The magnetic fields in galaxies and galaxy clusters are thought to result from the amplification of weak primordial magnetic fields, which, according to one class of theories, should exist in the voids between galaxies and galaxy clusters. Neronov and Vovk (p. 73) present evidence for the existence of intergalactic magnetic fields and derive a lower limit for their strength, based on an analysis of data from the Fermi Large Area Telescope. The results place constraints on mag-netogenesis models and suggest that magnetic fields originated in the early universe before galaxy formation took place.
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