BICEP / Keck XIII: Improved Constraints on Primordial Gravitational Waves using Planck, WMAP, and BICEP/Keck Observations through the 2018 Observing Season (2110.00483v1)

Abstract: We present results from an analysis of all data taken by the BICEP2, Keck Array and BICEP3 CMB polarization experiments up to and including the 2018 observing season. We add additional Keck Array observations at 220 GHz and BICEP3 observations at 95 GHz to the previous 95/150/220 GHz data set. The $Q/U$ maps now reach depths of 2.8, 2.8 and 8.8 $\mu{\mathrm K}{cmb}$ arcmin at 95, 150 and 220 GHz respectively over an effective area of $\approx 600$ square degrees at 95 GHz and $\approx 400$ square degrees at 150 & 220 GHz. The 220 GHz maps now achieve a signal-to-noise on polarized dust emission exceeding 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 and 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 no longer requires a prior on the frequency spectral index of the dust emission taken from measurements on other regions of the sky. This model is an adequate description of the data at the current noise levels. The likelihood analysis yields the constraint $r{0.05}<0.036$ at 95% confidence. Running maximum likelihood search on simulations we obtain unbiased results and find that $\sigma(r)=0.009$. These are the strongest constraints to date on primordial gravitational waves.

• The paper presents an improved constraint on the tensor-to-scalar ratio (r0.05 < 0.036) using enhanced multi-frequency CMB observations.
• The paper employs auto- and cross-spectra analyses on BICEP/Keck 95, 150, and 220 GHz data combined with WMAP and Planck frequencies for robust foreground separation.
• The paper’s findings narrow viable inflationary models and pave the way for advanced delensing techniques in future CMB surveys.

Improved Constraints on Primordial Gravitational Waves through BICEP/Observations

The paper from the BICEP/Collaboration presents refined constraints on primordial gravitational waves derived from cosmic microwave background (CMB) polarization data collected through 2018. By integrating observations from BICEP/95, 150, and 220 GHz bands with data from WMAP and Planck frequency bands, the research aims to investigate tensor perturbations predicted by inflationary models and enhance our understanding of the early universe.

Methodology and Data Analysis

The paper utilizes data from three main observing frequencies: 95, 150, and 220 GHz, each covering an extensive effective sky area. Through the enhancements in data collected from 2016 to 2018, particularly the incorporation of significant new observations at 220 and 95 GHz, the analysis aims to tighten constraints related to the tensor-to-scalar ratio, a key parameter in searching for primordial gravitational waves.

Key methods include the utilization of auto- and cross-spectra analysis of CMB polarization maps. This approach helps differentiate among several contributing components, namely lensed CMB, gravitational waves, and Galactic dust. A multicomponent model for the likelihood evaluation, excluding previous priors on dust frequency spectral indices, is applied for improved precision. Unlike preceding models, this paper incorporates a seven-parameter foreground model, reinforced by sensitivity improvements and noise reduction achieved through cross-frequency collaboration.

Main Findings

This paper reports that the constraint on the tensor-to-scalar ratio stands at $r_{0.05} < 0.036$ with a 95% confidence level, presently the most stringent constraint on primordial gravitational waves. This limits significantly narrows down the inflationary model possibilities. The new data lends substantial improvement over prior analyses, offering a tight bound on the presence and magnitude of gravitational wave signals in the CMB.

The likelihood analysis also reveals a robust detection of lensed B-mode polarization without any statistically significant finding for a primordial gravitational wave component. The favored models align well with expected lensed B-modes while the polarization from galactic dust remains a predominant source of noise. The noise limitations due to lensing are highlighted, marking the point where sample variance begins to dominate.

Implications and Future Directions

The refined measurements contribute critically to inflationary cosmology, aiding in the exclusion of certain theoretical models that predict stronger gravitational wave signatures. As an enhanced method of isolating B-mode signals from foregrounds, the results forge a path for distinguishing subtle inflationary B-mode signatures amidst lensed CMB and galactic impingements.

These observations imply practicality for future experimental designs targeting inflationary B-mode signals. There is a recognized need for advanced noise reduction and delensing techniques to further refine observational data, notably through enhanced instrument capability, more precise beam characterization, and wider survey areas. Amplified by the forthcoming deployment of new instrumentation and cross-collaborations with other probing observatories, the resultant progress holds promise for a deeper empirical understanding of cosmic beginnings and the physics of inflation. The data presented in this work serves as an invaluable guide for ongoing and future CMB surveys aimed at detecting primordial gravitational waves.