A Scale-Dependent Power Asymmetry from Isocurvature Perturbations

TL;DR

This paper explores a curvaton-based model with isocurvature perturbations to explain the scale-dependent power asymmetry observed in the CMB, consistent with quasar constraints and requiring specific conditions on dark matter contributions.

Contribution

It introduces a curvaton scenario with isocurvature perturbations that can produce the observed CMB asymmetry while satisfying quasar constraints, highlighting the importance of dark matter contributions.

Findings

Asymmetry is most prominent on large scales in the CMB.

Model requires a small but nonzero curvaton contribution to dark matter.

Future CMB observations can test the specific spectral predictions of this model.

Abstract

If the hemispherical power asymmetry observed in the cosmic microwave background (CMB) on large angular scales is attributable to a superhorizon curvaton fluctuation, then the simplest model predicts that the primordial density fluctuations should be similarly asymmetric on all smaller scales. The distribution of high-redshift quasars was recently used to constrain the power asymmetry on scales k ~ 1.5h/Mpc, and the upper bound on the amplitude of the asymmetry was found to be a factor of six smaller than the amplitude of the asymmetry in the CMB. We show that it is not possible to generate an asymmetry with this scale dependence by changing the relative contributions of the inflaton and curvaton to the adiabatic power spectrum. Instead, we consider curvaton scenarios in which the curvaton decays after dark matter freezes out, thus generating isocurvature perturbations. If there is a superhorizon fluctuation in the curvaton field, then the rms amplitude of these perturbations will be asymmetric, and the asymmetry will be most apparent on large angular scales in the CMB. We find that it is only possible to generate the observed asymmetry in the CMB while satisfying the quasar constraint if the curvaton's contribution to the total dark matter density is small, but nonzero. The model also requires that the majority of the primordial power comes from fluctuations in the inflaton field. Future observations and analyses of the CMB will test this model because the power asymmetry generated by this model has a specific spectrum, and the model requires that the current upper bounds on isocurvature power are nearly saturated.