The magnificent ACT of flavor-specific neutrino self-interaction. (arXiv:2303.08843v1 [astro-ph.CO])
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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. This trend
persists even when neutrino mass is taken into account and extra radiation is
added. We also study the prospect of constraining such strong interaction by
future terrestrial and space telescopes, and find that the upcoming CMB-S4
experiment will improve the upper limit on neutrino self-interaction by about a
factor of three.
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