Astronomical observations of cosmological sources are increasing becoming useful tools to learn about new physics beyond the Standard-Model of Particle Physics, complementary to Earth-bound particle accelerators. One particular phenomenon that could manifest if fundamental symmetries like Lorentz Invariance and Charge-Parity-Time (CPT) symmetry are violated in the new physics, is called ``vacuum birefringence'', which could cause light from distant sources traveling over cosmological distances through empty space to have its polarization altered ever so slightly in a cumulative fashion as it travels to us. Polarization measurements of extragalactic sources, including astrophysical tests with optical polarization measurements of cosmological sources, including Active Galactic Nuclei (AGN), can therefore be used to place interesting and useful limits on such new physics. Lower energy optical polarization measurements, which are routinely done with ground-based telescopes, can play an important role in constraining effects like vacuum birefringence, in comparison to baloon-borne or space-based higher energy x-ray/gamma-ray polarization measurements of distant source like Gamma-ray bursts, which are much more difficult and expensive to perform.
