Denis Barkats

We present the first constraints on multipole-dependent cosmic birefringence using CMB polarization data from the BK18 dataset, which combines observations from BICEP2, Array, and BICEP3 at frequencies of 95, 150, and 220 GHz. Photon coupling to an axionlike field leads to the rotation of CMB polarization, inducing nonzero E B cross-correlations. We show that a multipole-dependent rotation β ( ℓ ) imprints a distinct signature in the polarization spectra that can be constrained. Specifically, we consider an early dark energy (EDE) scenario in which a pseudoscalar field couples to photons through a Chern-Simons interaction, generating a polarization rotation with multipole dependence. We introduce a phenomenological β ( ℓ ) as a step function, obtaining constraints on the step function size consistent with zero, with uncertainties less than ∼ 0.15 ° (68% CL). In addition, using multi-frequency E E , B B , and E B cross spectra, along with robust BICEP/ foreground treatment and likelihood framework, we derive constraints on the axion-photon coupling amplitude g for several choices of EDE parameters. For the baseline best-fit value f EDE = 0.087 from the 2018 analysis, we obtain g = 0.11 ± 0.37 (68% CL), consistent with previous limits.

The BICEP/Keck collaboration searches for B-mode polarization in the Cosmic Microwave Background (CMB) as evidence of inflationary gravitational waves, with current best constraints placing $r_{0.05} < 0.035$. Building even deeper and larger maps requires scaling detector count at 150 GHz, where B-mode power is expected to peak at $\ell \sim 80$. The BICEP Array 150 GHz receiver (BA150) fields 7776 superconducting transition edge sensor (TES) detectors, over 15× the 512 deployed in the previous BICEP/Keck 150 GHz receiver, making it the densest time-division multiplexed readout receiver in the collaboration's history. We present the complete detector characterization of all 12 detector modules as deployed in BA150. We report thermal properties ($T_{c}$, $G_{c}$, $P_\rm{sat}$), end-to-end optical efficiencies of 20–48%, and fractional bandwidths of $\sim$28% across all modules. We additionally report the identification and mitigation of an out-of-band high-frequency photon leak spanning 180–265 GHz, and present preliminary investigations of elevated on-sky noise attributed to aliased high-frequency detector noise.

Millimeter-wave refracting telescopes targeting the degree-scale structure of the cosmic microwave background (CMB) have recently grown to diffraction-limited apertures of over 0.5 m. These instruments are entirely housed in vacuum cryostats to support their subkelvin bolometric detectors and to minimize radiative loading from thermal emission due to absorption loss in their transmissive optical elements. The large vacuum window is the only optical element in the system at ambient temperature, and therefore minimizing loss in the window is crucial for maximizing detector sensitivity. This motivates the use of low-loss polymer materials and a window as thin as practicable. However, the window must simultaneously meet the requirement to keep a sufficient vacuum, and therefore must limit gas permeation and remain mechanically robust against catastrophic failure under pressure. We report on the development of extremely thin composite polyethylene window technology that meets these goals. Two windows have been deployed for two full observing seasons on the BICEP3 and the 150 GHz BICEP Array receiver CMB telescopes at the South Pole. On BICEP3, the window has demonstrated a 6% improvement in detector sensitivity.

We present component-separated polarization maps of the cosmic microwave background (CMB) and Galactic thermal dust emission, derived using data from the BICEP/Keck experiments through the 2018 observing season and Planck. By employing a maximum-likelihood method that utilizes observing matrices, we produce unbiased maps of the CMB and dust signals. We outline the computational challenges and demonstrate an efficient implementation of the component map estimator. We show methods to compute and characterize power spectra of these maps, opening up an alternative way to infer the tensor-to-scalar ratio from our data. We compare the results of this map-based separation method with the baseline BICEP/Keck analysis. Our analysis demonstrates consistency between the two methods, finding an 84% correlation between the pipelines.

We characterize Galactic dust filaments by correlating BICEP/Keck and Planck data with polarization templates based on neutral hydrogen (H i) observations. Dust polarization is important for both our understanding of astrophysical processes in the interstellar medium (ISM) and the search for primordial gravitational waves in the cosmic microwave background (CMB). In the diffuse ISM, H i is strongly correlated with the dust and partly organized into filaments that are aligned with the local magnetic field. We analyze the deep BICEP/Keck data at 95, 150, and 220 GHz, over the low-column-density region of sky where BICEP/Keck has set the best limits on primordial gravitational waves. We separate the H i emission into distinct velocity components and detect dust polarization correlated with the local Galactic H i but not with the H i associated with Magellanic Stream i. We present a robust, multifrequency detection of polarized dust emission correlated with the filamentary H i morphology template down to 95 GHz. For assessing its utility for foreground cleaning, we report that the Hi morphology template correlates in B modes at a ∼10%–65% level over the multipole range 20 < ℓ < 200 with the BICEP/Keck maps, which contain contributions from dust, CMB, and noise components. We measure the spectral index of the filamentary dust component spectral energy distribution to be β = 1.54 ± 0.13. We find no evidence for decorrelation in this region between the filaments and the rest of the dust field or from the inclusion of dust associated with the intermediate velocity H i. Finally, we explore the morphological parameter space in the H i-based filamentary model.