Circular polarization of the CMB: Foregrounds and detection prospects

Abstract

The cosmic microwave background (CMB) is one of the finest probes of cosmology. Its all-sky temperature and linear polarization fluctuations have been measured precisely at a level of δT=T CMB ∼ 10 −6 . In contrast, circular polarization (CP) of the CMB has not been precisely explored. The current upper limit on the CP of the CMB is at a level of δV=T CMB ∼ 10 −4 and is limited on large scales. Some of the cosmologically important sources which can induce a CP in the CMB include early Universe symmetry breaking, a primordial magnetic field, galaxy clusters, and Pop III stars (also known as the first stars). Among these sources, Pop III stars are expected to induce the strongest signal with levels strongly dependent on the frequency of observation and on the number, N p , of the Pop III stars per halo. Optimistically, a CP signal in the CMB resulting from the Pop III stars could be at a level of δV=T CMB ∼ 2 × 10 −7 in scales of 1°at 10 GHz, which is much smaller than the currently existing upper limits on the CP measurements. Primary foregrounds in the cosmological CP detection will come from the galactic synchrotron emission, which is naturally (intrinsically) circularly polarized. We use data-driven models of the galactic magnetic field, thermal electron density, and relativistic electron density to simulate all-sky maps of the galactic CP. This work also points out that the galactic CP levels are important below 50 GHz and is an important factor for telescopes aiming to detect primordial B modes using CP as a systematic rejection channel. In this paper, we focus on a SNR evaluation for the detectability of the Pop III induced CP signal in the CMB. We find that a SNR higher than unity is achievable, for example, with a 10 m telescope and an observation time of 20 months at 10 GHz, if N p ≥ 100. We also find that, if frequency of observation and resolution of the beam is appropriately chosen, a SNR higher than unity is possible with N p ≥ 10 and resolution per pixel ∼1 μK at an observation time of 60 months.

The Galaxy in circular polarization: All-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays

“…We expect this ratio to increase with ν −0.5 , approaching 10 −3 at 40 MHz, which might be a detectable level for current instrumentation [45]. The frequency scaling of V /I ∝ ν −0.5 was already predicted by [36] for the GHz range.…”

“…Nevertheless, the provided CP prediction can be used for template based searches for the elusive CP signal. Our model shows similarities and differences to a CP prediction based on a 3D models of the Milky Way [36]. We expect our model to capture more details of the real CP sky, as its construction is based directly on observed data sets, without the detour of using those to construct a parametrized, and therefore coarse, 3D model.…”

A New Limit on CMB Circular Polarization from SPIDER

“…King & Lubin (2016) suggest that 3.4 n -is a reasonable model for synchrotron circular polarization. With this source spectrum, SPIDERʼs circular polarization limits in Figure 4 scale by 0.08 at 95 GHz and 0.11 at 150 GHz, still using CMB temperature units.…”

“…Additional sources include postulated extensions to QED such as Lorentz-invariance violating operators (Colladay & Kosteleckỳ1998;Alexander et al 2009), axion-like pseudoscalar particles (Finelli & Galaverni 2009), and nonlinear photon interactions (through effective EulerHeisenberg Lagrangians; Motie & Xue 2012). A brief review of some of these generation mechanisms can be found in King & Lubin (2016). Despite the wide range of physical processes they invoke, all of these mechanisms predict levels of circular polarization that are unlikely to be accessible with current technology.…”

Circular polarization of the cosmic microwave background from vector and tensor perturbations

“…In astrophysics, circular polarization may arise in masers [6,7], gamma-ray-burst afterglows [8][9][10][11], jets of active galactic nuclei [12][13][14][15], and pulsars [16][17][18][19]. In addition, circular polarization has recently been discussed also in the context of the CMB [20][21][22][23][24][25]. Circular polarization can be produced through Faraday conversion when a linearly polarized light ray propagates through a medium where the indexes of refraction differ along the two different transverse axes.…”