Cosmic Microwave Background Power Asymmetry from Non-Gaussian Modulation

Non-Gaussianity in the inflationary perturbations can couple observable scales to modes of much longer wavelength (even superhorizon), leaving as a signature a large-angle modulation of the observed cosmic microwave background power spectrum. This provides an alternative origin for a power asymmetry that is otherwise often ascribed to a breaking of statistical isotropy. The non-Gaussian modulation effect can be significant even for typical $\sim {10}^{-5}$ 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 has been claimed for inflation models with a non-Bunch-Davies initial state, for instance. Upper limits on the observed cosmic microwave background power asymmetry place stringent constraints on the duration of inflation in such models.

Figure 1

Expected amplitude $⟨A⟩$ of a dipole modulation ($L=1$) as a function of the CMB multipole $l$ for three sets of values for (${\alpha }_1$, $k_{\ell ,\min }$) as indicated in the figure. We have used Eq. (10) with $g_1=1$. Predictions for different $L$ can be obtained by multiplying with $g_L(L!{)}^2{4}^L/2(2L)!$.