Magnetic fields and the cosmic microwave background

Observations of the high degree of isotropy of the cosmic microwave background are commonly believed to indicate that the Universe is 'almost' Friedmann-Lemaitre-Robertson-Walker (at least since the time of last scattering). Theoretical support for this belief comes from the so-called Ehlers-Geren-Sachs theorem. We show that a generalization of this theorem rules out any strong magnetic fields in the Universe. Our theoretical result is model-independent and includes the case of inhomogeneous magnetic fields, complementing previous results. We thus prove that cosmic microwave background observations severely constrain all types of primordial and protogalactic magnetic fields in the universe.