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A Chern-Simons coupling of a new scalar field to electromagnetism may giverise to cosmological birefringence, a rotation of the linear polarization ofelectromagnetic waves as they propagate over cosmological distances. Prior workhas sought this rotation, assuming the rotation angle to be uniform across thesky, by looking for the parity-violating TB and EB correlations a uniformrotation produces in the CMB temperature/polarization. However, if the scalarfield that gives rise to cosmological birefringence has spatial fluctuations,then the rotation angle may vary across the sky. Here we search fordirection-dependent cosmological birefringence in the WMAP-7 data. We reportthe first CMB constraint on the rotation-angle power spectrum for multipolesbetween L = 0 and L = 512. We also obtain a 68% confidence-level upper limit of1 degree on the square root of the quadrupole of a scale-invariantrotation-angle power spectrum.

First CMB Constraints on Direction-Dependent Cosmological Birefringence from WMAP-7