A metastable relic that forms when a closed cosmological domain wall traps fermionic zero modes. Because the sign of the fermion mass flips across the wall, charged particles captured from the bulk live as massless modes confined to the wall surface; Fermi degeneracy then forces them up to a Fermi level whose energy scales as the square root of the trapped charge over the radius. The resulting outward pressure balances the wall tension, so the sphere stops shrinking at a finite radius instead of collapsing - the authors name this object a spheron, the domain-wall analogue of a cosmic-string vorton, though its energy scales with charge differently. Students learn how the charge capture rate and the friction from the thermal plasma set the over-damped shrinking regime, how the spheron later decays by charge leakage and by fission, and why the random angular momentum carried off by the decaying fermions gives the sphere an ellipticity and hence a time-varying quadrupole. The observable payoff is a second, mountain-shaped peak added to the stochastic gravitational-wave spectrum of a collapsing domain-wall network, distinct from the network's own peak and potentially within reach of LISA and the Einstein Telescope.
Domain wall (DW) networks may have formed in the early universe following the spontaneous breaking of a discrete symmetry. Notably, several particle physics models predict the existence of current-ca…