Are magnetic fields in cosmic voids primordial?

TL;DR

This paper investigates whether magnetic fields in cosmic voids are primordial or can originate from late-Universe processes. It proposes a mechanism in which the dipole component of galactic magnetic fields seeds space-filling void fields that, under non-ideal MHD propagation, could reach the strengths inferred from TeV-blazar observations, potentially obviating the need for primordial fields. Analytical estimates and numerical simulations show that a white-noise spectrum $P_B(k) \sim k^0$ is a robust signature, with $B(\lambda) \sim 10^{-13}$ to $10^{-16}$ G on Mpc scales depending on parameter choices, and a center-void RMS around $10^{-16}$ G. If validated, this mechanism provides a testable late-Universe origin for void fields and implies the need to disentangle galactic-dipole foregrounds from any primordial-field signal in cosmic voids.

Abstract

The nature of magnetic fields in the voids of the large-scale structure of the Universe has been a multifaceted open puzzle for decades. On one hand, their origin is not clear with most of the magnetogenesis models using physics beyond the standard model in the early Universe, and on the other hand, their existence and potential role in explaining the spectra of TeV blazars have been intensely debated in the past decade. Here, we propose a mechanism, within classical electrodynamics, that could fill the voids with late-Universe fields and, under certain conditions, dispel the need for primordial fields altogether to explain the void fields. Specifically, we use the dipole component of the galactic fields to generate space-filling magnetic fields in voids with white-noise spectrum and sufficient amplitude to explain the lack of GeV halos around TeV blazars observed by Fermi-LAT. A definitive test for such fields in the voids will be the white-noise spectral shape, which will constrain possible plasma processes in the voids to the ones that allow for the propagation of these dipole fields into the voids.

Figures (3)

• Figure 1: The characteristic magnetic field (purple lines), as defined in \ref{['ePspec1']}, from the galactic dipoles on different length scales ($\lambda_B=2\pi/k$) for a cube void of size $\simeq$ 29 Mpc surrounded by 204 dipoles uniformly distributed over the surface of the void; three different setups $A$, $B$, and $C$ detailed in \ref{['tab:input']} are shown. More details can be found in the paragraph after \ref{['ePspec1']}.
• Figure 2: Dipole magnetic field in the void as a function of the variance $\sigma$ of the distribution of the galactic magnetic field given by $\ln \left(B_{\rm g} (\sigma)/{3\,\mathrm{\mu G}}\right) \sim \mathcal{N} \left(0, \sigma\right)$, with the other parameters given by setup $A$ in \ref{['tab:input']}. The results are normalized to the case of $\sigma = 1$, and the geometric mean of the corresponding ratio is taken across wavelengths. The best fit to the curve is approximately $\sqrt{1+\sigma^{2.6}} - \sqrt{2}$ , which is depicted as the dashed grey line.
• Figure 3: If we define the mean of twenty ensembles as the full mean, and then consider various subsets, each containing $n$ ensembles, we can calculate the deviation of the mean of each subset from the full mean. The spread of this deviation from the full mean for the 95% of the ensembles is shown as the purple region for different values of $n$, with the horizontal axis representing $n$. The mean deviation of all the subsets with a given $n$ is shown as the blue line. The mean deviation from the full mean falls below 10% as soon as we consider at least two ensembles, whereas the 95 percentile deviation falls below 10% for seven ensembles in the subset.