Commenter Shantanu pointed to a web-site with talks available on-line from a symposium about Dark Energy now going on at the Space Telescope Science Institute. Yesterday Witten gave a talk entitled “Models of Dark Energy”, where he lays out very clearly the conventional wisdom of the string theory community about the dark energy problem and its implications for string theory.
Witten describes how the problem of a huge number of possible vacua has always been an embarrassment for string theory. Until about 10 years ago his attitude towards most constructions of string vacua was “who needs this mess”, thinking that once one figured out the vacuum energy problem, such constructions would all go away. He explains how the discovery of a small positive CC has changed his attitude, that he’s no longer sure that one can find a distinguished vacuum state, and thus maybe the anthropic landscape/multiverse crowd is right. He describes this possibility as involving both good news and bad news:
The good news (such as it is) then is that if we are really living in a “multiverse”, it may be that the theory as we know it is pretty close to the truth.
But there’s a hefty dose of bad news… If the vacuum of the real world is really a needle in a haystack, it is hard to see how we are supposed to be able to understand it. In other words, if an unimaginably large number of approximate “vacuum” states are realized in different parts of the Universe, none of them with any special meaning, and with the details of particle physics depending on where one happens to live, then what sort of understanding of particle physics can we hope to get? I don’t have an answer to this question, although we might learn something from the LHC that will help…
The crucial point of course is this last one: how can you ever test these ideas, making them real science and not metaphysics? At the end of his talk, Rachel Bean tried to pin him down on this question, leading to this exchange:
Bean: “If we have this landscape, this multiverse, … can we learn nothing, or is there some hope, do you have some hope, that if you were to find a universe that had remarkably small CC you could also make some allusion to the other properties of that universe for example the fine structure constant, or are we saying that all of these things are random variables, uncorrelated and we’ll never get an insight.”
Witten: “Well, I don’t know of course, I’m hoping that we’ll learn more, perhaps the LHC will discover supersymmetry and maybe other unexpected discoveries will change the picture. I wasn’t meaning to advocate anything.”
Bean: “I’m asking your opinion.”
Witten (after a silence): “I don’t really know what to think has got to be the answer…”
Besides the landscape problem, Witten also described attempts to model dark energy as an aspect of some differerent sort of physical field, saying that he has been working on this with a student, but that the problem is the strong experimental bounds on the existence of light fields coupling to ordinary matter.