Trees and the dynamics of polynomials

Laura G. DeMarco, Curtis T. McMullen

Published 2006-08-30, updated 2008-01-04Version 2

The basin of infinity of a polynomial map $f : {\bf C} \arrow {\bf C}$ carries a natural foliation and a flat metric with singularities, making it into a metrized Riemann surface $X(f)$. As $f$ diverges in the moduli space of polynomials, the surface $X(f)$ collapses along its foliation to yield a metrized simplicial tree $(T,\eta)$, with limiting dynamics $F : T \arrow T$. In this paper we characterize the trees that arise as limits, and show they provide a natural boundary $\PT_d$ compactifying the moduli space of polynomials of degree $d$. We show that $(T,\eta,F)$ records the limiting behavior of multipliers at periodic points, and that any divergent meromorphic family of polynomials $\{f_t(z) : t \mem \Delta^* \}$ can be completed by a unique tree at its central fiber. Finally we show that in the cubic case, the boundary of moduli space $\PT_3$ is itself a tree. The metrized trees $(T,\eta,F)$ provide a counterpart, in the setting of iterated rational maps, to the ${\bf R}$-trees that arise as limits of hyperbolic manifolds.