  We study the effect of free streaming in a universe with cosmic strings with
time-varying tension as well as with constant tension. Although current
cosmological observations suggest that fluctuation seeded by cosmic strings
cannot be the primary source of cosmic density fluctuation, some contributions
from them are still allowed. Since cosmic strings actively produce isocurvature
fluctuation, the damping of small scale structure via free streaming by dark
matter particles with large velocity dispersion at the epoch of
radiation-matter equality is less efficient than that in models with
conventional adiabatic fluctuation. We discuss its implications to the
constraints on the properties of particles such as massive neutrinos and warm
dark matter.
