Scalar induced gravitational waves review
  We provide a review on the state-of-the-art of gravitational waves induced by
primordial fluctuations, so-called induced gravitational waves. We present the
intuitive physics behind induced gravitational waves and we revisit and unify
the general analytical formulation. We then present general formulas in a
compact form, ready to be applied. This review places emphasis on the open
possibility that the primordial universe experienced a different expansion
history than the often assumed radiation dominated cosmology. We hope that
anyone interested in the topic will become aware of current advances in the
cosmology of induced gravitational waves, as well as becoming familiar with the
calculations behind.
