The Modified Dynamics as a Vacuum Effect
  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. 
Mordehai Milgrom (Weizmann Institute)