Sound speed induced production of primordial black holes
  We study different mechanisms by which the speed of primordial curvature
perturbations can produce an enhancement of the curvature spectrum, which could
lead to the production of primordial black holes (PBH). One possibility is the
growth of the sound speed in single scalar field models, which can induce
super-horizon growth of curvature perturbations. The other is the momentum
dependent effective sound (MESS) of curvature perturbations, which can arise in
multi-fields models or modified gravity theories.
  Future gravitational waves observatories such as LISA will allow to set
constraints of the space and time dependency of the sound speed, and on the
different theoretical scenarios from which it can originate.
