Grand unification of fermion generations: Neutrino masses and cosmological constraints

We discuss cosmological constraints on the mirror-symmetry-breaking scales in a recently proposed SO(18) model of family unification. We find two kinds of constraints: one based on considerations of cosmological baryogenesis restricts the scale of mirror-symmetry breaking to be close to the SO(18) unification scale; the second constraint needed to obtain a consistent picture of neutrino masses in this model requires an intermediate (B-L)-breaking scale below ${10}^6$–${10}^7$ GeV. We also show that constraints of grand unification require the breaking of the horizontal SO(8) scale to be above the scale of SO(10) symmetry.