CMB Power Asymmetry from Non-Gaussian Modulation
Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (superhorizon even), leaving as signature a large-angle modulation of the observed CMB power spectrum. This provides an alternative origin for a power asymmetry which is otherwise often ascribed to a breaking of statistical isotropy. The non-Gaussian modulation effect can be significant even for typical ~10^-5 perturbations, while respecting current constraints on non-Gaussianity, if the squeezed limit of the bispectrum is sufficiently infrared-divergent. Just such a strongly infrared-divergent bispectrum is predicted by inflation models with a non-Bunch-Davies initial state, for instance. Upper limits on the observed CMB power asymmetry place stringent constraints on the duration of inflation in such models.