BICEP2 / Keck Array x: Constraints on Primordial Gravitational Waves using Planck, WMAP, and New BICEP2/Keck Observations through the 2015 Season (1810.05216v1)

Abstract: We present results from an analysis of all data taken by the BICEP2/Keck CMB polarization experiments up to and including the 2015 observing season. This includes the first Keck Array observations at 220 GHz and additional observations at 95 & 150 GHz. The $Q/U$ maps reach depths of 5.2, 2.9 and 26 $\mu$K${cmb}$ arcmin at 95, 150 and 220 GHz respectively over an effective area of $\approx 400$ square degrees. The 220 GHz maps achieve a signal-to-noise on polarized dust emission approximately equal to that of Planck at 353 GHz. We take auto- and cross-spectra between these maps and publicly available WMAP and Planck maps at frequencies from 23 to 353 GHz. We evaluate the joint likelihood of the spectra versus a multicomponent model of lensed-$\Lambda$CDM+$r$+dust+synchrotron+noise. The foreground model has seven parameters, and we impose priors on some of these using external information from Planck and WMAP derived from larger regions of sky. The model is shown to be an adequate description of the data at the current noise levels. The likelihood analysis yields the constraint $r{0.05}<0.07$ at 95% confidence, which tightens to $r_{0.05}<0.06$ in conjunction with Planck temperature measurements and other data. The lensing signal is detected at $8.8 \sigma$ significance. Running maximum likelihood search on simulations we obtain unbiased results and find that $\sigma(r)=0.020$. These are the strongest constraints to date on primordial gravitational waves.

• The paper reports a major constraint with an upper limit of r₀.₀₅ < 0.07 (95% confidence), tightening to r₀.₀₅ < 0.06 when temperature data is included.
• It leverages auto- and cross-spectra from multi-frequency maps to disentangle signals from lensed CMB, dust, and synchrotron emission.
• Advanced receiver observations, including new 220 GHz data, enhance precision and pave the way for future CMB experiments like CMB-S4.

Analysis of BICEP/Keck Array Constraints on Primordial Gravitational Waves through CMB Polarization Observations

This paper presents a comprehensive analysis of constraints on primordial gravitational waves using data from BICEP (Background Imaging of Cosmic Extragalactic Polarization), Keck Array, WMAP (Wilkinson Microwave Anisotropy Probe), and Planck observations up to the 2015 observing season. The focus is on CMB polarization B-modes, which are indirect evidence of inflationary gravitational waves, a key feature predicted by the inflationary theory in cosmology.

Methodology and Data Collection

The paper utilizes BICEP/Keck data, including the introduction of new measurements at 220 GHz in addition to existing observations at 95 GHz and 150 GHz. The maps produced from these observations achieve impressive depths over an effective area of approximately 400 square degrees. The 220 GHz maps, in particular, show a signal-to-noise ratio that rivals Planck's 353 GHz observations on polarized dust emission, indicating substantial progress in measuring dust foregrounds.

Auto- and cross-spectra are computed between these maps and publicly available WMAP and Planck maps at a wide range of frequencies (23 to 353 GHz), analyzing the spectra against a model comprising lensed CMB, potential gravitational wave signals, dust, synchrotron emission, and noise. The model includes seven parameters, of which some are constrained using external data from larger sky regions observed by Planck and WMAP.

Results and Significance Levels

The analysis yields constraints on the tensor-to-scalar ratio, $r_{0.05}$, a parameter representing the intensity of primordial gravitational waves relative to the scalar density perturbations. A strong upper constraint of $r_{0.05}<0.07$ at 95% confidence is reported, tightening to $r_{0.05}<0.06$ with additional temperature data, marking the most stringent limits yet placed on $r$ from such measurements.

The lensing signal is detected at an 8.8 sigma level, which stands as the most robust detection of gravitational lensing using B-mode polarization to date. Maximum likelihood simulations suggest unbiased results with $\sigma(r) = 0.020$, underscoring the data's precision and accuracy in ruling out several inflationary models that predict higher values for $r$.

Implications and Future Directions

The paper's results have substantial implications for the theoretical understanding of the early universe, particularly in testing the validity of inflationary models. The constraints placed on $r$ present strong evidence against a large set of inflationary scenarios, particularly those predicting a large $r$, thereby narrowing the parameter space and guiding theoretical models towards regimes with lower energy scales for inflation.

Future developments, such as further analysis with additional receiver-years in the 95 and 220 GHz channels, promise yet more stringent constraints. The paper emphasizes ongoing improvements in handling foreground complexities, such as dust decorrelation and synchrotron emission, which are critical for isolating the primordial signals with greater precision. The theory and observation synergy here sets a clear roadmap for the next generation CMB experiments, including the planned CMB-S4 project aiming for even lower detection limits on $r$.

In conclusion, the work represents a critical advancement in CMB studies, achieving tight constraints on primordial gravitational wave backgrounds and sharpening cosmologists’ tools for exploring the inflationary epoch. Enhanced observational strategies and refined models will undoubtedly play a crucial role in the quest to uncover the early universe's secrets.