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Black holes offer a unique laboratory for fundamental physics and are crucial for understanding theories beyond Einstein’s theory of general relativity. In this paper, we focus on 4D effective field theories and string-theory inspired models that include scalar fields. We focus on one such model, axi-dilaton gravity, a quadratic gravity theory with two kinetically coupled scalar fields, an axion and a dilaton. To study the evolution and structure of these fields around black holes, we introduce canuda–axidil, the first open-source, parametrized numerical relativity code for quadratic and biscalar gravity. Using this code, we perform single black hole simulations to show the dynamical formation of axion and dilaton hairs and quantify the effect of higher-order terms in coupling and spin. Through these simulations, we measure the impact of black hole spin and curvature coupling strength on the profiles of the axion and dilaton and show that including kinetic coupling between the fields increases the observed deviations from general relativity. Furthermore, we simulate the axion and dilaton fields around a binary black hole coalescence demonstrating the growth of axion hair during the inspiral and the production of radiative modes for both fields. 

Black holes have a unique sensitivity to the presence of ultralight matter fields or modifications of the underlying theory of gravity. In the present paper we combine both features by studying an ultralight, dynamical scalar field that is nonminimally coupled to the gravitational Chern-Simons term. In particular, we numerically simulate the evolution of such a scalar field around a rotating black hole in the decoupling approximation and find a new kind of massive scalar hair anchored around the black hole. In the proximity of the black hole, the scalar exhibits the typical dipolar structure of hairy solutions in (massless) dynamical Chern-Simons gravity. At larger distances, the field transitions to an oscillating scalar cloud that is induced by the mass term. Finally, we complement the time-domain results with a spectral analysis of the scalar field characteristic frequencies. 

Abstract Gravitational waves emitted by black hole binary inspiral and mergers enable unprecedented strong-field tests of gravity, requiring accurate theoretical modeling of the expected signals in extensions of general relativity. In this paper we model the gravitational wave emission of inspiralling binaries in scalar Gauss–Bonnet gravity theories. Going beyond the weak-coupling approximation, we derive the gravitational waveform to relative first post-Newtonian order beyond the quadrupole approximation and calculate new contributions from nonlinear curvature terms. We also compute the scalar waveform to relative 0.5PN order beyond the leading −0.5PN order terms. We quantify the effect of these terms and provide ready-to-implement gravitational wave and scalar waveforms as well as the Fourier domain phase for quasi-circular binaries. We also perform a parameter space study, which indicates that the values of black hole scalar charges play a crucial role in the detectability of deviation from general relativity. We also compare the scalar waveforms to numerical relativity simulations to assess the impact of the relativistic corrections to the scalar radiation. Our results provide important foundations for future precision tests of gravity.