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The cosmic microwave background is [[polarization (waves)|polarized]] at the level of a few microkelvins. There are two types of polarization, called ''E''-modes and ''B''-modes. This is in analogy to [[electrostatics]], in which the electric field (''E''-field) has a vanishing [[curl (mathematics)|curl]] and the magnetic field (''B''-field) has a vanishing [[divergence]]. The ''E''-modes arise naturally from [[Thomson scattering]] in a heterogeneous plasma. The ''B''-modes, which have not been measured and are thought to have an amplitude of at most a 0.1 µK, are not produced from the plasma physics alone. They are a signal from [[cosmic inflation]] and are determined by the density of primordial [[gravitational wave]]s. Detecting the ''B''-modes will be extremely difficult, particularly given that the degree of foreground contamination is unknown, and the [[weak gravitational lensing]] signal mixes the relatively strong ''E''-mode signal with the ''B''-mode signal.<ref>{{cite journal|last=Lewis|first=A.|last2=Challinor|first2=A.|year=2006|title=Weak gravitational lensing of the CMB|journal=[[Physics Reports]]|volume=429|pages=1–65|doi = 10.1016/j.physrep.2006.03.002|id={{arxiv|astro-ph/0601594}}}}</ref>
==Osservazioni della radiazione di fondo==
{{main|Cosmic microwave background experiments}}
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