Λ CDM or self-interacting neutrinos: How CMB data can tell the two models apart

• Published in: Physical review. D 2019-09, Vol.100 (6), Article 063524
• Main Authors: Park, Minsu, Kreisch, Christina D., Dunkley, Jo, Hadzhiyska, Boryana, Cyr-Racine, Francis-Yan
• Format: Article
• Language: English
• Subjects: Big Bang theory; Correlation analysis; Cosmic microwave background; Cross-sections; Neutrinos; Polarization; Variation
• ISSN: 2470-0010; 2470-0029
• DOI: 10.1103/PhysRevD.100.063524

Of the many proposed extensions to the ΛCDM paradigm, a model in which neutrinos self-interact until close to the epoch of matter-radiation equality has been shown to provide a good fit to current cosmic microwave background (CMB) data, while at the same time alleviating tensions with late-time measurements of the expansion rate and matter fluctuation amplitude. Interestingly, CMB fits to this model either pick out a specific large value of the neutrino interaction strength, or are consistent with the extremely weak neutrino interaction found in ΛCDM, resulting in a bimodal posterior distribution for the neutrino self-interaction cross section. In this paper, we explore why current cosmological data select this particular large neutrino self-interaction strength, and by consequence, disfavor intermediate values of the self-interaction cross section. We show how it is the ℓ≳1000 CMB temperature anisotropies, most recently measured by the Planck satellite, that produce this bimodality. We also establish that smaller scale temperature data, and improved polarization data measuring the temperature-polarization cross-correlation, will best constrain the neutrino self-interaction strength. We forecast that the upcoming Simons Observatory should be capable of distinguishing between the models.