Hemispherical power asymmetry from scale-dependent modulated reheating

We propose a new model for the hemispherical power asymmetry of the CMB based on modulated reheating. Non-Gaussianity from modulated reheating can be small enough to satisfy the bound from Planck if the dominant modulation of the inflaton decay rate is linear in the modulating field sigma. sigma must then acquire a spatially-modulated power spectrum with a red scale-dependence. This can be achieved if the primordial perturbation of sigma is generated via tachyonic growth of a complex scalar field. Modulated reheating due to sigma then produces a spatially modulated and scale-dependent sub-dominant contribution to the adiabatic density perturbation. We show that it is possible to account for the observed asymmetry while remaining consistent with bounds from quasar number counts, non-Gaussianity and the CMB temperature quadupole. The model predicts that the adiabatic perturbation spectral index and its running will be modified by the modulated reheating component