Hairy black hole solutions in nonlocal quadratic gravity

Abstract: Hairy black hole solutions are constructed within quantum-inspired nonlocal quadratic gravity. Nonlocal effects induce Yukawa screening, which shifts the event horizon inward and modifies both the Bekenstein--Hawking entropy and the Hawking temperature, while also renormalizing the chemical potential. Nonlocal corrections also reduce the magnitude of the negative specific heat, making small hairy black holes more stable, with Helmholtz and Gibbs free energies consistent with the absence of first-order phase transitions. The nonlocal spin-2 propagator contains, in addition to the massless graviton, a massive pole with positive residue and positive norm. Consequently, hairy black holes in nonlocal quadratic gravity are free of ghost instabilities at the quadratic and classical levels, in the effective field theory.