  Collisionless dark matter galactic halos are expected to exhibit damped
oscillations as a result of ongoing late time accretion. An n-body model of the
cosmological assembly of a Milky Way-like halo is used to quantify the time
dependence of its gravitational field. The simulation contains stellar streams
whose incommensurate perpendicular velocities are found to have an
approximately exponential distribution with a scale of 10-20\kms, depending on
how the stars are selected, comparable to those reported for the Orphan stream.
The fluctuations in the quadrupole moment of the dark matter halo are
sufficient to largely explain the tangential velocities. If velocity
measurements of a larger sample of Milky Way streams finds (or does not find)
the expected distribution of transverse velocities it will lead to limits on
the cross-section of self-interacting dark matter, in which kinetic viscosity
can damp the oscillations more rapidly than the mixing processes of
collisionless dark matter alone.
