Neutrinos produced in the very early Universe decoupled from the primordial plasma when the Universe was less than one second old and temperatures had dropped to of order a few MeV. The Universe today is bathed in a sea of these ancient neutrinos that is known as the cosmic neutrino background (CNB). Recent work by KICP senior member Scott Dodelson and Mika Vesterinen has shed new light on the distance to the surface of last scattering for CNB neutrinos - the distance they have traveled over the past 13.7 billion years. Dodelson and Vesterinen find that the last scattering surface (LSS) of the cosmic neutrino background is much broader and much closer than that of the cosmic microwave background.

Figure one illustrates the distance to the LSS for neutrinos of various masses and for the CMB photons.

This result is at first glance counterintuitive since the photons in the CMB were emitted at t = 380,000 years, much later than the neutrinos, and thus have had less time to travel since decoupling. However, neutrinos are massive (at least one species of neutrino must have a mass of 0.05 eV or larger), and thus propagate at subluminal velocities. The effects calculated by Dodelson and Vesterinin are dramatic - for neutrinos with a mass of 1 eV the surface of last scattering is only several hundred Mpc away, hundreds of times closer than the CMB surface of last scattering.

Current detectors are not able to observe the CNB, but if (far) future experiments can detect these neutrinos they offer a unique window into the evolution of structure. Anisotropies in the CNB could be directly compared with data from galaxy surveys, providing snapshots of the overdensities in a given location at two very different times.

Publication: Dodelson and Vesterinen, Cosmic Neutrino Last Scattering Surface, Phys. Rev. Lett. 103, 171301(2009)