  We show that the assumption of non-zero topological susceptibility of the
vacuum in a fermion-free version of a theory, such as gravity or QCD, suffices
to conclude the following: Once N massless fermion flavors are added to the
theory, they break the chiral flavor symmetry dynamically, down to a subgroup
that would be anomaly-free under gauging; In both theories, the
pseudo-Goldstone corresponding to axial U(1)-symmetry becomes massive; In QCD
as well as in gravity the massless fermions are eliminated from the low energy
spectrum of the theory. All the above conclusions are reached without making an
assumption about confinement. Some key methods of our approach are:
Reformulation of topological susceptibility in the language of a three-form
gauge theory; Utilization of gravity in the role of a spectator interaction for
the chiral anomaly-matching in QCD; Gauging chiral symmetries and matching
their anomalies using the spectator Green-Schwarz type axions. Our observations
suggest that breaking of chiral symmetries in QCD and in gravity can be
described in unified topological language, and seemingly-disconnected
phenomena, such as, the generation of eta'-meson mass in QCD and breaking of
global chiral symmetry by gravity may share a secret analogy. The described
phenomenon may shed a new light - via contribution of micro black holes into
the gravitational topological susceptibility of the vacuum - on incompatibility
between black holes and global symmetries. It appears that explicit breaking is
not the sole possibility, and like QCD, gravity may break global symmetries
dynamically. As an useful byproduct, matching of gravitational anomalies
provides a selection tool for compositeness, eliminating possibility of
massless composite fermions where standard gauge anomaly matching would allow
for their existence.
