# $\Lambda$CDM or self-interacting neutrinos? - how CMB data can tell the two models apart

## TL;DR

This paper investigates how current and future CMB data, especially from Planck and the Simons Observatory, can differentiate between the standard $
ull$CDM model and models with self-interacting neutrinos, focusing on the bimodal posterior distribution of the interaction strength.

## Contribution

The study explains why high-$\ell$ CMB anisotropies cause bimodality in neutrino self-interaction models and forecasts how upcoming data can better constrain or distinguish these models.

## Key findings

- High-$\ell$ CMB data cause bimodality in neutrino interaction strength.
- Future polarization data will improve constraints on neutrino self-interactions.
- Simons Observatory can distinguish between $
ull$CDM and self-interacting neutrino models.

## Abstract

Of the many proposed extensions to the $\Lambda$CDM 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 $\Lambda$CDM, 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 $\ell \gtrsim 1000$ 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.