Annual modulation of cosmic relic neutrinos
The cosmic neutrino background (CνB), produced about one second after the big bang, permeates the Universe today. New technological advancements make neutrino capture on beta-decaying nuclei (NCB) a clear path forward towards the detection of the CνB. We show that gravitational focusing by the Sun causes the expected neutrino capture rate to modulate annually. The amplitude and phase of the modulation depend on the phase-space distribution of the local neutrino background, which is perturbed by structure formation. These results also apply to searches for sterile neutrinos at NCB experiments. Gravitational focusing is the only source of modulation for neutrino capture experiments, in contrast to dark-matter direct-detection searches where the Earth’s time-dependent velocity relative to the Sun also plays a role. The cosmic neutrino background (CνB) is a central prediction of standard thermal cosmology [1]. It is similar to the cosmic microwave background (CMB) as both are relic distributions created shortly after the big bang. However, while the CMB formed when the Universe was roughly 400,000 years old, the CνB decoupled from the thermal Universe only ∼1 second after the big bang. Indirect evidence for the CνB arises from the contribution of relic neutrinos to the energy density of the Universe.