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Circular polarization of the CMB: Foregrounds and detection prospects

PHYSICAL REVIEW D 94, 023501 (2016) Circular polarization of the CMB: Foregrounds and detection prospects Soma King 1 and Philip Lubin 2 Department of Physics and Astronomy, University of California Davis, Davis, California 95616, USA Department of Physics, University of California Santa Barbara, Santa Barbara, California 93106, USA (Received 26 April 2016; published 5 July 2016) 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. DOI: 10.1103/PhysRevD.94.023501 I. INTRODUCTION The cosmic microwave background (CMB) has been the finest probe of cosmology. A complete characterization of the CMB is made by quantifying the four Stokes parameters [1] associated with its unpolarized intensity, linear polarization (LP) and circular polarization (CP). Temperature and polarization anisotropies of the CMB have provided invaluable insights into the Universe that we live in. The venture into the CMB studies began with its discovery in 1965 [2,3], which showed the CMB radiation to be a black body with a mean temperature of T ∼ 2.7255 K. Subsequently, the first detection of angular variation of the CMB mean temperature was measured with the COBE satellite [4], and today the temperature fluctua- tions have been measured with an unprecedented precision [5–9]. The CMB is also linearly polarized due to Thomson scattering at the surface of last scattering. This polarization level is at ∼5% of the temperature anisotropies. Polarization fluctuations have been measured with a high precision, and many future experiments are aimed at increasing the precision of polarization measurements [10–14]. The Planck experiment reached a sensitivity level, δT=T CMB ∼ 10 −6 , for temperature and polarization, a much higher level of sensitivity compared to the WMAP satellite. Together, the CMB temperature and the LP data over- whelmingly support the cosmological standard model or the Λ cold dark matter [15] model. It is only natural to wonder about what kinds of information the CP of the CMB stores. The CP of the CMB has not been explored extensively and the current level of CMB CP measurement stands at a level ΔV=T CMB ∼ 10 −4 in the scales of 8° and 24° [16]. This upper limit is much higher than the level of the CP signal expected from any cosmologically relevant sources. Previously, Ref. [17] made one of the first efforts to measure the CP of the CMB. Recently, CLASS [18] and PIPER [19] experiments proposed measuring the LP and CP of the CMB; however, the experiment is designed to focus on primarily the measurements of the LP of the CMB. These experiments aim to detect primordial B modes using variable-delay polarization (VPM) instruments where the observing strategy relies on the expectation of CP of the sky to be null to constrain systematic uncertainties. In this paper, we provide galactic CP maps for frequencies relevant for both CLASS at 40 GHz and PIPER at 220 GHz, showing that the galactic CP effects are important for frequencies below 50 GHz. There are various cosmologically important sources which may induce a CP in the CMB via different © 2016 American Physical Society