  The cosmic microwave background (CMB) encodes information about the content
and evolution of the universe. The presence of light, weakly interacting
particles impacts the expansion history of the early universe, which alters the
temperature and polarization anisotropies of the CMB. In this way, current
measurements of the CMB place interesting constraints on the neutrino energy
density and mass, as well as on the abundance of other possible light
relativistic particle species. We present the status of an on-going 1500 sq.
deg. survey with the SPT-3G receiver, a new mm-wavelength camera on the 10-m
diameter South Pole Telescope (SPT). The SPT-3G camera consists of 16,000
superconducting transition edge sensors, a 10x increase over the previous
generation camera, which allows it to map the CMB with an unprecedented
combination of sensitivity and angular resolution. We highlight projected
constraints on the abundance of sterile neutrinos and the sum of the neutrino
masses for the SPT-3G survey, which could help determine the neutrino mass
hierarchy.
