Gauge coupling unification

Gauge coupling unification is the convergence of several low-energy gauge couplings to one coupling at a high scale because the gauge factors embed into one simple group.

An SU(5) grand unified theory embeds $SU(3{)}_c\times SU(2{)}_L\times U(1{)}_Y$ into $SU(5)$. Canonical normalization of its hypercharge generator predicts $g_s^2=g_w^2=(5/3)g_Y^2$ at the unification scale.