Lambert, Simon2008-12-192008-12-1920082008-12-19http://hdl.handle.net/1828/1303I examine the observational effects of a light scalar field with a scalar coupling to masses and a pseudoscalar coupling to light and particle spins. The pseudoscalar coupling to light induces a coupling to atomic spins both by inducing a coupling to particle spins directly, and by interactions with electromagnetic fields in the atom. Experiments measuring the interaction of spins to the gradient of the field are the only known way to measure the strength of the interaction with spins. However, limits on the interaction with light derived from these experiments are barely competitive with the separate astronomical limits on the scalar interaction and the interaction with light. Assuming a low mass of the field, as would be the case if the field acts as quintessence, the polarization rotation of the CMB provides a much tighter limit on the product of the pseudoscalar and scalar interaction strengths.enAvailable to the World Wide Webscalar-tensorgravitypseudoscalarquintessencepolarizationspinBrans-DickeUVic Subject Index::Sciences and Engineering::Physics::AstrophysicsThesis