# The origin, evolution and signatures of primordial magnetic fields

**Authors:** Kandaswamy Subramanian  
**arXiv:** [1504.02311](https://arxiv.org/abs/1504.02311)  
**Date:** 2015-04-09  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1504.02311v1/

## Key points

- Magnetism is pervasive in the cosmos, observed from galaxies to galaxy clusters
- Primordial magnetic fields exist at micro Gauss levels on scales up to ten kiloparsecs
- Evidence suggests weak magnetic fields in intergalactic voids extending over megaparsec scales
- Magnetic fields may be relics from the early universe, amplified and sustained by dynamo processes
- Theoretical framework of magnetohydrodynamics in an expanding universe explored
- Generation of magnetic fields during inflation and other phase transitions discussed
- Evolution of primordial magnetic fields through different cosmic eras examined, including radiation-dominated epoch
- Observational signatures on cosmic microwave background and constraints from Planck data considered
- Impact of primordial magnetic fields on structure formation, particularly on dwarf galaxy scales highlighted
- Effects on cosmological phenomena such as reionization, redshifted 21 cm line observations, weak lensing distortions discussed
- Constraints from radio and $\gamma$-ray astronomy summarized for observational implications

## Summary

In this comprehensive review, Kandaswamy Subramanian explores the fascinating world of primordial magnetic fields and their impact on the universe. The author begins by highlighting the pervasive nature of magnetism in the cosmos, from galaxies to galaxy clusters. Magnetic fields at the micro Gauss level are observed on scales up to ten kiloparsecs. Recent evidence suggests that even the vast intergalactic voids may harbor a weak magnetic field of around $10^{-16}$ Gauss, extending over megaparsec scales. Delving deeper into the origins of these magnetic fields, Subramanian discusses the possibility that they are relics from the early universe. They could potentially be amplified and sustained by dynamo processes within collapsed structures. The review then delves into the theoretical framework of magnetohydrodynamics in an expanding universe. It explores how magnetic fields could have been generated during inflation and other phase transitions. The evolution of these primordial magnetic fields through different cosmic eras is meticulously traced. This includes their linear and nonlinear progression during the radiation-dominated epoch with considerations for viscous effects. The author also examines sensitive observational signatures of primordial magnetic fields on the cosmic microwave background, discussing current constraints derived from Planck data. As the universe evolves further post-recombination, primordial magnetic fields play a crucial role in shaping structure formation. They particularly impact dwarf galaxy scales. Subramanian outlines their effects on various cosmological phenomena such as reionization, redshifted 21 cm line observations, weak lensing distortions, and features in the Lyman-$\alpha$ forest. Constraints from radio and $\gamma$-ray astronomy are summarized to provide a comprehensive overview of observational implications. Additionally, astrophysical batteries and dynamos' roles in reshaping these primordial fields are briefly touched upon before concluding with final reflections on the enduring mysteries surrounding primordial magnetic fields. This detailed exploration sheds light on a fundamental aspect of cosmic magnetism and its profound influence on our understanding of the universe's evolution.