Bmodes

Bmodes refer to a pattern of polarization in the cosmic microwave background (CMB) and in astronomy more generally that has a curl-like, or twist, structure. In the CMB, polarization can be decomposed into two components: E-modes, which are curl-free, and B-modes, which possess a handedness. B-modes are of particular interest because they carry information about processes in the early universe and the distribution of matter along the line of sight.

Two main origins generate B-modes in the CMB. Gravitational lensing by large-scale structure converts some of the more easily measured E-mode polarization into B-modes at smaller angular scales. Primordial gravitational waves produced during cosmic inflation could imprint a distinct B-mode signal at large angular scales, with an amplitude characterized by the tensor-to-scalar ratio, r. Reionization can also contribute to very large-scale B-modes.

Measuring B-modes is technically challenging due to their faintness and contamination from foregrounds, such as polarized emission from Galactic dust and synchrotron radiation. Accurate, multi-frequency observations are required to separate the cosmological signal from foregrounds. Experiments pursuing B-modes include ground-based projects like BICEP/Keck, POLARBEAR, SPTpol, and ACTpol, as well as space-based missions such as Planck (which constrained B-modes) and planned efforts like LiteBIRD and CMB-S4. Data analysis focuses on the angular power spectrum, delensing to remove lensing-induced B-modes, and cross-checks to control systematics.

Significance of B-modes lies in testing inflationary cosmology and constraining the energy scale of the early universe. So far, no definitive detection of primordial B-modes has been confirmed; current results place upper limits on r at the few percent level.