Reheating the Universe Once More: The Dissipation of Acoustic Waves as a
  Novel Probe of Primordial Inhomogeneities on Even Smaller Scales
  We provide a simple but robust bound on the primordial curvature perturbation
in the range $10^4 {\rm Mpc}^{-1} < k < 10^5 {\rm Mpc}^{-1}$, which has not
been constrained so far unlike low wavenumber modes. Perturbations on these
scales dissipate the energy of their acoustic oscillations by the Silk damping
after primordial nucleosynthesis but before the redshift $z \sim 2 \times 10^6$
and reheat the photon bath without invoking CMB distortions. This {\it acoustic
reheating} results in the decrease of the baryon-photon ratio. By combining
independent measurements probing the nucleosynthesis era and around the
recombination epoch, we find an upper bound on the amplitude of the curvature
perturbation over the above wavenumber range as ${\cal P}_\zeta < 0.06$.
Implications for super massive black holes are also discussed.
