Can cosmological perturbations produce early universe vorticity?

TL;DR

This paper reviews how cosmological perturbations at second order can generate vorticity in the early universe, exploring its potential observational signatures and implications for primordial magnetic fields.

Contribution

It demonstrates that second-order perturbations can source vorticity in cosmology, providing estimates for its magnitude and discussing observational relevance.

Findings

Vorticity is generated at second order by entropy and density gradients.

Estimated vorticity power spectrum depends on initial perturbation spectra.

Potential link between vorticity and primordial magnetic field generation.

Abstract

In this special issue article, based on the talk with the same title in session B5 (Theoretical and Mathematical Cosmology) at GR19, we review the case of vorticity generation in cosmology using cosmological perturbation theory. We show that, while at linear order the vorticity evolution equation has no source term in the absence of anisotropic stress, at second order vorticity is sourced by gradients in entropy and energy density perturbations. We then present some estimates for the magnitude and scale dependence of the vorticity power spectrum using simple input power spectra for the energy density and entropy perturbations. Finally, we close with possible directions for future work followed by some hints toward the observational importance of the vorticity so generated, and the possibility of primordial magnetic field generation.