Gravitational waves from spontaneously scalarized black holes

Certain scalar-tensor theories remain viable despite stringent observational constraints from the Solar System due to a Z_2 symmetry that keeps the scalar field dormant in the weak-field regime. However, extreme-gravity environments can trigger a phase transition that promotes the spontaneous growth of the scalar field around compact objects like black holes and neutron stars. This is the phenomenon of spontaneous scalarization. In this talk, I will discuss how we can construct cheap, approximate models of the gravitational waveforms emitted by binary systems in certain scalar-tensor-Gauss-Bonnet theories that allow for the spontaneous scalarization of black holes. We will then confront these waveforms with existing data from the LIGO Scientific, Virgo, and KAGRA Collaborations to establish some of the first constraints on this class of theories.