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.
Comments
37 responses to “Is our universe natural?”
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
Sean/Mark.,
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.
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.
Sean,
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?
[…] Moving to non-Mormon blogs, the firestorm over superstring theory continues unabated. Some physicists really getting upset at how the Discovery Institute keeps trying to link Susskind’s thoughts on superstring theory with intelligent design. For a nice overview including links to a New Scientist interview, check out Not Even Wrong. Also up is a discussion of the same issues and the question of whether our universe is natural. It’s discussion that arose from the author’s Nature review. Lots of other links on this topic, but those two should get you started if you are interested. […]
I just noticed that Sean’s paper cites
http://arxiv.org/abs/hep-th/0503249
Any comment as to why that paper is relevant, Sean? 🙂
[…] It’s remarkable how polarizing the whole idea of the string-theory landscape and the anthropic principle really is. It’s not a simple split of string theorists vs. cosmologists vs. everyone else; there are string theorists who love the lanscape, as well as ones who hate it, and likewise for cosmologists or anyone else paying attention. I’ve been arguing that the landscape/multiverse might very well exist and is interesting to think about, but that it’s absolutely impossible right now (and might always be) to use it to calculate anything, or even to sensibly re-calibrate our notions of what is “natural.” I was happy to learn that Paul Steinhardt and Neil Turok are basically in agreement with this view, and are even writing a paper that attempts to make it crystal clear that the landscape does not correctly predict the cosmological constant ala Weinberg. In fact, if we’re allowed to take it seriously at all, it makes quite a strong and vividly different prediction altogether: the cosmological constant should be quite large (many times the matter density, although presumably not at the Planck scale), and we should live in a single lonely galaxy in an empty universe dominated by vacuum energy. Their paper is in preparation, and I hope to say more about it when it comes out. In the meantime, there is serious and hard work to be done to understand the generation and evolution of cosmological perturbations, so it hasn’t all devolved into a shouting match over whether talking about unobservable parts of the universe should count as science. […]
[…] Of course, you do have to make the case that your personally favorite approach is a promising one, to the public and to colleagues in other specialties as well as to graduate students. This is not always a job that string theorists have done well. Some of them, I’ve heard rumors, can even occasionally be a mite arrogant. Let’s admit, this is something of an occupational hazard among academics; if universities fired all the arrogant people, the remaining faculty would be stuck teaching twenty courses a semester. And, while I think that an enormous landscape of stringy vacua might very well exist, I think that supporters of the idea have dramatically failed to take seriously the difficulty of actually calculating anything on that basis. Discussions about these crucial issues have all too often degenerated into sophomore-level philosophy-of-science debates, which haven’t done credit to either side. The truth is, we’re not doing science in a new way, it’s the same old way — trying to come up with the simplest possible consistent and coherent framework that explains the phenomena we observe. […]
[…] I think this is a new category for my CV — “articles subjected to close reading by creationists.” (That, and pioneering the concept of the least bloggable unit.) Here is the first entry: my humble little essay for Nature entitled “Is Our Universe Natural?” has been lovingly dissected at “Creation-Evolution Headlines.” In which they claim that my paper “arms the intelligent design movement in the current fight over the definition of science.” Okay, now those are fighting words. […]
[…] Ol’ Wystan is right; we do have a better time than most of the universe. It would be no fun to constantly worry that “a lover’s kiss / Would either not be felt / Or break the loved one’s neck.” And in a sense, it’s surprising (one might almost say unnatural) that our local conditions allow for the build-up of the delicate complexity necessary to nurture passion and poetry among we creatures of median size. […]
[…] While we’re on the subject of tenure here at CV, there is yet another tenure flap happening in the physics department at Iowa State University. Which is my alma mater (always embarrassing to admit that, so I might as well get it out of the way early on). The department has denied tenure to Professor Guillermo Gonzalez. Prof Gonzalez, by all reports, is the author of nearly 70 peer-reviewed scientific papers, co-author of a major college-level astronomy textbook, his work led to the discovery of two new planets, and he has had his research featured in Science, Nature, and on the cover of Scientific American. Recently, he discovered what is known as the Galactic Habitable Zone, which essentially proposes that life forms when there is the right balance of unique conditions. A hypothesis not too different from our own discussions of the anthropic principle here in theoretical high energy physics… […]
[…] It’s interesting to see how Jo Anne acknowledges the extent of Prof Gonzalez’s science credentials: The department has denied tenure to Professor Guillermo Gonzalez. Prof Gonzalez, by all reports, is the author of nearly 70 peer-reviewed scientific papers, co-author of a major college-level astronomy textbook, his work led to the discovery of two new planets, and he has had his research featured in Science, Nature, and on the cover of Scientific American. Recently, he discovered what is known as the Galactic Habitable Zone, which essentially proposes that life forms when there is the right balance of unique conditions. A hypothesis not too different from our own discussions of the anthropic principle here in theoretical high energy physics! […]