Standard thermodynamic treatments of quantum field theory in the presence of
black-hole backgrounds reproduce the black hole entropy by usually specializing
to the leading order of the heat-kernel or the high-temperature expansion. By
contrast, this work develops a hybrid framework centered on geometric spectral
asymptotics whereby these assumptions are shown to be unwarranted insofar as
black hole thermodynamics is concerned. The approach--consisting of the
concurrent use of near-horizon and heat-kernel asymptotic expansions--leads to
a proof of the holographic scaling of the entropy as a universal feature driven
by conformal quantum mechanics.Comment: 13 pages, JHEP style. Added section 3 in the new version and a few
typos were correcte