Real quadratic singular moduli and $p$-adic families of modular forms

The classical theory of elliptic curves with complex multiplication is a fundamental tool for studying the arithmetic of abelian extensions of imaginary quadratic fields. While no direct analogue is available for real quadratic fields, a (conjectural) theory of "real multiplication" was recently proposed by Darmon and Vonk, relying on $p$-adic methods, and in particular on the new notion of rigid meromorphic cocycles. A rigid meromorphic cocycle is a class in the first cohomology of the group $\mathrm{SL}_2(\mathbb{Z}[1/p])$ acting on the non-zero rigid meromorphic functions on the Drinfeld $p$-adic upper half plane by M\"obius transformation. The values of rigid meromorphic cocycles at real quadratic points can be thought of as analogues of singular moduli for real quadratic fields. In this survey article, we will discuss aspects of the theory of complex multiplication and compare them with conjectural analogues for real quadratic fields, with an emphasis on the role played by families of modular forms in both settings.