New window on primordial non-gaussianity.

We know very little about primordial curvature perturbations on scales smaller than about a Mpc. Measurements of the μ distortion of the cosmic microwave background spectrum provide the unique opportunity to probe these scales over the unexplored range from 50 to 10(4)  Mpc(-1). This is a very clean probe, in that it relies only on well understood linear evolution. Also, just the information about the low multipoles (l∼100) of μ is necessary. We point out that correlations between μ distortion and temperature anisotropies can be used to test gaussianity at these very small scales. In particular the μT two-point correlation is proportional to the very squeezed limit of the primordial bispectrum and hence measures f(NL)(loc), while μμ is proportional to the primordial trispectrum and measures τ(NL). We present a Fisher matrix forecast of the observational constraints on f(NL)(loc) and stress that a cosmic variance limited experiment could in principle reach Δf(NL)(loc)∼O(10(-3)).