  We describe neutrino radiation in general relativity by introducing the
energy-momentum tensor of a null fluid into the Einstein equations.
Investigating the geometry and analysis at null infinity, we prove that a
component of the null fluid enlarges the Christodoulou memory effect of
gravitational waves. The description of neutrinos in general relativity as a
null fluid can be regarded as a limiting case of a more general description
using the massless limit of the Einstein-Vlasov system. The present authors
with co-authors have work in progress to generalize the results of this paper
using this more general description. Gigantic neutrino bursts occur in our
universe in core-collapse supernovae and in the mergers of neutron star
binaries.
