Polarization (cosmology)
According to the standard Big Bang theory, the early universe was sufficiently hot for all the matter in it to be fully ionised.
[6] With a standard optical telescope, the background space between stars and galaxies is almost completely dark.
However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object.
[8] The launch of the IXPE telescope in late 2021 made polarization measurements in the 2–8 KeV band also a reality (more than 40 years after the pioneering observations of the OSO-8 satellite) and its polarimetric observations confirmed theoretical predictions, according to which X-ray radiation from magnetar sources is also highly polarized, up to ≈ 80%, the highest value detected so far.
[9] Photons propagating in a strongly magnetized environment are expected to be linearly polarized in two normal modes called the ordinary (O) and the extraordinary (X) one, parallel or perpendicular to the plane of the local magnetic field and the photon propagation direction, respectively.
[10][11] In the 2–10 keV band (which is the one accessible to current instrumentation), radiation emitted from the bare, condensed surface of magnestars is expected to be only mildly polarized (≲30%), with either O- or X-mode dominating, depending on both the photon energy and propagation direction with respect to the star magnetic field.
Hence, the authors have cautioned against over-interpretation of current results and wait for more detailed polarization measurements from future missions such as POLAR-2[16] and LEAP.
[20] Till date (as of 2021), all GRB polarization measurements performed have made use of Compton scattering in the detector.
[21] B-mode polarization can also be used as an indirect probe into the cosmic neutrino background because unlike E-mode polarization, it is possible to generate the B-mode by Compton scattering in case of tensor mode of metric perturbation but not in the case of scalar mode of metric perturbation.
[24] Plane waves fluctuations (like density or scalar perturbations in the early universe) produce polarization patterns of a particular type, known as E mode.
[24] Gravitational wave can cause an anisotropic stretching of space, and this asymmetry causes a "handedness" to the pattern of polarization.
If reflected across a line going through the center the E-patterns are unchanged, while the positive and negative B-patterns get interchanged.
The names derive from an analogy to the decomposition of a vector field into curl-less (here "E" for electric or "G" for gradient) and divergence-less ("B" for magnetic or "C" for curl) components.
In general, the polarization of monochromatic light is completely described via four Stokes parameters, which form a (non-orthonormal) vector space when the various waves are incoherent.
However, the Milky-Way "dust" polarization (the "foreground" to cosmologists) can produce B-modes, so it must be well-understood and subtracted to obtain the cosmological signal.
Hence, when the neutrinos have decoupled with their entropy is separately conserved, the photons are in equilibrium with electrons and positrons.
[27] Observation of these primordial photons is meant to reveal the two polarization patterns E and B modes which help to understand the physics of the early universe and its late-time evolution.
Unfortunately, galactic nuclei and dust emit very strongly in the wavelength < 3 x 10^-2 cm, completely swamping the primordial signal.
[27] In the early universe, several processes keep the radiation and matter tightly coupled until a temperature of about a few eV due to sufficient number of free charged particles.
These constituents interact amongst themselves and with the photons through various electromagnetic processes, like Bremsstrahlung, Compton (and Thomson) scattering, recombination reaction (
[27] The formation of atoms affects the photons, which were in thermal equilibrium with the rest of the matter through various scattering processes.
