Universal electromagnetic fields

We study universal electromagnetic (test) fields, i.e. p-forms fields that solve simultaneously (virtually) any generalized electrodynamics (containing arbitrary powers and derivatives of in the field equations) in n spacetime dimensions. One of the main results is a sufficient condition: any null that solves Maxwell's equations in a Kundt spacetime of aligned Weyl and traceless-Ricci type III is universal (in particular thus providing examples of p-form Galileons on curved Kundt backgrounds). In addition, a few examples in Kundt spacetimes of Weyl type II are presented. Some necessary conditions are also obtained, which are particularly strong in the case n  =  4  =  2p: all the scalar invariants of a universal 2-form in four dimensions must be constant, and vanish in the special case of a null . (paper)