Primordial magnetogenesis

Abstract: Magnetic fields appear everywhere in the universe. From stars and galaxies, all the way to galaxy clusters and remote protogalactic clouds magnetic fields of considerable strength and size have been repeatedly observed. Despite their widespread presence, however, the origin of cosmic magnetic fields is still a mystery. The galactic dynamo is believed capable of amplifying weak magnetic seeds to strengths like those measured in ours and other galaxies, but the question is where do these seed fields come from? Are they a product of late, post-recombination, physics or are they truly cosmological in origin? The idea of primordial magnetism is attractive because it makes the large-scale magnetic fields, especially those found in early protogalactic systems, easier to explain. As a result, a host of different scenarios have appeared in the literature. Nevertheless, early magnetogenesis is not problem free, with a number of issues remaining open and a matter of debate. We review the question of primordial magnetic fields and consider the limits set on their strength by the current observational data. The various mechanisms of pre-recombination magnetogenesis are presented and their advantages and shortcomings are debated. We consider both classical and quantum scenarios, that operate within as well as outside the standard model, and also discuss how future observations could be used to decide whether the large-scale magnetic fields we see in the universe today are truly primordial or not.

• The paper examines theoretical mechanisms, including both inflationary and post-inflationary processes, for generating primordial magnetic fields.
• It details challenges such as coherence scaling, adiabatic dilution, and stringent observational limits from CMB and BBN data.
• The review highlights implications for galaxy formation and emphasizes the need for advanced simulations to unravel cosmic magnetism.

Overview of "Primordial Magnetogenesis"

The paper "Primordial Magnetogenesis" by Alejandra Kandus, Kerstin E. Kunze, and Christos G. Tsagas offers a comprehensive review of mechanisms responsible for the generation of primordial magnetic fields (PMFs) and their implications in cosmology. It scrutinizes the theoretical developments and observational constraints that have been established, providing an extensive account of the possible origins and amplification processes of cosmic magnetic fields.

Cosmic magnetic fields, with strengths on the micro-Gauss scale, are observed across a wide array of astrophysical structures from stars and galaxies to galaxy clusters. The enigmatic origin of these fields has led to the exploration of various primordial scenarios. Broadly, the paper categorizes magnetogenesis processes into those occurring before and after recombination, with a particular focus on early Universe processes that could seed the observed fields.

Early-Time Magnetogenesis and Its Challenges

The review outlines the complexities inherent in generating significant magnetic fields in the early Universe. Challenges are segmented into difficulties related to coherence scaling and amplification. Magnetic fields produced in the post-inflationary phases (between inflation and recombination) typically have coherence lengths constrained by causality, often resulting in scales too small to sustain a galactic dynamo. On the other hand, magnetic fields generated during inflation, though they might be sufficiently coherent, generally decay too fast—rendering them weak due to the adiabatic dilution caused by the expansion of the Universe.

Inflation and Post-Recombination Mechanisms

Inflation is a critical period during which quantum fluctuations can be stretched to cosmological scales, potentially seeding large-scale magnetic fields. However, this generally results in fields too weak to be astrophysically relevant. The paper explores scenarios both within and outside of conventional electromagnetism where amplified PMFs might be possible, such as scenarios involving modified gravity theories, which offer intriguing extensions by coupling the electromagnetic field with other scalar or gravitational fields.

In the post-inflationary Universe, mechanisms such as the Biermann battery, which relies on pressure gradients to generate currents leading to magnetogenesis, are analyzed. However, these fields are typically generated on much smaller scales and need further amplification via mechanisms such as the galactic dynamo to account for observed magnitudes.

Constraints from Observational Data

Observational constraints from cosmic microwave background (CMB) anisotropies, big bang nucleosynthesis (BBN), and large-scale structure provide stringent upper limits on the strength of PMFs. The paper emphasizes the need for these constraints in evaluating the validity of different magnetogenesis models.

Observational evidence of micro-Gauss fields in high-redshift galaxies suggests a possibility of primordial origins, though definitive signatures in the CMB spectrum remain essential for confirmation. Future observations from facilities like the SKA and ongoing investigations into the potential influence of PMFs on the CMB are highlighted as pivotal in understanding cosmic magnetism.

Implications and Future Prospects

Primordial magnetic fields, if verified, have far-reaching implications for the understanding of early universe physics and structure formation. The review speculates on their potential role in processes such as galaxy formation and cosmic ray propagation. Theoretical models are increasingly incorporating nontrivial PMF components to evaluate their impact across cosmic history.

Advancements in computational power and algorithm sophistication are also driving more detailed simulations that factor in these magnetic contributions. Such developments are expected to yield insights into how cosmic magnetism interacts with gravitational and hydrodynamic processes in the Universe's evolution.

In conclusion, while the paper recognizes the definitive role of primordial fields remains unsolved, it affirms the theoretical plausibility and ongoing evidential gathering in the quest to unravel the origins of cosmic magnetism. Further breakthroughs in both theoretical modeling and observational astronomy are necessary to solidify the position of PMFs in our cosmic narrative.