Cosmological implications of low scale quark-lepton unification

Abstract

There is a unique $\hbox{SU(4)}\otimes\hbox{SU(2)}_L \otimes \hbox{SU(2)}_R$ gauge model which allows quarks and leptons to be unified at the TeV scale -- thereby making the model testable and avoiding the gauge hierarchy problem. In its minimal form, this model could quite naturally accommodate simultaneous solutions to the solar and LSND neutrino oscillation data. The atmospheric neutrino anomaly can be easily accommodated by mirror-symmetrising the minimal model. The model also contains three right-handed neutrinos, with masses in the range 1 keV to $\sim$ 1 GeV. We investigate the implications of these right-handed neutrinos for early Universe cosmology. It is shown that the minimal model is inconsistent with some of the standard assumptions of the big bang model. This motivates an examination of non-standard big bang cosmology, such as a low reheating temperature scenario with $T_{RH} \sim$ MeV. In such a Universe, peaceful co-existence between low-scale quark-lepton gauge unification and early Universe cosmology is possible.