Dark-Matter Induced Scalarization of Black Holes in Extended Scalar-Tensor-Gauss-Bonnet Theories

Abstract: In the extended scalar-tensor-Gauss-Bonnet theory, spontaneous scalarization in the GB(^-) regime typically occurs only in rotating black holes, while it is absent in spherically symmetric black holes, a phenomenon known as spin-induced scalarization. However, we find that when the spacetime is permeated by perfect fluid dark matter, spontaneous scalarization can also be induced by dark matter in the GB(^-) regime. Analytical calculations reveal that this scalarization occurs when the dark matter parameter (b/M) exceeds a critical value ((b/M)_\text{crit}\simeq1.86287), a threshold determined by the lower boundary of the unstable region for scalar perturbations as the coupling constant approaches negative infinity. Additionally, we verified these findings through numerical analysis of the time evolution of scalar perturbations, identifying the unstable parameter region. The results show that when coupling constant (-\lambda/M^2) is small, spontaneous scalarization only occurs near the extremal black hole limit. As (-\lambda/M^2) increases, the scalarization region expands; however, its lower boundary remains above (b/M \simeq 1.86287), consistent with theoretical predictions.