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Primordial-tensor foreground separation is the attempt to distinguish a polarization signal from early-universe gravitational waves from other signals in microwave observations. A measured B-mode polarization pattern can contain polarized Galactic emission and conversion of other Cosmic microwave background polarization by gravitational lensing, as well as a possible primordial contribution.

Frequency coverage helps because the cosmic microwave background, dust and synchrotron radiation do not share one emission spectrum. Spatial correlations and instrumental tests provide additional information. A joint component fit must propagate uncertainty in the foreground model; a B-mode excess at one frequency is not by itself a detection of cosmic inflation.^[ref]