Resurrecting Gravitational Vector Modes and their Magnetogenesis

TL;DR

This paper investigates primordial gravitational vector modes and their potential to generate magnetic fields, exploring exotic initial conditions and their observational signatures in the CMB and BAO data.

Contribution

It introduces new initial conditions that sustain vector modes on super Hubble scales and compares their observational signatures with current cosmological data.

Findings

V-modes are consistent with current data.

Proposed initial conditions cannot generate sufficient magnetic fields.

V-modes should be further constrained to understand early Universe physics.

Abstract

We revisit the presence of primordial gravitational vector modes (V-modes) and their sourcing of primordial magnetic fields (PMF), i.e. magnetogenesis. As the adiabatic vector mode generically decays with expansion, we consider exotic initial conditions which circumvent this issue and lead to observational imprints. The first initial condition is an isocurvature mode between photons and neutrinos vorticities, and the second one is a non-trivial initial condition on the neutrino octupole. Both types of conditions sustain a constant vector mode on super Hubble scales at early times. We also consider a third scenario in which the adiabatic vector modes are rapidly sourced, at a given early but finite time, by an exotic component which develops an anisotropic stress. We find the best fitting parameters in these three cases to CMB and BAO data. We compare the resulting $B$-mode spectra of the CMB to data from BICEP/Keck and SPTpol. We find that none of the proposed initial conditions can produce large enough PMFs to seed every type of magnetic fields observed. However, V-modes are still consistent with the data and ought to be constrained for a better understanding of the primordial Universe before its hot big-bang phase.