Gravitational anomaly and fundamental forces

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 early universe microwave data, which might have been detected already.