The magnificent ACT of flavor-specific neutrino self-interaction

Understand

We revisit the cosmology of neutrino self-interaction and use the latest cosmic microwave background data from the Atacama Cosmology Telescope (ACT) and the Planck experiment to constrain the interaction strength.

• In both flavor-universal and nonuniversal coupling scenarios, we find that the ACT data prefers strong neutrino self-interaction that delays neutrino free streaming until just before the matter-radiation equality.
• When combined with the Planck 2018 data, the preference for strong interaction decreases due to the Planck polarization data.
• For the combined dataset, the flavor-specific interaction still provides a better fit to the CMB data than $\Lambda$CDM.