I present an argument, based on the topology of the universe, why there are
three generations of fermions. The argument implies a preferred unified gauge
group of SU(5), but with SO(10) representations of the fermions. The breaking
pattern SU(5)→SU(3)×SU(2)×U(1) is preferred over the
pattern SU(5)→SU(4)×U(1). On the basis of the argument one expects
an asymmetry in the microwave data, which might have been detected already.Comment: 6 page