Probing small-scale non-Gaussianity from anisotropies in acoustic
  reheating
  We give new constraints on small-scale non-Gaussianity of primordial
curvature perturbations by the use of anisotropies in acoustic reheating.
Mixing of local thermal or local kinetic equilibrium systems with different
temperatures yields a locally averaged temperature rise, which is proportional
to the square of temperature perturbations damping in the photon diffusion
scale. Such secondary temperature perturbations are indistinguishable from the
standard temperature perturbations linearly coming from primordial curvature
perturbations and hence should be subdominant compared to the standard ones. We
show that small-scale higher order correlation functions (connected
non-Gaussian and disconnected Gaussian parts) of primordial curvature
perturbations can be probed by investigating auto power spectrum of the
generated secondary perturbations and the cross power spectrum with the
standard perturbations. This is simply because these power spectra come from
higher order correlation functions of primordial curvature perturbations with
non-linear parameters such as $f_{\rm NL}$ and $\tau_{\rm NL}$ since secondary
temperature perturbations are second order effects. Thus, the observational
results $l(l+1)C^{TT}_l\simeq 6\times 10^{-10}$ at large scales give a robust
and universal upper bound on small-scale non-Gaussianities of primordial
curvature perturbations.
