Primordial circular polarization in the cosmic microwave background

Abstract

Circular ("V-mode") polarization is expected to be vanishing in the CMB, since it is not produced in Thomson scattering. However, considering that the conventional CMB anisotropies are generated via an early universe mechanism such as inflation or a bouncing scenario, it is possible that circular polarization could be primordially produced and survive to the surface of last scattering. We study this in detail, and find a large class of inflationary models that produce a nearly scale invariant spectrum of scalar V-mode anisotropies. We study the inflationary production and subsequent evolution via the Boltzmann hierarchy, and show that V-mode polarization present in the CMB is suppressed by a factor of at least 10 10 20 relative to the primordial V , consistent with expectation of negligible V-mode polarization from inflation. We consider alternative possibilities for sourcing V primordially, such as the V-mode polarization induced by circularly polarized primordial gravitational waves, or producing V after inflation, via new interactions at recombination.

CMB V modes from photon-photon forward scattering revisited

“…In fact, after using the spin-weighted spherical harmonic expansion and performing the momenta integrals in Eqs. (30), (31), and (32) with the same prescriptions as before, we find…”

“…From the physical point of view, the set of coupled Eqs. (30), (31), and (32) just derived gives rise both to the transformation of Q modes into U modes and vice-versa (Faraday rotation), and to the conversion of linear polarization to circular polarization and vice-versa (Faraday conversion). In this paper, we are interested only in the Faraday conversion effect; thus, we decouple the Q and U modes by assuming g…”

“…Notice that in Eqs. (30), (31), and (32), the source terms proportional to s 1 and s 2 are linear in the perturbations. However, as we have just shown, such contributions at the end vanish, leaving therefore only the remaining source terms that are second order in the cosmological fluctuations.…”

Spin Hall effect of light in inhomogeneous axion field

“…These include primordial magnetic fields [30], polarized Compton scattering [31], primordial gravitational waves [32], Faraday conversion by external magnetized plasma [33,34], the magnetic field of the so called first stars [35], and, finally, different modifications and symmetry breaking mechanisms beyond the standard model [36,37]. In view of these, recently there has been considerable effort directed towards the determination of upper limits on the observation of circularly polarized CMB radiation [38,39].…”

CMB polarization as a tool to constrain the optical properties of the Universe

“…It can be generated by known physics as CMB photons propagate across the Universe [5][6][7][8][9], but only in tiny amounts. However, physics beyond the standard model might be responsible for the generation of a larger amount of circular polarization [10][11][12][13][14][15][16][17][18]. Observing circular polarization in the CMB, therefore, could provide evidence for new physics or constrain certain classes of extensions of the standard model of particle physics.…”