A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales with POLARBEAR (1403.2369v3)

Abstract: We report a measurement of the B-mode polarization power spectrum in the cosmic microwave background (CMB) using the POLARBEAR experiment in Chile. The faint B-mode polarization signature carries information about the Universe's entire history of gravitational structure formation, and the cosmic inflation that may have occurred in the very early Universe. Our measurement covers the angular multipole range 500 < l < 2100 and is based on observations of an effective sky area of 25 square degrees with 3.5 arcmin resolution at 150 GHz. On these angular scales, gravitational lensing of the CMB by intervening structure in the Universe is expected to be the dominant source of B-mode polarization. Including both systematic and statistical uncertainties, the hypothesis of no B-mode polarization power from gravitational lensing is rejected at 97.1% confidence. The band powers are consistent with the standard cosmological model. Fitting a single lensing amplitude parameter A_BB to the measured band powers, A_BB = 1.12 +/- 0.61 (stat) +0.04/-0.12 (sys) +/- 0.07 (multi), where A_BB = 1 is the fiducial WMAP-9 LCDM value. In this expression, "stat" refers to the statistical uncertainty, "sys" to the systematic uncertainty associated with possible biases from the instrument and astrophysical foregrounds, and "multi" to the calibration uncertainties that have a multiplicative effect on the measured amplitude A_BB.

• The paper demonstrates a robust detection of CMB B-mode polarization from gravitational lensing, rejecting the null hypothesis at 97.1% confidence.
• It employs a meticulous methodology with blind analysis, Monte Carlo techniques, and systematic checks using 1,274 TES bolometers over a 25-square-degree field.
• The findings support the standard cosmological model and open avenues for refining neutrino mass constraints and probing early universe inflation physics.

Measurement of CMB B-Mode Polarization Using POLARBEAR

The paper "A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales with POLARBEAR" presents a detailed analysis of B-mode polarization in the cosmic microwave background (CMB) observed by the POLARBEAR experiment. The presented research focuses on a critical aspect of cosmology: the measurement and implications of B-modes, which provide insights into the early universe and gravitational structure formation.

Gravitational lensing of the CMB photons, primarily due to intervening large-scale structures, is regarded as a significant source of B-modes at the scales measured in this research. POLARBEAR reports a precise B-mode power spectrum measurement covering angular multipoles from 500 to 2100, using a 3.5 arcminute resolution at a frequency of 150 GHz. This experiment focuses on observing a 25-square-degree region in the sky from the James Ax Observatory in the Atacama Desert.

Methodology and Results

The research utilizes an extensive setup with a two-mirror telescope system and 1,274 TES (Transition Edge Sensor) bolometers in use, yielding significant sensitivity and angular resolution benchmarks. The collected data reveal a statistically robust rejection of the absence of B-mode polarization due to gravitational lensing, achieving a confidence level of 97.1%. The band powers were found consistent with the standard cosmological model.

A meticulous approach was adopted to manage systematic uncertainties and avoid observer bias. For example, in their analysis framework, the team employed blind analysis methods, which ensured data analysis was conducted without introducing unintended biases until the interpretation stage. They also tested various instrumental and observation splits to ensure robustness and consistency in their results.

Analytical and Monte Carlo methods were employed to calculate the mode-mixing effects and filter transfer functions, confirming the reliability of observed results. Systematic uncertainties included differential pointing, gain modulation, and beam asymmetries, which were carefully evaluated and shown to yield negligible impact compared to the experiment's statistical uncertainties.

Implications

The implications of observing B-modes in the CMB due to gravitational lensing are profound. The measurement enables a detailed reconstruction of the integrated large-scale structure (LSS) through which CMB photons traverse. Thus, these measurements serve as robust probes for examining fundamental physics, including neutrino mass constraints, and also provide potential insights into inflationary physics through indirect means, such as aiding the removal of cosmic variance limitations in searching for primordial B-modes.

The precision achieved by POLARBEAR in measuring B-modes is an important milestone delineating the potential of CMB polarization measurements in providing a window into both early universe conditions, like cosmic inflation, and high-energy scale physics beyond the standard model via gravitational lensing.

Future Directions

The research underscores the importance of B-mode measurements as a complementary technique to non-Gaussian analyses of the CMB, enhancing the capacity to paper uncharted physics domains. Future advancements will likely necessitate further refinement in handling systematic uncertainties and achieving even lower noise levels, potentially through diverse observational campaigns and instrumentation improvements.

Moreover, the synthesis of CMB B-mode data with forthcoming galaxy surveys and their cross-correlation could solidify constraints on late-time cosmological parameters, including the sum of neutrino masses and dark energy dynamics, enhancing our understanding of the universe's evolution.

This paper effectively marks an advancement in the field of precision cosmology, underlining both the progress made and the challenges ahead in comprehensively revealing the B-mode sector of the cosmic microwave background.