Searching for dark energy with the Sun

General extensions of General Relativity (GR) based on bona fide degrees of freedom predict a fifth force which operates within massive objects, opening up an exciting opportunity to perform precision tests of gravity at stellar scales. Here, focusing on general scalar-tensor theories for dark energy, we utilize the Sun as our laboratory and search for imprints of the fifth-force effect on the solar equilibrium structure. With analytic results and numerical simulations, we explain how the different solar regions offer powerful ways to test gravity. Accounting for the delicate interplay between fifth force and solar microphysics such as opacity, diffusion, equation of state and metallicity, we demonstrate that the fifth force still leaves a sharp signature on the solar sound speed, in a region where simple estimates of input physics uncertainties become negligible. For general scalar-field extensions of GR, known as (U-)DHOST, based solely on the observational helioseismic errors, our analysis at the equilibrium level allows to place an approximate constraint on the fifth-force coupling strength of $-10^{-3} \lesssim \mathcal{Y} \lesssim 5\cdot 10^{-4}$ at $2\sigma$. This result improves previous stellar constraints by $\sim 3$ orders of magnitude, and should be confirmed and improved by future helioseismic inversions in modified gravity combined with an elaborate accounting of theoretical uncertainties. Our analysis can be applied to a wide set of theories beyond GR, and also paves the way for helioseismic analyses in this context. In this regard, we discuss how the solar radiative and convective zone can be employed as promising laboratories to test generic theories of gravity.