http://arxiv.org/abs/2010.03383
Primordial magnetic fields are often thought to be the early Universe seeds that have bloomed into what we observe today as galactic and extra-galactic magnetic fields. Owing to their minuscule strength, primordial magnetic fields are very hard to detect in cosmological and astrophysical observations. We show 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 primordial magnetic fields, even if the latter have a comoving amplitude $B_0 \lesssim0.01~\mbox{nG}$. In our model Dark Matter is a stable singlet scalar $\chi$, which interacts with electromagnetism through the Rayleigh operator as $\chi^2 F_{\mu \nu} F^{\mu \nu}/\Lambda^2$. For primordial magnetic fields present in the early Universe this operator forces the $Z_2$-symmetry of the model to be spontaneously broken. Later, when the primordial magnetic field redshifts below a critical value, the symmetry is restored through an “inverse phase transition”. At that point the field $\chi$ begins to oscillate and acts as a “magnetomorphic” Dark Matter component, inheriting the properties of the primordial magnetic field space distribution. In particular, for a nearly flat spectrum of magnetic field fluctuations, the scalar $\chi$ carries a \emph{statistically anisotropic isocurvature mode}. We discuss the parameter space of the model and consider the possibility that the bulk of the Dark Matter is composed of the same particles $\chi$ produced via the freeze-in mechanism.
S. Ramazanov, F. Urban and A. Vikman
Thu, 8 Oct 20
20/54
Comments: 29 pages and 4 figures
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