Title: Observing primordial magnetic fields through Dark Matter

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Abstract:
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Primordial magnetic fields (PMF) are often thought to be the early Universe seeds that have bloomed into what we observe today as galactic and extragalactic magnetic fields. Owing to their minuscule strength, PMF are very hard to detect in cosmological and astrophysical observations. I will discuss our proposal from arXiv:2010.03383 where we showed how this changes if a part of neutral Dark Matter has a magnetic susceptibility. In this way, by studying Dark Matter one can obtain information about the properties of PMF, even if the latter have a comoving amplitude 0.01 nG. In our model Dark Matter is a stable neutral scalar field, which interacts with electromagnetism through the so-called Rayleigh operator. For large PMF present in the early Universe this interaction breaks the symmetry of the model, so that the scalar field develops a nonvanishing and adiabatically changing vacuum expectation value. Later, when the PMF redshifts below a critical value, the symmetry is restored through an “inverse phase transition”. At that point the scalar field begins to oscillate and acts as a “magnetomorphic” Dark Matter component, inheriting the properties of the PMF space distribution. In particular, for a nearly flat spectrum of magnetic field fluctuations, the scalar field carries a statistically anisotropic isocurvature mode which could be observed in future observations.