The polarization of the CMB can provide clues about the universe's inflationary epoch and gravitational waves.

Just like light reflected off a surface, the photons of the CMB can be polarized, meaning their electromagnetic waves oscillate in a preferred direction. There are two main types of CMB polarization: E-modes and B-modes. E-modes are generated by density variations (the same ones that create temperature anisotropies) and are readily observed. B-modes, however, are a much fainter and more challenging signal to detect. They are particularly exciting because they can be produced by two phenomena: gravitational lensing of E-modes by foreground matter, or, more tantalizingly, by primordial gravitational waves generated during the universe's rapid 'inflationary' epoch. Detecting primordial B-modes would provide direct evidence for cosmic inflation, a theoretical period of exponential expansion immediately after the Big Bang that elegantly solves several outstanding cosmological puzzles. While experiments like BICEP2 initially reported a detection of primordial B-modes, subsequent analysis by the Planck mission showed that much of the signal was attributable to galactic dust. The search for clear, unequivocal primordial B-modes continues with ongoing experiments, representing one of the holy grails of modern cosmology and offering a window into physics at incredibly high energies in the very, very early universe.