  Why does there appear in the modified dynamics (MOND) an acceleration
constant, a0, of cosmological significance? An intriguing possibility is that
MOND, indeed inertia itself--as embodied in the actions of free particles and
fields, is due to effects of the vacuum. Either cosmology enters local dynamics
by affecting the vacuum, and inertia in turn, through a0; or, the same vacuum
effect enters both MOND (through a0) and cosmology (e.g. through a cosmological
constant). For the vacuum to serve as substratum for inertia a body must be
able to read in it its non-inertial motion; this indeed it can, by detecting
Unruh-type radiation. A manifestation of the vacuum is also seen, even by
inertial observers, in a non-trivial universe (marked, e.g., by curvature or
expansion). A non-inertial observer in a nontrivial universe will see the
combined effect. An observer on a constant-acceleration (a) trajectory in a de
Sitter universe with cosmological constant L sees Unruh radiation of
temperature T\propto [a^2+a0^2]^{1/2}, with a0=(\L/3)^{1/2}. The temperature
excess over what an inertial observer sees, T(a)-T(0), turns out to depend on a
in the same way that MOND inertia does.
  An actual inertia-from-vacuum mechanism is still a far cry off.
