  The mechanism for generating neutrino masses remains a puzzle in particle
physics. If neutrino masses follow from a Dirac mass term, then neutrino states
exist with opposite chirality compared to their weakly-interacting
counterparts. These inactive states do not interact with their active
counterparts at measurable scales in the standard model. However, the existence
of these states can have implications for cosmology as they contribute to the
radiation energy density at early times, and the matter energy density at late
times. How Dirac neutrinos may populate thermal states via an anomalous
magnetic moment operator is the focus of this work. A class of models where all
neutrinos have a magnetic moment independent of flavor or chirality is
considered. Subsequently, the cross sections for neutrinos scattering on
background plasma particles are calculated so that the relic inactive neutrino
energy is derived as a function of plasma temperature. To do so, one needs
cross sections for scattering on all electrically charged standard-model
particles. Therefore, the scattering cross section between a neutrino and
$W$-boson via the magnetic moment vertex is derived. Current measurements put a
constraint on the size of the neutrino magnetic moment from the cosmological
parameter $N_{\rm eff}$ and light-element primordial abundances. Finally, how
the extra Dirac states contribute to the matter energy density at late times is
investigated by examining neutrino free-streaming.

High Energy Physics - Phenomenology;Cosmology and Nongalactic Astrophysics