Lorentz Violation , Electrodynamics , and the Cosmic Microwave

The properties of light have proved to be a valuable testing ground for special relativity for more than a century. Contemporary experiments are motivated in part by a possible breakdown of special relativity with origins in Planck-scale physics [1, 2, 3]. These experiments include modern versions of the classic Michelson-Morley and Kennedy-Thorndike experiments that use highly stable resonant cavities to search for violations of rotation and boost symmetries [4]. However, the highest sensitivities to relativity violations in electrodynamics are found in searches for vacuum birefringence in light from very distant sources [5, 6, 7, 8]. Birefringence studies take advantage of the extremely long baselines that allow the miniscule effects of a Lorentz violation to accumulate to (potentially) detectable levels over the billions of years it takes for the light to reach Earth. The cosmic microwave background (CMB) is the oldest light available to observation and therefore provides an excellent source for birefringence searches. Here we summarize a recent search for signals of Lorentz violation using CMB polarimetry [8]. General Lorentz violation is described by a framework known as the Standard Model Extension (SME) [3]. The SME provides the theoretical backbone for studies in a number of areas [2], including photons [4, 6, 7, 8, 9]. Most tests of Lorentz violation focus on the minimal SME, which assumes usual gauge invariance and energy-momentum conservation and restricts attention to superficially renormalizable operators. Operators of dimension d ≤ 4 are of renormalizable dimension and are included the minimal SME. Two types of Lorentz-