Stochastic Tsunamis: Diffuse Scalar Background from Black Hole Formation

Massive astrophysical objects can source huge static configurations of a scalar field. When such an object ends up forming a black hole, for instance, via a core-collapse supernova, the scalar field loses its source abruptly; then the static configuration becomes dynamical and propagates away in a burst, a "scalar tsunami". These bursts accumulate over cosmological history, forming a relic stochastic diffuse scalar background peaked in the $1-10^3~\text{Hz}$ range. We propose this as a novel mechanism for the generation of such a background, compute its spectrum, and compare it with the sensitivity of future experiments. We show how this extends the experimental sensitivity to scalar masses $m_\phi\lesssim 10^{-13}~\text{eV}$, ten orders of magnitude larger than those accessible via individual transient events previously considered.