Modified gravity and cosmic magnetic fields in weakly nonlinear structure formation Mobile researcher: Lukas Hollenstein, France

Astronomers observe the presence of magnetic fields in all cosmic structures, and still their origin is poorly understood to date. The only viable mechanism known so far that generates magnetic fields from zero initial conditions relies on weakly nonlinea r evolution of cosmic density fluctuations around recombination, the epoch at which the cosmic microwave background radiation decouples from the matter. However, the magnetic fields generated in this way are too weak. Another pressing problem in cosmology is the observed acceleration of the expansion rate of today's Universe which can be explained by s small cosmological constant, dynamical dark energy fields or large-scale modifications to General Relativity. These different scenarios can be distinguishe d by simultaneously observing the evolution of the average geometry as well as linear and nonlinear structure formation. Typical modified gravity models come with additional degrees of freedom that often have nonminimal couplings to electromagnetism. We expect such couplings to impact the nonlinear generation of magnetic fields. Therefore we investigate this mechanism of magnetogenesis in scenarios of modified gravity with and without nonminimal couplings of electromagnetism to gravity. If such couplings turn out to have a strong impact on the amplification of the first magnetic fields these models are either favoured or can even be constrained by this effect. On the other hand, the weakly nonlinear evolution of structure in modified gravity models need s to be understood to make predictions for a complete set of consistency relations for different scenarios late-time acceleration that I proposed recently. We can directly apply the intermediate results obtained to compute the evolution of the magnetic fi elds to predict the respective contributions to the consistency relations. The knowledge of these contributions is essential for the correct physical interpretation of future observations.