘Science’ is used? It is not personal speculation.

  • Mark

    I’m just not going to get into this. But I do still ask that you not constantly use the blog as a place to pitch these thoughts.

  • Science

    sorry :-(

  • Sean

    Lambchop — eternal inflation makes infinitely big regions because, you know, it’s eternal. Once inflation starts, there are quantum fluctuations in the “inflaton” field whose energy is driving the expansion. In some regions of space the field rolls down and reheats, giving rise to a standard-issue radiation-dominated universe. In other regions a quantum fluctuation pushes the field to larger values, where inflation continues. And then the story continues in perpetuity: where the field had fluctuated up, part of it reheats and part of it keeps inflating. So every kind of vacuum that is populated by this form of inflation will generally be produced with an infinite volume.

  • Dissident

    Sean, was your mention of “the recently-discovered vacuum energy (10-3 eV)” part of the requested “speculative and provocative” deal? Cause AFAIK, no such thing has been discovered; it’s just the most common hypothesis about observational data which, just like weak lensing, come with no label attached by the Creator, saying “this is caused by dark stuff”. One need only browse recent threads at

    to find both alternative ideas and problems with the current consensus hypothesis (such as, “Recent Supernovae Ia observations tend to rule out all the cosmologies”).

  • Dissident

    #13: …after an infinite amount of time…


  • Mark

    I’m not trying to kick you off Science, just trying to keep things focused. Hope you’ll stay around.

  • Grey

    hi Sean!

    Just a small comment

    “the entropy of our universe is much much lower than it needs to be to support the existence of life.”

    But it needs to create life from the most primortial elements, not just sustain a pre-existing life. I think that factor makes a huge difference on the need for low entroy. It is pretty fantastic that we complex and creative creature exist at all.

  • http://www.anthropic-principle.ORG island

    According to the criteria just described, the universe we observe seems dramatically unnatural. The entropy of the universe isn’t nearly as large as it could be, although it is at least increasing; for some reason…

    I can never seem to wrap my head around this because it seems to me that our universe *is* the most natural configuration if you consider that flatness represent’s the damping of an imperfect quantum harmonic oscillator, where non-unitary dissipative evolution prevents an inherently imbalanced density matrix from evolving inhomogeneously, since the second law dictates a tendency toward uniform mass distribution.

    Entropy as a measure of the uniformity of the distribution of energy tells us that complete entropy, (with all matter and energy distributed over maximally expanded spacetime), requires the formation of far-from-equilibrium dissipative structures in order for the total mass distribution in an imperfect world to become *progressively* more-uniform, so a near-flat universe is most natural… if there is inherent asymmetry.

  • Count Iblis

    Science, I’ll reply later on your blog.

  • LambchopofGod

    Sean said:
    So every kind of vacuum that is populated by this form of inflation will generally be produced with an infinite volume.

    OK,thanks! So I guess the next question is: why should we believe in inflation that is eternal rather than inflation that happens *once* after a [finite] universe was born? I’ve seen claims that eternal inflation is “generic” but I can’t even imagine what that could mean…

  • Count Iblis


    It’s because the rate of expansion of the false vacuum is so extremely fast. If you start out with a small piece of false vacuum that is inflating then there is always a small probability that a piece of it won’t have decayed to the normal state.

    The survival probability of a piece of the false vacuum goes to zero exponentially, but the total volume of the false vacuum increases exponentially. The product of the two factors grows exponentially, so the total volume of inflating false vacuum keeps on growing forever.

    See also here:

  • Elliot


    Again forgive the layman’s naivete here but.

    Since the “inflaton” field drives the inflationary expansion and Dark Energy seems to be driving the acceleration of expansion in our universe, is there any line of reasoning that might suggest these two are related or are we talking apples and oranges here?



  • Sean

    Grey– trust me, the entropy of the universe is much lower than it needs to be to support life. For example, the 100,000,000,000 galaxies we don’t live in seem pretty superfluous.

    Island– if you want to think about the spatial geometry of the universe as an harmonic oscillator, it’s an upside-down oscillator; flatness is an unstable point. Except for accelerated expansion (ala inflation or dark energy), there’s nothing that drives spatial hypersurfaces to be flat.

  • Sean

    Lambchop– what Count Iblis said, plus the inevitability of quantum fluctuations. If inflation happened classically, with the inflaton simply obeying its equation of motion, it would eventually end. But the fluctuations mean that there is always some probability that the rate of inflation increases rather than decreases. So in a typical model, there will always be some place where inflation is still going on; it’s not an add-on to the model, it’s just a consequence of quantum mechanics.

    Elliot– people have thought about linking inflation and dark energy, e.g. Peebles and Vilenkin. But the energy scales are so fantastically different that it doesn’t seem like a match made in heaven.

  • http://www.anthropic-principle.ORG island

    Island— if you want to think about the spatial geometry of the universe as an harmonic oscillator, it’s an upside-down oscillator; flatness is an unstable point.

    Except for accelerated expansion (ala inflation or dark energy), there’s nothing that drives spatial hypersurfaces to be flat.

    I’m pretty sure that a balance between gravity and the expansion will accomplish that in this case, which was my point, where the simplist version of infationary theory was tacked-on to big bang theory in order to explain a lack of causal connection in the universe given the uniformity of cosmic background radiation and the size of the universe… which I just did, as well, but without the need for the bandaid theory.

    Occam would tell you that causality is not violated if the universe has volume when a big bang occurs, so you should not assume idealized theoretical projections beyond that, (which require that we accept even more unproven theoretical speculation), without very hard proof, which I do not believe that you have.

    I don’t believe that you can prove that Einstein’s static model was unstable either, since matter generation from vacuum energy in that model will cause a counterbalancing increase in negative pressure and positive gravitational curvature. The fact that Einstein did not know about real particle potential in the quantum vacuum does not change the fact that he’s still right until somebody proves him wrong, since this process causes expansion in this model.

    It is easily shown that a counterbalanced increase between negative pressure and positive gravitaional curvature causes tension between the vacuum and ordinary matter to increase while the universes is held flat and stable by this effect which will lead to the previously described scenario for pre-existing volume when the forces that bind the universe give-way to the straw that breaks its back, so there will again be no violation of causilty, AGAIN… and occam is still a happy camper.

    I’ve made this point before, but it didn’t capture your interest, so I can only assume that this means that you don’t see how this might affect other assumptions that have been made which are also taken for granted to disprove or supercede this model.

    I disagree with that too, because the interpretations of what the evidence means change in this model, as well.

  • Plato

    Equilibrium points unstable? I think is what Sean is saying?

    Anyway I wanted to divert attention for 1 second and show the status of discrete functions in our cosmo? Does it have sound reasoning?

    Are topological functions natural? :) I was creatively inspired. :)

    Now back to regular programming. Thanks

  • inflation-newcomer

    just clarify one thing.. (not exactly related to the post)
    is the cosmic no-hair conjecture proved??..
    i mean.. do all initial conds (including anisotropic ones) approach the de-sitter solution ???
    Upon digging up stuff, I was not able to find agreement on this issue. Pple like Wald, G.Ellis seem to insist that this hasn’t been done… while linde etc insist that it is over! And most inflation reviews and books happily use the friedman equation with (at best) just a passing comment on the issue of anisotropies.

  • Sean

    The cosmic no-hair theorem, which roughly states that an expanding universe in the presence of a positive cosmological constant will generically approach empty de Sitter space, has by no means been rigorously proven in all interesting cases. Nevertheless, something like it is probably true, at least in an open universe. Jennifer Chen and I argue in favor of it in our first paper.

  • Count Iblis


    in your paper hep-th/0410270 with Chen, you discuss a bit about unitary time evolution. This reminded me of this problem (I don’t work in this field so perhaps it is trivial):

    If you start out with a region of some finite volume then presumably here are only a finite number of fundamental states available for that volume. If this region undergoes expansion and becomes larger, then shouldn’t there be less degrees of freedom available per unit volume if you demand unitary time evolution?

    But if one thinks of the standard model as an effective low energy theory obtained by integrating out the fundamental degrees of freedom, then this means that the coupling constants would have to change according to a renormalization group transformation.

    Or is this just an artefact of treating the expansion clasically? Is the density of states in the original space time for which the scalar field configurations are compatible with the expansion just the same as in the final space time?

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  • LambchopofGod

    I just noticed that Sean’s paper cites

    Any comment as to why that paper is relevant, Sean? :-)

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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