Regularity for quasilinear elliptic equations in metric measure spaces

In the present article we prove second-order and Lipschitz regularity for quasilinear elliptic equations in metric spaces endowed with a lower bound on the Ricci curvature. The estimates we obtain are quantitative and cover a large class of elliptic equations with polynomial growth. As a particular case we settle the Lipschitz regularity of $p$-harmonic functions for all values of $p\in(1,\infty)$, proving also a Cheng-Yau type inequality. These results are the first in this setting that simultaneously address a wide family of elliptic operators and extend beyond the classical H\"older regularity theory. Our strategy rests on the use of Galerkin's method, which we employ as an alternative to the traditional difference quotients technique.