New Window on Primordial Non-Gaussianity

We know very little about primordial curvature perturbations on scales smaller than about a Mpc. Measurements of the mu 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 similar to 100) of mu is necessary. We point out that correlations between mu distortion and temperature anisotropies can be used to test Gaussianity at these very small scales. In particular the mu T two-point correlation is proportional to the very squeezed limit of the primordial bispectrum and hence measures f(NL)(loc), while mu mu is proportional to the primordial trispectrum and measures tau(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 Delta f(NL)(loc) similar to O(10(-3)).