Thermal dark energy

We present a novel source of dark energy, which is motivated by the prevalence of hidden sectors in string theory models and is consistent with all of the proposed swampland conjectures. Thermal effects hold a light hidden sector scalar at a point in field space that is not a minimum of its zero temperature potential. This leads to an effective ``cosmological constant, '' with an equation of state w=-1, despite the scalar's zero temperature potential having only a four-dimensional Minkowski or anti--de Sitter vacuum. For scalar masses, which could be technically natural via sequestering, there are large regions of phenomenologically viable parameter space such that the induced vacuum energy matches the measured dark energy density. Additionally, in many models a standard cosmological history automatically leads to the scalar having the required initial conditions. We study the possible observational signals of such a model, including at fifth force experiments and through N₄₅₅ measurements. Similar dynamics that are active at earlier times could resolve the tension between different measurements of H₀ and can lead to a detectable stochastic gravitational wave background.