Magnetic Fields in the Early Universe

TL;DR

Magnetic fields in the early Universe are explored as potential primordial relics that could influence structure formation, CMB anisotropies, and fundamental processes like nucleosynthesis and baryogenesis. The review surveys observational evidence, dynamo versus primordial origins, and the evolution of magnetic fields, highlighting flux freezing and helicity conservation as key survival mechanisms. It delineates how magnetic fields modify cosmological perturbations, leaves imprints on the CMB through altered acoustics, polarization, and Faraday rotation, and sets BBN constraints that limit their strength and scale. The generation mechanisms span vorticity, phase transitions, inflation, and cosmic strings, with helicity playing a central role in linking magnetism to baryogenesis. Overall, while many magnetogenesis scenarios remain speculative, observable imprints in the CMB, BBN outcomes, and large-scale structure provide promising avenues to probe primordial magnetism and its ties to fundamental physics.

Abstract

This review concerns the origin and the possible effects of magnetic fields in the early Universe. We start by providing to the reader with a short overview of the current state of art of observations of cosmic magnetic fields. We then illustrate the arguments in favour of a primordial origin of magnetic fields in the galaxies and in the clusters of galaxies. We argue that the most promising way to test this hypothesis is to look for possible imprints of magnetic fields on the temperature and polarization anisotropies of the cosmic microwave background radiation (CMBR). With this purpose in mind, we provide a review of the most relevant effects of magnetic fields on the CMBR. A long chapter of this review is dedicated to particle physics inspired models which predict the generation of magnetic fields during the early Universe evolution. Although it is still unclear if any of these models can really explain the origin of galactic and intergalactic magnetic fields, we show that interesting effects may arise anyhow. Among these effects, we discuss the consequences of strong magnetic fields on the big-bang nucleosynthesis, on the masses and couplings of the matter constituents, on the electroweak phase transition, and on the baryon and lepton number violating sphaleron processes. Several intriguing common aspects, and possible interplay, of magnetogenesis and baryogenesis are also dicussed.

Figures (6)

• Figure 1: The effect of a cosmic magnetic field on the multipole moments. The solid line shows the prediction of a standard CDM cosmology ($\Omega=1$,$h=0.5$, $\Omega_{\rm B}=0.05$) with an $n=1$ primordial spectrum of adiabatic fluctuations. The dashed line shows the effect of adding a magnetic field equivalent to $2 \times 10^{-7}$ Gauss today. From Ref.AdamsDGR
• Figure 2: The evolution of the polarization brightnesses, for $k=0.16\,{\rm Mpc}^{-1}$ and $\mu = 0.5$ (in arbitrary units). Also plotted as a dotted line is the differential visibility function $\dot\tau e^{-\tau}$ in units of ${\rm Mpc^{-1}}$. From Ref.KosLoe96.
• Figure 3: Numerical integration for the multipoles of the anisotropy correlation function in a standard CDM model without a primordial magnetic field $(F=0)$, and with $F=1,\ 4,\ 9$, which correspond to $\nu_0=\nu_{\rm d},\ \nu_{\rm d}/2,\ \nu_{\rm d}/3$ respectively, with $\nu_{\rm d}\approx$ 27 GHz $(B_*/0.01{\rm Gauss})^{1/2}$. From Ref.Harari97.
• Figure 4: The neutron-depletion rate $\Gamma_{n \to p}$, normalized to the free-field rate, is plotted as a function of the temperature for several values of $\gamma$. From Ref.GraRub95
• Figure 5: The $^4$He predicted abundance is represented in function of the parameter $\gamma$, considered at $T = 10^9~^oK$, in three different cases: only the effect of the magnetic field energy density is considered (dashed line); only the effect of the field on the electron statistics is considered (dotted-dashed line); both effects are considered (continuous line). The dotted line represents the observational upper limit. From Ref.GraRub96