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.