Multifield inflation after Planck: Isocurvature modes from nonminimal couplings

Recent measurements by the Planck experiment of the power spectrum of temperature anisotropies in the cosmic microwave background radiation reveal a deficit of power in low multipoles compared to the predictions from best-fit Lambda CDM cosmology. If the low-l anomaly persists after additional observations and analysis, it might be explained by the presence of primordial isocurvature perturbations in addition to the usual adiabatic spectrum, and hence may provide the first robust evidence that early-Universe inflation involved more than one scalar field. In this paper we explore the production of isocurvature perturbations in nonminimally coupled two-field inflation. We find that this class of models readily produces enough power in the isocurvature modes to account for the Planck low-l anomaly, while also providing excellent agreement with the other Planck results.