  Gravitational waves induced by large primordial curvature fluctuations may
result in a sizable stochastic gravitational wave background. Interestingly,
curvature fluctuations are gradually generated by initial isocurvature
fluctuations, which in turn induce gravitational waves. Initial isocurvature
fluctuations commonly appear in multi-field models of inflation as well as in
the formation of scattered compact objects in the very early universe, such as
primordial black holes and solitons like oscillons and cosmic strings. Here we
provide a review on isocurvature induced gravitational waves and its
applications to dark matter and the primordial black hole dominated early
universe.
