Polarization of the Cosmic Microwave Background
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Our universe is immersed in a weak bath of microwaves—short-wavelength radio waves that represent the afterglow of the Big Bang. These microwave photons are present everywhere in the sky, and for the most part they are very similar wherever one looks. But very sensitive instruments are able to detect slight differences in the temperatures of the photons, suggesting that the early universe wasn't perfectly uniform. It turns out that the microwaves from different points on the sky also have slightly different polarizations—the preferential orientation of their electric fields. Measurements of the polarizations should reveal fundamental properties of the cosmos, such as the dynamics and composition of the early universe and the presence of primordial gravitational waves.