An Etymological Dictionary of Astronomy and Astrophysics

A → polarization component in the → cosmic microwave background radiation that depends only on → curl, is independent of → gradient, and has → handedness that distinguishes left from right. The B-mode is due to only → vector perturbations or → tensor perturbations. It has two types, the first type, which constitutes the majority of the B-mode polarization,
results from → lensing by galaxies that twist the → E-mode polarized light on its journey from the other side of the → observable Universe.
The second type can be produced only by → gravitational waves, not by density perturbations.
This type of B-mode is incredibly faint, producing temperature variations of about 0.4 microK and accounting for just one part in 10 million in the CMB temperature distribution. It is expected to be generated during cosmic → inflation shortly after the → Big Bang. The → BICEP2 team announced in March 2014 that they had detected the second type of B-modes, consistent with inflation and gravitational waves in the → early Universe. The detected degree scale B-mode polarization has a tensor-to-scalar ratio, r = 0.2 (+0.07, -0.05), which is a measure of the amplitude of the primordial gravitational waves.

See also: B, indicating magnetic-field like; → mode; → polarization.