Screening of dipolar emission in two-scale Gauss-Bonnet gravity

Abstract

We study black holes in shift-symmetric scalar Gauss-Bonnet gravity extended by a cubic Galileon interaction with a distinct energy scale. Introducing this hierarchy profoundly modifies the theory's phenomenology. The cubic interaction allows for smaller black holes, and can generate a screening mechanism near the horizon, making large Gauss-Bonnet couplings consistent with gravitational-wave bounds. Observable quantities such as the scalar charge, the innermost stable circular orbit, and its frequency are most affected for small black holes. The resulting multi-scale effective field theory remains technically natural and offers new avenues to probe gravity in the strong-field regime.

Figures (2)

• Figure 1: Normalized charge $Q/\sqrt{\alpha_{\text{GB}}}$ as a function of the normalized ADM mass $G M/\sqrt{\alpha_{\text{GB}}}$, for different values of the energy scale $\hat{\Lambda}$. The inset is a zoom-in of the region around the minimum mass for $\sigma=0$ indicated by the black dashed line. We choose $\Lambda_{\text{GB}}=1/r_h$ with $\sigma=-1$. The presence of the cubic interaction does not significantly affect the values of the charge, but it does affect the allowed range of masses.
• Figure 2: Relative difference in the location of the ISCO and the frequency at the ISCO between GR and the theory \ref{['eq:action_2']}, with $\Lambda_{\text{GB}}r_h = 0.1$ and $\hat{\Lambda}r_h=10^{-7}$. The deviations from GR are negligible, unless $Q/\sqrt{\alpha_\text{GB}}\gg 1$.