Static spheres in black hole spacetimes: pairing, energy conditions, and an upper bound on the innermost radius

Abstract

In this work, we investigate the existence, relation to the energy conditions, and radial bounds of static spheres in general static, spherically symmetric, asymptotically flat black hole spacetimes. By analyzing the global behavior of a radial function constructed from the mass and radial pressure functions, we prove that a static sphere necessarily requires a negative radial pressure (tension). Furthermore, we show that non-degenerate static spheres must always appear in pairs: an inner unstable sphere and an outer stable one. Assuming the Weak Energy Condition (WEC) always holds, the inner static sphere is characterized by a violation of the strong energy condition (SEC) inequality $\rho+p+2p_T<0$, where $\rho$, $p$, and $p_T$ denote the energy density, radial pressure, and tangential pressure, respectively; the SEC inequality is restored ($\rho+p+2p_T> 0$) at the outer sphere; the degenerate marginal case satisfies $\rho+p+2p_T=0$. In addition, focusing on the innermost static sphere and assuming that the WEC holds while the SEC is uniformly violated between the event horizon and this sphere, we derive a rigorous upper bound on its radius, \[ r^-_{\mathrm{sp}}\le \left[r_H^3+\frac{3r_H\bigl(1-8\pi r^2_H\rho(r_H)\bigr)}{8\pi\kappa}\right]^{1/3}, \] where $r_H$ is the horizon radius, $\rho(r_H)$ the energy density at the horizon, and $\kappa$ characterizes the strength of the SEC violation. These results establish a direct, analytic link between the energy conditions and the existence of static spheres, and provide a quantitative constraint on the matter environment of black holes possessing such orbits. The findings have potential applications in testing black hole solutions in general relativity and modified theories of gravity, as well as in interpreting related astronomical observations.