Future constraints on neutrino isocurvature perturbations in the curvaton scenario

TL;DR

This paper investigates how future cosmic microwave background experiments can constrain neutrino isocurvature perturbations in the curvaton scenario, focusing on parameters like b3, b4N_eff, and neutrino degeneracy.

Contribution

It provides forecasted constraints on neutrino isocurvature perturbations and related parameters for upcoming experiments, extending understanding of neutrino properties in the early universe.

Findings

Planck can limit b3 to 5.3x10^-3 and b4N_eff to 0.16.

SPIDER can limit b3 to 1.2x10^-2 and b4N_eff to 0.40.

CMBPol can further tighten constraints to b3 < 1.5x10^-3 and b4N_eff < 0.043.

Abstract

In the curvaton scenario, residual isocurvature perturbations can be imprinted in the cosmic neutrino component after the decay of the curvaton field, implying in turn a non-zero chemical potential in the neutrino distribution. We study the constraints that future experiments like Planck, SPIDER or CMBPol will be able to put on the amplitude of isocurvature perturbations in the neutrino component. We express our results in terms of the square root \gamma of the non-adiabaticity parameter \alpha and of the extra relativistic degrees of freedom \Delta N_eff. Assuming a fiducial model with purely adiabatic fluctuations, we find that Planck (SPIDER) will be able to put the following upper limits at the 1sigma level: \gamma < 5.3x10^-3 (1.2x10^-2) and \Delta N_eff < 0.16 (0.40) . CMBPol will further improve these constraints to \gamma < 1.5x10^-3 and \Delta N_eff < 0.043. Finally, we recast these bounds in terms of the background neutrino degeneracy parameter \xi\ and the corresponding perturbation amplitude \sigma_\xi, and compare with the bounds on \xi\ that can be derived from Big Bang Nucleosynthesis.