Primordial fluctuations of a vast range of wavelengths were generated in the early Universe. The power spectrum of those on large scales has been well determined thanks to the observations of the anisotropy of the cosmic microwave background (CMB) radiation. Probing small-scale primordial fluctuations also gives valuable information about the early Universe. Diffusion damping of short-scale acoustic waves in the early Universe, and primordial black holes are among helpful methods in probing small-scale primordial fluctuations.
References include arXiv:1403.5407, 1703.10559, 1710.06945.