Cosmology predicts the existence of a Cosmic Neutrino Background (CNB), consisting of primordial neutrinos that decoupled 1s after the Big Bang. Detecting the CNB is extremely challenging, but would provide a wealth of information about the very early properties of our Universe.
Like the Cosmic Microwave Background (CMB), the CNB carries both primordial and secondary gravitational perturbations, the latter imprinted by the large-scale structure at late times. The first image below shows the distribution of neutrinos in the local universe, as determined from cosmological simulations whose initial conditions were constrained to reproduce the observed galaxy distribution (white dots).
The simulations are based on the same constraints and methods as Virgo’s SIBELIUS-DARK simulation (McAlpine et al., 2022). The simulations were started from an ensemble of initial conditions that represent plausible initial states of the Universe, given the observed galaxy distribution and the assumed cosmological model. The neutrino masses are uncertain and were also assumed in the analysis. In the first image, a neutrino mass of 0.06 eV was used.
The second image below shows the neutrino flux on the sky as it would be observed on Earth if the mass were 0.01 eV. Overlaid in purple is the projected dark matter distribution along the line-of-sight. The anti-correlation between local neutrino flux and distant matter perturbations is caused by the distribution observed in the previous image: neutrinos clustered around groups of distant galaxies do not arrive on Earth.
The existence of neutrino masses gives the CNB additional structure compared to the CMB, as neutrinos travelling at different speeds sample the same matter perturbations at different times. The anti-correlation with the matter distribution depends on the neutrino momentum and distance to the perturbation. Measuring the momentum and angular distribution of the flux could thus enable a tomographic reconstruction of the matter distribution in space and time.
Willem Elbers et al. (2023), Where shadows lie: reconstruction of anisotropies in the neutrino sky, Journal of Cosmology and Astroparticle Physics, 10/010. Available at https://doi.org/10.1088/1475-7516/2023/10/010.