Cosmic Microwave Background Polarization Experiments Research Articles

Cross-correlation of CMB Polarization Lensing with High-z Submillimeter Herschel-ATLAS Galaxies

Abstract We report a 4.8σ measurement of the cross-correlation signal between the cosmic microwave background (CMB) lensing convergence reconstructed from measurements of the CMB polarization made by the Polarbear experiment and the infrared-selected galaxies of the Herschel-ATLAS survey. This is the first measurement of its kind. We infer a best-fit galaxy bias of , corresponding to a host halo mass of at an effective redshift of z ∼ 2 from the cross-correlation power spectrum. Residual uncertainties in the redshift distribution of the submillimeter galaxies are subdominant with respect to the statistical precision. We perform a suite of systematic tests, finding that instrumental and astrophysical contaminations are small compared to the statistical error. This cross-correlation measurement only relies on CMB polarization information that, differently from CMB temperature maps, is less contaminated by galactic and extragalactic foregrounds, providing a clearer view of the projected matter distribution. This result demonstrates the feasibility and robustness of this approach for future high-sensitivity CMB polarization experiments.

Development of a contact-less cryogenic rotation mechanism employed for a polarization modulator unit in cosmic microwave background polarization experiments

We present the design and the performance of a contact-less cryogenic rotation mechanism used in cosmic microwave background (CMB) experiments. A precise measurement of the CMB polarization is possible to verify the cosmic inflation theory that describes the very beginning (10−38 seconds) of the early universe. The polarization modulator, that rotates a half wave plate continuously at the aperture of the telescope, is one of the key instruments in the experiments. In order to reduce noise and systematic uncertainties, the polarization modulator is required a stable rotation with minimal heat dissipation in a cryogenic environment less than 20 K. Thus, we adopted the rotation mechanism that combines completely contact-less bearing and motor, a superconducting magnetic bearing, and a hollow bore synchronous motor. The heat dissipation and the load torque due to the friction can be minimized by avoiding physical contacts. We constructed the prototype of the rotation mechanism and carried out mechanical and thermal performance tests. A continuous rotation test in cryogenic temperature is performed, and it is confirmed that the rotation stability is less than 1% with the rotation frequency between 0.5 Hz and 3.0 Hz. We also conducted a thermal performance test, and obtained the heat dissipation at the rotor of 9.0 mW. We discussed the reduction of heat dissipation using a developed magnetic circuit with improved magnetic field uniformity.

Future CMB constraints on cosmic birefringence and implications for fundamental physics

The primary scientific target of the ground- and space-based cosmic microwave background (CMB) polarization experiments currently being built and proposed is the detection of primordial tensor perturbations. As a byproduct, these instruments will significantly improve constraints on cosmic birefringence, or the rotation of the CMB polarization plane. If convincingly detected, cosmic birefringence would be a dramatic manifestation of physics beyond the standard models of particle physics and cosmology. We forecast the bounds on the cosmic polarization rotation (CPR) from the upcoming ground-based Simons Observatory (SO) and the space-based LiteBIRD experiments, as well as a ``fourth-generation'' ground-based CMB experiment like CMB-S4 and the midcost space mission PICO. We examine the detectability of both a stochastic anisotropic rotation field, as well as an isotropic rotation by a constant angle. CPR induces new correlations of CMB observables, including spectra of parity-odd type in the case of isotropic CPR, and mode-coupling correlations in the anisotropic rotation case. We find that LiteBIRD and SO will reduce the $1\ensuremath{\sigma}$ bound on the isotropic CPR from the current value of 30 arcmin to 1.5 and 0.6 arcmin, respectively, while a CMB-S4-like experiment and PICO will reduce it to $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{arcmin}$. The bounds on the amplitude of a scale-invariant CPR spectrum will be reduced by 1, 2, and 3 orders of magnitude by LiteBIRD, SO, and CMB-S4-like/PICO, respectively. We discuss potential implications for fundamental physics by interpreting the forecasted bounds on CPR in terms of the corresponding constraints on pseudoscalar fields coupled to electromagnetism, primordial magnetic fields (PMF), and violations of Lorentz invariance. We find that CMB-S4-like and PICO can reduce the $1\ensuremath{\sigma}$ bound on the amplitude of the scale-invariant PMF from 1 to 0.1 nG, while also probing the magnetic field of the Milky Way. The upcoming experiments will also tighten bounds on the axion-photon coupling, with SO improving the bound from ${f}_{a}\ensuremath{\gtrsim}50{H}_{I}$ at present, where ${H}_{I}$ is the energy scale of inflation, to ${f}_{a}\ensuremath{\gtrsim}500{H}_{I}$, and CMB-S4-like and PICO raising it to ${f}_{a}\ensuremath{\gtrsim}\mathrm{few}\ifmmode\times\else\texttimes\fi{}{10}^{3}{H}_{I}$, placing stringent constraints on the string theory axions.

Measurements of Tropospheric Ice Clouds with a Ground-based CMB Polarization Experiment, POLARBEAR

Abstract The polarization of the atmosphere has been a long-standing concern for ground-based experiments targeting cosmic microwave background (CMB) polarization. Ice crystals in upper tropospheric clouds scatter thermal radiation from the ground and produce a horizontally polarized signal. We report a detailed analysis of the cloud signal using a ground-based CMB experiment, Polarbear, located at the Atacama desert in Chile and observing at 150 GHz. We observe horizontally polarized temporal increases of low-frequency fluctuations (“polarized bursts,” hereafter) of ≲0.1 K when clouds appear in a webcam monitoring the telescope and the sky. The hypothesis of no correlation between polarized bursts and clouds is rejected with >24σ statistical significance using three years of data. We consider many other possibilities including instrumental and environmental effects, and find no reasons other than clouds that can explain the data better. We also discuss the impact of the cloud polarization on future ground-based CMB polarization experiments.

Detector and Readout Assembly and Characterization for the Simons Array

The Simons Array (SA) is a cosmic microwave background (CMB) polarization experiment comprised of three telescopes that will observe the CMB at 90, 150, 220, and 270 GHz with more than 22,000 Transition Edge Sensor (TES) bolometers. The cryogenic receivers inside each telescope are named POLARBEAR-2a, POLARBEAR-2b, and POLARBEAR-2c (PB-2a, PB-2b, and PB-2c, respectively). To allow for the large number of detectors, SA uses frequency-division multiplexing with multiplexing factor of 40. We describe the process developed to assemble the readout circuit repeatably for SA. After assembly, we characterize the readout circuit and TESs at cryogenic temperatures in a condition of negligible incident optical power. Impedances in the readout circuit bias our estimates of TES parameters, and we describe a method to account for this.

Low-Temperature Detectors for CMB Imaging Arrays

We review advances in low temperature detector (LTD) arrays for Cosmic Microwave Background (CMB) polarization experiments, with a particular emphasis on imaging arrays. We briefly motivate the science case, which has spurred a large number of independent experimental efforts. We describe the challenges associated with CMB polarization measurements and how these challenges impact LTD design. Key aspects of an ideal CMB polarization imaging array are developed and compared to the current state-of-the-art. These aspects include dual-polarization-sensitivity, background-limited detection over a 10:1 bandwidth ratio, and frequency independent angular responses. Although existing technology lacks all of this capability, today's CMB imaging arrays achieve many of these ideals and are highly advanced superconducting integrated circuits. Deployed arrays map the sky with pixels that contain elements for beam formation, polarization diplexing, passband definition in multiple frequency channels, and bolometric sensing. Several detector architectures are presented. We comment on the implementation of both transition-edge-sensor bolometers and microwave kinetic inductance detectors for CMB applications. Lastly, we discuss fabrication capability in the context of next-generation instruments that call for $\sim 10^6$ sensors.

Detecting electron density fluctuations from cosmic microwave background polarization using a bispectrum approach

Recent progress in high sensitivity Cosmic Microwave Background (CMB) polarization experiments opens up a window on large scale structure (LSS), as CMB polarization fluctuations on small angular scales can arise from a combination of LSS and ionization fluctuations in the late universe. Gravitational lensing effects can be extracted from CMB datasets with quadratic estimators but reconstructions of electron density fluctuations (EDFs) with quadratic estimators are found to be significantly biased by the much larger lensing effects in the secondary CMB fluctuations. In this paper we establish a bispectrum formalism using tracers of LSS to extract the subdominant EDFs from CMB polarization data. We find that this bispectrum can effectively reconstruct angular band-powers of cross correlation between EDFs and LSS tracers. Next generation CMB polarization experiments in conjunction with galaxy surveys and cosmic infrared background experiments can detect signatures of EDFs with high significance.

Design and Thermal Characteristics of a 400 mm Diameter Levitating Rotor in a Superconducting Magnetic Bearing Operating Below at 10 K for a CMB Polarization Experiment

We present the design and thermal characteristics of a levitating rotor in a superconducting magnetic bearing (SMB) system that is designed for a polarization modulator of a cosmic microwave background (CMB) polarization experiment. Thermal characteristics of the levitating rotor in the SMB are important to reduce the detector noise. We have constructed two types of SMB systems that both employ a NdFeB permanent magnet as a rotor and a YBCO superconductor array as a stator. One is the 1/6 scale prototype model and the other one is the flight representative full-scale prototype. In order to build a thermal model of the SMB system, we estimate thermal properties of a NdFeB magnet based on the measured data using the 1/6 scale prototype model. The thermal model is expanded to the full-scale prototype, and we obtained the requirement of heat loads from the SMB system is to be less than 1 mW. We also measured the thermal conductivity between the levitating rotor and a holder mechanism, which is the thermal path when the rotor is held in place at the time of field cooling. The measured conductivity is 1.34 mW/K, which is almost same as the 1/6 scale prototype model.

Fast algorithm for the computation of the CMB polarization TE power spectrum using non-circular beam

A precise measurement of the Cosmic Microwave Background (CMB) anisotropy has been one of the foremost concerns in modern cosmology as it provides a valuable information on the cosmology of the Universe. The estimation of the CMB power spectrum is complicated by different systematics. For the polarization experiments, the signals are rather fainter in comparison with the CMB total intensity, which might lead to an important bias in the estimation of the angular power spectrum. One of the most important source of bias in CMB polarization experiment is the beam asymmetry. In this paper, we present a semi-analytical framework using the pseudo-Cl estimator to compute the power spectrum TE of the temperature anisotropy and the E-component of the polarization radiation field using non-circular beams. We assume that the beam is non-rotating. We adopt a model of beams obtained from a perturbative expansion of the beam around a circular (axisymmetric) beam in harmonic space. We compute the resulting bias matrix which relates the true power spectrum with the observed one by using an efficient algorithm for rapid computation. We show that for a multipole up to lmax=500, the bias matrix can be computed in less than one second with a single CPU processor at 2.53 GHz. We find that the systematic effect induced by the beam asymmetry in the temperature and polarization power spectrum at the peak of the bias matrix for WMAP and Planck experiments can be as large as a few 10 to 20% (upper limit for Planck LFI 30 GHz).

Observing patchy reionization with future CMB polarization experiments

We study the signal from patchy reionization in view of the future high accuracy polarization measurements of the Cosmic Microwave Background (CMB). We implement an extraction procedure of the patchy reionization signal analogous to CMB lensing. We evaluate the signal to noise ratio (SNR) for the future Stage IV (S4) CMB experiment. The signal has a broad peak centered on the degree angular scales, with a long tail at higher multipoles. The CMB S4 experiment can effectively constrain the properties of reionization by measuring the signal on degree scales. The signal amplitude depends on the properties of the structure determining the reionization morphology. We describe bubbles having radii distributed log-normally. The expected S/N is sensitive to the mean bubble radius: R̄=5 Mpc implies S/N ≈ 4, R̄=10 Mpc implies S/N ≈ 20. The spread of the radii distribution strongly affects the integrated SNR, that changes by a factor of 102 when σlnr goes from ln 2 to ln 3. Future CMB experiments will thus place important constraints on the physics of reionization.

Probing primordial gravitational waves: Ali CMB Polarization Telescope.

In this paper, we will give a general introduction to the Ali CMB Polarization Telescope (AliCPT) project, which is a Sino–US joint project led by the Institute of High Energy Physics and involves many different institutes in China. It is the first ground-based Cosmic Microwave Background (CMB) polarization experiment in China and an integral part of China's Gravitational-wave Program. The main scientific goal of the AliCPT project is to probe the primordial gravitational waves (PGWs) originating from the very early Universe. The AliCPT project includes two stages. The first stage, referred to as AliCPT-1, is to build a telescope in the Ali region of Tibet at an altitude of 5250 meters. Once completed, it will be the highest ground-based CMB observatory in the world and will open a new window for probing PGWs in the northern hemisphere. The AliCPT-1 telescope is designed to have about 7000 transition-edge sensor detectors at 95 GHz and 150 GHz. The second stage is to have a more sensitive telescope (AliCPT-2) with more than 20 000 detectors. Our simulations show that AliCPT will improve the current constraint on the tensor-to-scalar ratio r by one order of magnitude with three years' observation. Besides the PGWs, AliCPT will also enable a precise measurement of the CMB rotation angle and provide a precise test of the CPT symmetry. We show that three years' observation will improve the current limit by two orders of magnitude.

Mitigating Complex Dust Foregrounds in Future Cosmic Microwave Background Polarization Experiments

Abstract Polarized Galactic foregrounds are one of the primary sources of systematic error in measurements of the B-mode polarization of the cosmic microwave background (CMB). Experiments are becoming increasingly sensitive to complexities in the foreground frequency spectra that are not captured by standard parametric models, potentially affecting our ability to efficiently separate out these components. Employing a suite of dust models encompassing a variety of physical effects, we simulate observations of a future seven-band CMB experiment to assess the impact of these complexities on parametric component separation. We identify configurations of frequency bands that minimize the “model errors” caused by fitting simple parametric models to more complex “true” foreground spectra, which bias the inferred CMB signal. We find that: (a) fits employing a simple two-parameter modified blackbody (MBB) dust model tend to produce significant bias in the recovered polarized CMB signal in the presence of physically realistic dust foregrounds; (b) generalized MBB models with three additional parameters reduce this bias in most cases, but non-negligible biases can remain, and can be hard to detect; (c) line-of-sight effects, which give rise to frequency decorrelation, and the presence of iron grains are the most problematic complexities in the dust emission for recovering the true CMB signal. More sophisticated simulations will be needed to demonstrate that future CMB experiments can successfully mitigate these more physically realistic dust foregrounds.

Optimal strategy for polarization modulation in the LSPE-SWIPE experiment

Context. Cosmic microwave background (CMB) B-mode experiments are required to control systematic effects with an unprecedented level of accuracy. Polarization modulation by a half wave plate (HWP) is a powerful technique able to mitigate a large number of the instrumental systematics. Aims. Our goal is to optimize the polarization modulation strategy of the upcoming LSPE-SWIPE balloon-borne experiment, devoted to the accurate measurement of CMB polarization at large angular scales. Methods. We departed from the nominal LSPE-SWIPE modulation strategy (HWP stepped every 60 s with a telescope scanning at around 12 deg/s) and performed a thorough investigation of a wide range of possible HWP schemes (either in stepped or continuously spinning mode and at different azimuth telescope scan-speeds) in the frequency, map and angular power spectrum domain. In addition, we probed the effect of high-pass and band-pass filters of the data stream and explored the HWP response in the minimal case of one detector for one operation day (critical for the single-detector calibration process). We finally tested the modulation performance against typical HWP-induced systematics. Results. Our analysis shows that some stepped HWP schemes, either slowly rotating or combined with slow telescope modulations, represent poor choices. Moreover, our results point out that the nominal configuration may not be the most convenient choice. While a large class of spinning designs provides comparable results in terms of pixel angle coverage, map-making residuals and BB power spectrum standard deviations with respect to the nominal strategy, we find that some specific configurations (e.g., a rapidly spinning HWP with a slow gondola modulation) allow a more efficient polarization recovery in more general real-case situations. Conclusions. Although our simulations are specific to the LSPE-SWIPE mission, the general outcomes of our analysis can be easily generalized to other CMB polarization experiments.

Thermal analysis of a prototype cryogenic polarization modulator for use in a space-borne CMB polarization experiment

We report a thermal analysis of a polarization modulator unit (PMU) for use in a space-borne cosmic microwave background (CMB) project. A measurement of the CMB polarization allows us to probe the physics of early universe, and that is the best method to test the cosmic inflation experimentally. One of the key instruments for this science is to use a halfwave plate (HWP) based polarization modulator. The HWP is required to rotate continuously at about 1 Hz below 10 K to minimize its own thermal emission to a detector system. The rotating HWP system at the cryogenic environment can be realized by using a superconducting magnetic bearing (SMB) without significant heat dissipation by mechanical friction. While the SMB achieves the smooth rotation due to the contactless bearing, an estimation of a levitating HWP temperature becomes a challenge. We manufactured a one-eighth scale prototype model of PMU and built a thermal model. We verified our thermal model with the experimental data. We forecasted the projected thermal performance of PMU for a full-scale model based on the thermal model. From this analysis, we discuss the design requirement toward constructing the full-scale model for use in a space environment such as a future CMB satellite mission, LiteBIRD.

Advances in Bolometer Technology for Fundamental Physics

In the last decade, cryogenic bolometers have provided increasingly improved resolution and sensitivity in particle and radiation detectors. Thermal particle detectors have proven their outstanding capabilities in different fields of fundamental physics, especially in rare event detection. Cryogenic incoherent detector arrays designed to detect millimeter-wave photons have helped enable precision measurements of anisotropies in the cosmic microwave background (CMB), providing a unique probe of early universe physics and helping to constrain parameters of particle physics such as the sum of the neutrino masses. We review the latest achievements of cryogenic particle detectors for direct detection searches for dark matter and double-[Formula: see text] decay, as well as for CMB measurements, and we discuss expected improvements aiming to increase the sensitivities of these experiments. An important challenge is the large-scale implementation of arrays of detectors such as transition edge sensors, especially in CMB polarization experiments. We describe the challenges of scaling up to these larger arrays, including fabrication throughput and development of new multiplexing electronics.

A New Limit on CMB Circular Polarization from SPIDER

Abstract We present a new upper limit on cosmic microwave background (CMB) circular polarization from the 2015 flight of Spider, a balloon-borne telescope designed to search for B-mode linear polarization from cosmic inflation. Although the level of circular polarization in the CMB is predicted to be very small, experimental limits provide a valuable test of the underlying models. By exploiting the nonzero circular-to-linear polarization coupling of the half-wave plate polarization modulators, data from Spider's 2015 Antarctic flight provide a constraint on Stokes V at 95 and 150 GHz in the range 33 < ℓ < 307 . No other limits exist over this full range of angular scales, and Spider improves on the previous limit by several orders of magnitude, providing 95% C.L. constraints on ℓ ( ℓ + 1 ) C ℓ VV / ( 2 π ) ranging from 141 to 255 μK2 at 150 GHz for a thermal CMB spectrum. As linear CMB polarization experiments become increasingly sensitive, the techniques described in this paper can be applied to obtain even stronger constraints on circular polarization.

Vibrational characteristics of a superconducting magnetic bearing employed for a prototype polarization modulator

We present the vibrational characteristics of a levitating rotor in a superconducting magnetic bearing (SMB) system operating at below 10 K. We develop a polarization modulator that requires a continuously rotating optical element, called half-wave plate (HWP), for a cosmic microwave background polarization experiment. The HWP has to operate at the temperature below 10 K, and thus an SMB provides a smooth rotation of the HWP at the cryogenic temperature of about 10 K with minimal heat dissipation. In order to understand the potential interference to the cosmological observations due to the vibration of the HWP, it is essential to characterize the vibrational properties of the levitating rotor of the SMB. We constructed a prototype model that consists of an SMB with an array of high temperature superconductors, YBCO, and a permanent magnet ring, NdFeB. The rotor position is monitored by a laser displacement gauge, and a cryogenic Hall sensor via the magnetic field. In this presentation, we present the measurement results of the vibration characteristics using our prototype SMB system. We characterize the vibrational properties as the spring constant and the damping, and discuss the projected performance of this technology toward the use in future space missions.

Estimation of the Heat Dissipation and the Rotor Temperature of Superconducting Magnetic Bearing Below 10 K

We present the thermal characteristics of a superconducting magnetic bearing (SMB) system that is designed for a polarization modulator of a cosmic microwave background polarization experiment. The SMB system consists of a ring-shaped magnet and an array of YBCO tiles. Both have an opening diameter of about 400 mm. The ring-shaped magnet is formed by an array of segmented magnets. The magnetic field inhomogeneity of this rotor magnet is originated from the magnetic field inhomogeneity of each magnet and the spatial gap between the segments. This inhomogeneity can contribute as a source of friction, and thus a source of heat in an operation at below 10 K. We estimate the heat dissipation from friction of the SMB by conducting the spin-down measurements, and we projected the heat dissipation of 2–20 mW from the hysteresis contribution depending on the levitation height. We discuss the consistency of the spin-down measurements to Bean's model between the magnetic field inhomogeneity and the hysteresis loss. We also discuss the potential temperature rise of the rotor magnet when it is levitating at the temperature at around 10 K. Thus, we address the means to estimate the rotor temperature by the physical contact to the rotor. The extrapolated temperature recovers the rotor temperature with the difference of less than 2 K without any correction.

POLOCALC: A Novel Method to Measure the Absolute Polarization Orientation of the Cosmic Microwave Background

We describe a novel method to measure the absolute orientation of the polarization plane of the Cosmic Microwave Background (CMB) photons with arcsecond accuracy, enabling unprecedented measurements for cosmology and fundamental physics. Existing and planned CMB polarization instruments looking for primordial B-mode signals need an independent, experimental method for systematics control on the absolute polarization orientation. The lack of such a method limits the accuracy of the detection of inflationary gravitational waves, the constraining power on the neutrino sector through measurements of gravitational lensing of the CMB, the possibility of detecting Cosmic Birefringence (CB), and the ability to measure primordial magnetic fields. Sky signals used for calibration and direct measurements of the detector orientation cannot provide an accuracy better than 1[Formula: see text]. Self-calibration methods provide better accuracy, but may be affected by foreground signals and rely heavily on model assumptions, losing constraining power on fundamental processes, like CB, Faraday Rotation and chiral gravity models. The POLarization Orientation CALibrator for Cosmology, POLOCALC, will dramatically improve instrumental accuracy by means of an artificial calibration source flying on high-altitude balloons and aerial drones. Polarization angle calibration requires observation of a well-characterized distant source at high elevation angles. A balloon-borne calibrator will provide a source in the far field of larger telescopes, while an aerial drone can be used for tests and smaller polarimeters. POLOCALC will also allow a unique method to measure the telescopes’ polarized beam. Even a two-hour balloon flight will allow enough time to perform polarization angle calibration and polarized beam function measurements. The source will make use of both narrow and broadband microwave emitters between 40[Formula: see text]GHz and 150[Formula: see text]GHz coupled to precise polarizing filters. The orientation of the source polarization plane will be registered to absolute celestial coordinates by star cameras and gyroscopes with arcsecond accuracy. This project can become a rung in the calibration ladder for the field: any existing or future CMB polarization experiment observing our novel polarization calibrator will enable measurements of the polarization angle for each detector with respect to absolute sky coordinates.

Scale invariance of the primordial tensor power spectrum

Future cosmic microwave background polarization experiments will search for evidence of primordial tensor modes at large angular scales, in the multipole range $4 \leq \ell \leq 50.$ Because in that range there is some mild evidence of departures from scale invariance in the power spectrum of primordial curvature perturbations, one may wonder about the possibility of similar deviations appearing in the primordial power spectrum of tensor modes. Here we address this issue and analyze the possible presence of features in the tensor spectrum resulting from the dynamics of primordial fluctuations during inflation. We derive a general, model independent, relation linking features in the spectra of curvature and tensor perturbations. We conclude that even with large deviations from scale invariance in the curvature power spectrum, the tensor spectrum remains scale invariant for all observational purposes.