Scalar Vacuum Polarization in Loop Quantum Gravity Black Holes

Abstract

A quantum macroscopic ``Kruskal'' black hole solution that incorporates quantum geometry effects has been derived in Loop Quantum Gravity as the counterpart to the classical Schwarzschild solution with a distinct imprint outside the event horizon, even at scales much larger than the Planck length. This resulting black hole quantum geometry is supported by an effective energy density of quantum origin, outside the horizon, which prevents asymptotic flatness at large distances and confines massive particles to finite radii, thereby preventing their escape to infinity. In this work we adapt to these solutions the extended Anderson-Candelas-Christensen-DeWitt approach to compute the quantum vacuum polarization in order to provide an accurate measure of the quantum activity around these black holes. We carry out a numerical implementation of the formalism and present, to our knowledge for the first time, the scalar vacuum polarization $\langle\phi^2\rangle$ exterior to this quantum-corrected geometry. We find that the quantum-gravity exponent $\epsilon$ enhances the near-horizon polarization and induces, farther out, a small negative tail that we identify -- through a parameter-free DeWitt--Schwinger comparison -- with the field's response to the nonzero curvature of the background (absent for Ricci-flat Schwarzschild). The correction scales linearly with $\epsilon$, the parameter tracking the quantum gravitational corrections, so that for astrophysically realistic (i.e., tiny) $\epsilon$, the result is numerically indistinguishable from Schwarzschild. The calculation furnishes a consistency check on the quantum activity around these solutions, the fluctuations tracking the local curvature without anomalous growth in the exterior.