Shimura curves and the abc conjecture

We develop a general framework to study Szpiro's conjecture and the $abc$ conjecture by means of Shimura curves and their maps to elliptic curves, introducing new techniques that allow us to obtain several unconditional results for these conjectures. We first prove various general results about modular and Shimura curves, including bounds for the Manin constant in the case of additive reduction, a detailed study of maps from Shimura curves to elliptic curves and comparisons between their degrees, and lower bounds for the Petersson norm of integral modular forms on Shimura curves. Our main applications for Szpiro's conjecture and the $abc$ conjecture include improved effective bounds for the Faltings height of elliptic curves over $\mathbb{Q}$ in terms of the conductor, bounds for products of $p$-adic valuations of the discriminant of elliptic curves over $\mathbb{Q}$ which are polynomial on the conductor, and results that yield a modular approach to Szpiro's conjecture over totally real number fields with the expected dependence on the discriminant of the field. These applications lie beyond the scope of previous techniques in the subject. A main difficulty in the theory is the lack of $q$-expansions, which we overcome by making essential use of suitable integral models and CM points. Our proofs require a number of tools from Arakelov geometry, analytic number theory, Galois representations, complex-analytic estimates on Shimura curves, automorphic forms, known cases of the Colmez conjecture, and results on generalized Fermat equations.