Currently the cleanest probe of the early universe is the CMB, relic light generated ~380,000 years after the Big Bang. Today the CMB can be observed at millimeter wave frequencies and its intensity is mostly uniform across the sky, presenting variations at a level of one part in one hundred thousand. The uniformity of the CMB provided irrefutable evidence of the hot, dense origin of the Universe ('Big Bang'). Quantum fluctuations present in the early Universe manifested as anisotropies in the CMB; measuring the power spectrum of these anisotropies has provided invaluable measurements of the conditions in our early Universe: its age, geometry, and the percentage of baryonic matter, dark matter and dark energy in the universe.
Anisotropies can be produced by two primary mechanisms: scalar fluctuations (as described above) and gravity wave (tensor) fluctuations generated during inflation. The CMB is polarized via Compton scattering off of quadrupolar anisotropies in the temperature distribution. The polarization pattern is decomposed into the spin-2 spherical harmonics basis: the quantum fluctuations leave 'curl-free' modes (E-modes), while inflationary gravity waves leave both 'curl-free' and 'divergence-free' modes (B-modes). The inflationary gravity wave E-mode signal is at least an order of magnitude smaller than the E-mode generated from quantum fluctuations, so the only direct signature we could expect to see from inflation in the CMB polarization spectrum will come via B-modes. In many inflationary models, a measurement of the B-mode spectrum will yield the the energy scale of inflation:
The current standard model of cosmology includes a period of extremely rapid expansion at ~ t-35 seconds, and the leading theoretical framework describing this period of expansion is inflation. Inflation has yet to be verified, the best tool we have for testing inflation is the polarized CMB. Planck was developed to make high precision measurements of the temperature and polarization of the CMB, joining a body of work originated by ground-based, balloon-borne, and space based experiments, each adding a level of sensitivity to previous measurements. As polarization experiments become more sensitive they will have increasing discriminatory and verification power of inflationary models.