  The dark matter halos in $\Lambda$CDM cosmological simulations are triaxial
and highly flattened. In many cases, these triaxial equilibria are also
tumbling slowly, typically about their short axes, with periods of order a
Hubble time. Halos may therefore exert a slowly changing external torque on
spiral galaxies that can affect their dynamical evolution in interesting ways.
We examine the effect of the external torques exerted by a tumbling quadrupolar
tidal field on the evolution of spiral galaxies using N-body simulations with
realistic, disk galaxy models. We measure the amplitude of the external
quadrupole moments of dark halos in cosmological simulations and use these to
force disk galaxy models in a series of N-body experiments for a range of
pattern speeds. We find that the torques are strong enough to induce long lived
transient warps in disks similar to those observed in real spirals and also
induce the bar instability at later times in some galaxy models that are
otherwise stable for long periods of time in isolation. We also observe forced
spiral structure near the edge of the disk where normally self-gravity is too
weak to be responsible for such structure. This overlooked influence of dark
halos may well be responsible for many of the peculiar aspects of disk galaxy
dynamics.
