Can String Theory Explain Dark Energy? [indent]A new paper by Cambridge physicist Stephen Hawking and Thomas Hertog of CERN (hertog@mail.cern.ch) suggests that it can. The leading explanation for the observed acceleration of the expansion of the universe is that a substance, dark energy, fills the vacuum and produces a uniform repulsive force between any two points in space  a sort of antigravity. Quantum field theory allows for the existence of such a universal tendency. Unfortunately, its prediction for the value of the density of dark energy (a parameter referred to as the cosmological constant) is some 120 orders of magnitude larger than the observed value. In 2003, cosmologist Andrei Linde of Stanford University and his collaborators showed that string theory allows for the existence of dark energy, but without specifying the value of the cosmological constant. String theory, they found, produces a mathematical graph shaped like a mountainous landscape, where altitude represents the value of the cosmological constant. After the big bang, the value would settle on a low point somewhere between the peaks and valleys of the landscape. But there could be on the order of 10500 possible low points  with different corresponding values for the cosmological constant  and no obvious reason for the universe to pick the one we observe in nature. Some experts hailed this multiplicity of values as a virtue of the theory. For example, Stanford University's Leonard Susskind in his book "The Cosmic Landscape: String Theory and the Illusion of Intelligent Design," argues that different values of the cosmological constant would be realized in different parallel worlds  the pocket universes of Linde's "eternal inflation" theory. We would just happen to live in one where the value is very small. But critics see the landscape as exemplifying the theory's inability to make useful predictions. The Hawking/Hertog paper is meant to address this concern. It looks at the universe as a quantum system in the framework of string theory. Quantum theory calculates the odds a system will evolve a certain way from given initial conditions, say, photons going through a double slit and hitting a certain spot on the other side. You repeat your experiment often enough and then you check that the odds you predicted were the correct ones. In Richard Feynman's formulation of quantum theory, the probability that a photon ends up at a particular spot is calculated by summing up over all possible trajectories for the photon. A photon goes through multiple paths at once and can even interfere with its other personas in the process. Hawking and Hertog argue that the universe itself must also follow different trajectories at once, evolving through many simultaneous, parallel histories, or "branches." (These parallel universes are not to be confused with those of eternal inflation, where multiple universes coexist in a classical rather than in a quantum sense.) What we see in the present would be a particular, more or less probable, outcome of the "sum" over these histories. In particular, the sum should include all possible initial conditions, with all possible values of the cosmological constant. But applying quantum theory to the entire universe  where the experimenters are part of the experiment  is tricky. Here you have no control over the initial conditions, nor can your repeat the experiment again and again for statistical significance. Instead, the HawkingHertog approach starts with the present and uses what we know about our branch of the universe to trace its history backwards. Again, there will be multiple possible branches in our past, but most can be ignored in the Feynman summation because they are just too different from the universe we know, so the probability of going from one to the other is negligible. For example, Hertog says, knowledge that our universe is very close to being flat could allow one to concentrate on a very small portion of the string theory landscape whose values for the cosmological constant are compatible with that flatness. That could in turn lead to predictions that are experimentally testable. For example, one could calculate whether our universe is likely to produce the microwave background spectrum we actually observe. [/indent] ___
