  While the precise relationship between the Milky Way disk and the symmetry
planes of the dark matter halo remains somewhat uncertain, a time-varying disk
orientation with respect to an inertial reference frame seems probable.
Hierarchical structure formation models predict that the dark matter halo is
triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30
muas/yr). These models also predict a time-dependent accretion of gas, such
that the angular momentum vector of the disk should be misaligned with that of
the halo. These effects, as well as tidal effects of the LMC, will result in
the rotation of the angular momentum vector of the disk population with respect
to the quasar reference frame. We assess the accuracy with which the positions
and proper motions from Gaia can be referred to a kinematically non-rotating
system, and show that the spin vector of the transformation from any rigid
self-consistent catalog frame to the quasi-inertial system defined by quasars
should be defined to better than 1 muas/yr. Determination of this inertial
frame by Gaia will reveal any signature of the disk orientation varying with
time, improve models of the potential and dynamics of the Milky Way, test
theories of gravity, and provide new insights into the orbital evolution of the
Sagittarius dwarf galaxy and the Magellanic Clouds.
