On the Cosmological Constant and Dark Energy in String Theory
Why doesn’t the vacuum energy, implied by QFT and the Standard Model of Particle Physics, curve spacetime catastrophically towards an exponentially rapid expansion or collapse? Supersymmetry is known to ameliorate this long-standing Cosmological Constant Problem, but only down to the scale set by the susy-breaking mass splittings; at least ~TeV for the visible sector. String theory offers new possibilities. I will describe how the symmetries and dynamics of string theory can lead to a suppression of the one-loop vacuum energy down to the observed scale of Dark Energy.  I will illustrate these ideas with an explicit string construction, based on non-supersymmetric string theory, and discuss the remaining opportunities and challenges for theory and observation.