How Cities Grow: Universal Roughness, Urban Sprawl, and an Intermezzo on Scaling

by Prof. Marc Barthelemy (IPhT (Paris))

Thursday 28 May 2026, 11:00 → 12:00 Europe/Rome

Urban sprawl reshapes cities worldwide, yet its spatial dynamics are still largely described in qualitative or descriptive terms.

In this seminar, I will present a quantitative framework for urban expansion inspired by non-equilibrium surface growth physics. Using high-resolution built-up area data and historical population records for 19 cities across multiple continents, the work reveals anisotropic, branch-like expansion patterns and identifies two competing growth mechanisms: smooth local accretion at the urban fringe and abrupt coalescence with surrounding clusters. Despite the diversity of urban trajectories, a universal local roughness exponent close to 1/2 emerges, while global growth exponents vary widely, highlighting the roles of anisotropy, coalescence, and heterogeneity in shaping urban form.

I will also discuss what urban scaling laws can, and cannot, mean in this context, emphasizing the distinction between aggregate cross-sectional regularities and the actual temporal dynamics of individual cities.

Location: Aula Vigna (Via Jappelli 1)

locandina_Barthelemy.pdf