  We study the possible origin of hydrodynamic turbulence in cold accretion
disks such as those in star-forming systems and quiescent cataclysmic
variables. As these systems are expected to have neutral gas, the turbulent
viscosity is likely to be hydrodynamic in origin, not magnetohydrodynamic.
Therefore MRI will be sluggish or even absent in such disks. Although there are
no exponentially growing eigenmodes in a hydrodynamic disk, because of the
non-normal nature of the eigenmodes, a large transient growth in the energy is
still possible, which may enable the system to switch to a turbulent state. For
a Keplerian disk, we estimate that the energy will grow by a factor of 1000 for
a Reynolds number close to a million.
