Near-extremal critical dynamics driven by a phantom field in Einstein-Maxwell-scalar theory
We investigate the fully nonlinear dynamics of Einstein-Maxwell-scalar black holes in the low-mass, near-extremal region of the solution space, which is difficult to access with conventional positive-energy perturbations. An independent phantom field is introduced as a controllable negative-energy channel, and its initial amplitude is varied while the remaining parameters of the initial data are held fixed. The phantom contribution is concentrated during the early-time infall and reduces the quasi-local mass in the black-hole region at fixed electric charge, thereby allowing the evolution to approach near-extremal configurations. Near a critical amplitude $p_\ast$, evolutions on both sides of the threshold enter the same long-lived scalarized intermediate stage before separating toward either a near-extremal Reissner-Nordstrom black hole or a nonextremal hairy black hole. The residence time near this intermediate stage obeys the logarithmic scaling law of Type I critical behavior, and the growth rate extracted from the departure is consistent with the inverse lifetime coefficient. The intermediate stage is characterized by a small horizon surface gravity and finite horizon curvature, with no curvature blow-up resolved at the horizon during the simulated interval. These results demonstrate that phantom-assisted negative-energy transport provides a dynamical route into the near-extremal sector of the EMS solution space and reveals a threshold-mediated transition between bald and hairy black-hole end states.