Cosmological scaling of precursor domain walls

Domain wall (DW) networks have a large impact on cosmology and present interesting dynamics that can be controlled by various scaling regimes. In the first stage after spontaneous breaking of the discrete symmetry, the network is seeded with `DW precursors’, the zeros of a tachyonic field. At sufficiently weak coupling, this stage can be quite long. The network is then driven to a non-relativistic scaling regime: in flat spacetime the correlation length grows like $L\sim t^{\kappa }$ with $\kappa =1\mathrm{/}2$. We focus on the precursor regime in cosmology, assuming a power-law scale factor $a\propto t^{\alpha }$. We obtain the scaling exponent as a function of the external parameter, $\kappa (\alpha )$, by explicit computation in $1+1$ and $2+1$ dimensions, and find a smooth transition from nonrelativistic scaling with $\kappa \simeq 1\mathrm{/}2$ for $\alpha \lesssim 1\mathrm{/}2$ to DW gas regime $\kappa \simeq \alpha$ for $\alpha \gtrsim 1\mathrm{/}2$, confirming previous arguments. The precise form of the transition $\kappa (\alpha )$ is surprisingly independent of dimension, suggesting that similar results should also be valid in $3+1$ dimensions.