In the first few fractions of a second after the Big Bang the universe was rather small and in full causal contact and therefore also in thermal equilibrium. Then, about 10-35 seconds after the Big Bang, the universe underwent a period of accelerated expansion known as cosmic inflation, after which many regions that were initially in causal contact lost contact with each other. However, because of the original thermal equilibrium, temperature and matter distribution across the universe would be expected to remain generally uniform even after inflation, resulting in an isotropic plasma. Yet because of the tremendous expansionary effects of inflation, tiny quantum fluctuations in the causally-connected universe were expanded to classical levels, resulting in slight variations in matter distribution and spatial geometry. Since radiation was coupled to matter prior to recombination, information about any such inhomogeneities in the early plasma would be carried by the CMB, which would be expected to be isotropic otherwise. Over time, the effects of these variations, or anisotropies, became amplified, as denser regions gravitationally attracted more matter and grew denser. This process eventually led, as the universe continued expanding, to the formation of larger structures, resulting in the large-scale structure that we see today. The information that the CMB contains about matter distribution and spatial geometry can be extracted from temperature and polarization observations and calculations. Temperature is a clear indicator of matter density and distribution as hotter regions are also denser. Polarization contains information about both matter distribution (“E mode” polarization) and spatial geometry, specifically gravitational waves (“B mode” polarization). The variations in all these parameters are quite small: CMB temperature variations are on the order of less than a 100 micro-Kelvins, E-mode variations of under 1 micro-Kelvin, and B-mode variations of a few tens of nano-Kelvins. So far, only temperature and E-mode variations have been detected in the CMB. B-mode variations are currently below the detection thresholds and remain a theoretical prediction.