Neutrinos in the early Universe decoupled essentially in helicity eigenstates.

As they propagate through the Universe, their helicities could be modified via

two effects. First, neutrinos with a finite magnetic moment would rotate their

spins with respect to their momenta as they encounter cosmic magnetic fields,

modifying their helicities. Second, the bending of neutrino's spin by a

gravitational field lags the bending of its momentum, again modifying its

helicity. We study both effects and investigate the implications of the

helicity modification on the detection of relic neutrinos using the Inverse

Tritium Beta Decay (ITBD) reaction. We find that the ITBD rate depends

sensitively on the neutrino mass hierarchy and on the Dirac or Majorana nature

of the neutrinos. We also consider other aspects of ITBD related to the neutrinoless

double beta decay and the apparent puzzle of angular momentum conservation.

This talk is based on work in collaboration with Gordon Baym.