Primordial Weibel instability

MIRÓN-GRANESE, NAHUEL; CALZETTA, ESTEBAN; KANDUS, ALEJANDRA

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS

IOP PUBLISHING LTD

Año: 2022 vol. 2022

1475-7516

We study the onset of vector instabilities in the post-inflationary epoch of theUniverse as a mechanism for primordial magnetic fields amplification. We assume the presenceof a charged spectator scalar field arbitrarily coupled to gravity during Inflation in itsvacuum de Sitter state. Gravitational particle creation takes place at the transition from Inflationto the subsequent Reheating stage and thus the vacuum field state becomes an excitedmany particles one. Consequently this state can be described as a real fluid, and we build outthe hydrodynamic framework using second order theories for relativistic fluids with a relaxationtime prescription for the collision integral. Given the high-temperature regime and thevanishing scalar curvature of the Universe during Reheating (radiation-dominated-type era),the fluid can be regarded as a conformal one. The large quantum fluctuations induced bythe rapid transition from inflationary to effectively radiation dominated expansion becomestatistical fluctuations whereby both a charge excess and anisotropic pressures are producedin any finite domain. The precise magnitude of the effect for each scale is determined bythe size of the averaging domain and the coupling to curvature. We look at domains whichare larger than the horizon at the beginning of Reheating, but much smaller than our ownhorizon, and show that in a finite fraction of them the anisotropy and charge excess providesuitable conditions for a Weibel instability. If moreover the duration of reheating is shorterthan the relaxation time of the fluid, then this instability can compensate or even overcomethe conformal dilution of a primordial magnetic field. We show that the non-trivial topologyof the magnetic field encoded in its magnetic helicity is also amplified if present.