Inefficiency of chiral dynamos in protoneutron stars and the early universe

TL;DR

This paper analyzes the limitations of chiral dynamos in generating magnetic fields in protoneutron stars and the early universe, highlighting how chiral flipping suppresses dynamo action under realistic conditions.

Contribution

It derives a quantitative limit on the CPI rate considering source timescale and chiral flipping, revealing significant suppression in astrophysical environments.

Findings

CPI rate is limited by source timescale and chiral flipping.

Chiral flipping suppresses dynamo if its rate exceeds a certain threshold.

Suppression is significant in protoneutron stars and marginal near the early universe's electroweak transition.

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.