Inefficiency of chiral dynamos in protoneutron stars and the early universe

Abstract

The chiral plasma instability (CPI) has been invoked as a possible mechanism for generating primordial magnetic fields in the universe and ultrastrong fields in neutron stars. We investigate chiral dynamos where the chirality imbalance is pumped by a source on a timescale $t_0$ and show that the CPI rate $\gamma$ is limited to $\gamma_0/(1+{\cal Q}^2)$, where ${\cal Q}= (\gamma_0 t_0)^{1/3}$ and $\gamma_0$ corresponds to models with instantaneously created chirality imbalance $(t_0=0)$. We then find that chiral flipping with rate $\Gamma_{\mathrm f}$ hinders the chiral dynamo if $\Gamma_{\mathrm f} >\gamma_0/(1+{\cal Q}^2)$ and completely suppresses it if $\Gamma_{\mathrm f} >\gamma_0/(1+{\cal Q}^{3/2})$. Realistic $t_0$ typically give ${\cal Q}\gg 1$, which makes the dynamo greatly vulnerable to suppression by chiral flipping. The suppression is strong in protoneutron stars and may be (barely) avoided near the electroweak transition in the early universe.