The Well-Tempered Cosmological Constant

Abstract

Self tuning is one of the few methods for dynamically cancelling a large cosmological constantand yet giving an accelerating universe. Its drawback is that it tends to screen all sources of energydensity, including matter. We develop a model that tempers the self tuning so the dynamical scalarfield still cancels an arbitrary cosmological constant, including the vacuum energy through any highenergy phase transitions, without affecting the matter fields. The scalar-tensor gravitational actionis simple, related to cubic Horndeski gravity, with a nonlinear derivative interaction plus a tadpoleterm. Applying shift symmetry and using the property of degeneracy of the field equations we findfamilies of functions that admit de Sitter solutions with expansion rates that are independent ofthe magnitude of the cosmological constant and preserve radiation and matter dominated phases. That is, the method can deliver a standard cosmic history including current acceleration, despite the presence of a Planck scale cosmological constant.