Regular black holes from dynamical-tension string hedgehogs

Abstract

A spherical cloud of radial strings with non-zero constant tension has an energy density proportional to $r^{-2}$ and therefore produces a singular conical geometry rather than an asymptotically flat regular black hole. We show that this obstruction can be removed within the modified-measure formulation of strings. A continuum of radial worldsheets whose tensions are generated by a composite bulk tension scalar reduces, on a constrained $\mathrm{SO}(3)$ hedgehog branch, to Einstein gravity coupled to a nonlinear function $K(Y)$ and an auxiliary three-form. This construction gives model-independent conditions directly in terms of the radial tension $\mathcal{T}(r)$. Finite curvature at the centre requires $\mathcal{T}=\mathcal{O}(r^2)$, while the null energy condition forbids a faster leading power. Thus a positive regular centre satisfying the null energy condition is necessarily de Sitter. Finite ADM mass requires the tension to be screened at infinity, whereas the dominant energy condition cannot hold globally for a non-trivial positive profile screened faster than $r^{-2}$. An exact cubic profile realises these properties and gives a continuous family of asymptotically flat geometrically regular black holes whose first correction to Schwarzschild appears at order $r^{-4}$.