Global view of neutrino interactions in cosmology: The freestreaming window as seen by Planck

Understand

Neutrinos are expected to freestream (i.e.

• not interact with anything) since they decouple in the early Universe at a temperature $T\sim 2~{\rm MeV}$.
• However, there are many relevant particle physics scenarios that can make neutrinos interact at $T< 2~{\rm MeV}$.
• In this work, we take a global perspective and aim to identify the temperature range in which neutrinos can interact given current cosmological observations.