Strength in Numbers: Studying the Cosmic Neutrino Background with Gravity
In the standard model of cosmology the three active neutrino species are thermally produced in the big bang and survive to today with a number density similar to that of the photons thermally produced in the big bang, the cosmic microwave background. The low energies of these neutrinos and the weakness of their interactions make their direct detection very difficult, if not impossible. Amazingly, we can infer their existence using an even weaker force: gravity.
I will explain what we are learning about neutrinos (and any other dark and relativistic particles that may have been produced in the big bang) from their gravitational influence on the production of light elements, cosmic microwave background anisotropy, galaxy clustering and the distance-redshift relation. I will focus mostly on results from Planck, and also point toward what we can hope to learn about neutrinos from future cosmological probes.