  We use the field theoretical model to perform relativistic calculations of
neutrino energy losses caused by the direct Urca processes on nucleons in the
degenerate baryon matter. By our analysis, in a free nucleon gas under beta
equilibrium, the direct neutron decay is forbidden if the number density of
neutrons exceeds the critical value $n_{n}^{c}=5.9\times 10^{31} cm^{-3}$. In
superdense nuclear matter, $n>n_{0}$, the weak decay of neutrons is possible
only due to strong interactions, caused by an exchange of isovector mesons.
Mean field of isovector mesons in the medium creates a large energy gap between
spectrums of protons and neutrons, which is required by kinematics of beta
decay. Our expression for the neutrino energy losses, obtained in the mean
field approximation, incorporates the effects of nucleon recoil, parity
violation, weak magnetism, and pseudoscalar interaction. For numerical testing
of our formula, we use a self-consistent relativistic model of the
multicomponent baryon matter. The relativistic emissivity of the direct Urca
reactions is found substantially larger than predicted in the non-relativistic
approach. We found that, due to weak magnetism effects, relativistic
emissivities increase by approximately 40-50%.
