2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/194185Possible Lorentz-violating effects in the cosmic microwave background are studied. We provide a systematic classification of renormalizable and nonrenormalizable operators for Lorentz violation in electrodynamics and use polarimetric observations to search for the associated violations.4 pages two-column REVTeX, accepted in Physical Review LettersAstrophysicsHigh Energy Physics - Phenomenologytext