The cosmic microwave background (CMB) provides a unique window onto fundamental physics, allowing us to study particles and interactions that are otherwise extremely difficult to probe in laboratory experiments. One important example is the cosmic neutrino background, which influences the evolution of the early Universe, primarily through gravity despite interacting only very weakly with ordinary matter.
In this talk, I will discuss how the free-streaming nature of neutrinos is expected to leave a subtle but distinctive imprint on the CMB by inducing a phase-shift in the acoustic oscillations of the photon–baryon plasma prior to recombination, providing a clean and robust probe of neutrino physics. I will present a framework to extract this effect directly from CMB observations, and show that current data from Planck, ACT, and SPT detect it with high significance, consistent with the Standard Model prediction of three free-streaming neutrino species.
I will then discuss how the same measurement can be used to test beyond standard model scenarios in which neutrinos interact more strongly, delaying their decoupling from the primordial plasma. By directly relating the observed phase shift to neutrino interactions, cosmological data can place competitive constraints on new neutrino physics using this single, well-understood observable and without the need for dedicated model-dependent analyses.