PI: James Bolton

In Pavičević et al. (2025, PRL, 135, 071001) we have presented the first constraints on primordial magnetic fields from the Lyman-alpha forest using full cosmological hydrodynamical simulations. The simulations were performed using the DiRAC Data Intensive Service (CSD3) hosted at the University of Cambridge. Primordial magnetic fields (PMFs) are of interest to cosmologists, as they can provide insights on new fundamental physics in the early Universe, such as inflation or phase transitions. By comparing our DiRAC simulations with observational data obtained from the Keck and VLT telescopes, the Lyman-alpha forest predicted by our simulations is in better agreement with the observed data when PMFs are included. The figure shows the predicted Lyman-alpha forest absorption from a simulation with PMFs (blue curve) and without (red curve). In particular, we find that a standard Lambda-CDM model of the Universe with a very weak magnetic field of around 0.2 nano-gauss fits observational data much better than a model without PMFs. Our measurement may also have exciting consequences for high-redshift galaxy formation and galactic feedback: for our preferred PMF model the dark matter halo mass function will be enhanced toward low masses, leading to the earlier formation of galaxies. This could have a measurable impact on the interpretation of high-redshift galaxy luminosity functions as observed by the JWST.