Discovering new forces with gravitational waves from supermassive black holes

Supermassive black hole binary mergers generate a stochastic gravitational wave background detectable by pulsar timing arrays. While the amplitude of this is subject to significant uncertainties, frequency dependence robust prediction general relativity. We show that effects new forces beyond Standard Model can modify and introduce unique features into spectral shape. In particular, we consider possibility holes in binaries are charged under long-range force, find arrays capable robustly detecting such forces. their environments acquire charge due high-energy particle production or dark sector interactions, making measurement shape powerful test fundamental physics.