CMB dipole asymmetry from a fast roll phase

Understand

The observed CMB (cosmic microwave background) dipole asymmetry cannot be explained by a single field model of inflation - it inevitably requires more than one field where one of the fields is responsible for amplifying the super-Hubble fluctuations beyond the pivot scale.

• Furthermore the current constraints on $f_NL$ and $tau_NL$ require that such an amplification cannot produce large non-Gaussianity.
• In this paper we propose a model to explain this dipole asymmetry from a spectator field, which is responsible for generating all the curvature perturbations, but has a temporary fast roll phase before the Hubble exit of the pivot scale.
• The current data prefers spectator scenario because it leaves no isocurvature perturbations.