[Paper Review] The Latest Constraints on Inflationary B-modes from the BICEP/Keck Telescopes

Keck Collaboration, P. A. R. Ade+93 more|ORCA Online Research @Cardiff (Cardiff University)|Mar 30, 2022

Particle Accelerators and Free-Electron Lasers1 references18 citations

TL;DR

This paper presents the latest constraints on primordial gravitational waves from B-mode polarization in the cosmic microwave background using data from the BICEP/Keck telescopes. The analysis tightens the upper limit on the tensor-to-scalar ratio to r₀.₀₅ < 0.036 at 95% confidence, significantly improving over prior results and excluding key inflation models like natural and monomial inflation.

ABSTRACT

For the past decade, the BICEP/Keck collaboration has been operating a series of telescopes at the Amundsen-Scott South Pole Station measuring degree-scale $B$-mode polarization imprinted in the Cosmic Microwave Background (CMB) by primordial gravitational waves (PGWs). These telescopes are compact refracting polarimeters mapping about 2% of the sky, observing at a broad range of frequencies to account for the polarized foreground from Galactic synchrotron and thermal dust emission. Our latest publication "BK18" utilizes the data collected up to the 2018 observing season, in conjunction with the publicly available WMAP and Planck data, to constrain the tensor-to-scalar ratio $r$. It particularly includes (1) the 3-year BICEP3 data which is the current deepest CMB polarization map at the foreground-minimum 95 GHz; and (2) the Keck 220 GHz map with a higher signal-to-noise ratio on the dust foreground than the Planck 353 GHz map. We fit the auto- and cross-spectra of these maps to a multicomponent likelihood model ($Λ$CDM+dust+synchrotron+noise+$r$) and find it to be an adequate description of the data at the current noise level. The likelihood analysis yields $σ(r)=0.009$. The inference of $r$ from our baseline model is tightened to $r_{0.05}=0.014^{+0.010}_{-0.011}$ and $r_{0.05}&lt;0.036$ at 95% confidence, meaning that the BICEP/Keck $B$-mode data is the most powerful existing dataset for the constraint of PGWs. The up-coming BICEP Array telescope is projected to reach $σ(r) \lesssim 0.003$ using data up to 2027.

Motivation & Objective

• To improve constraints on primordial gravitational waves by analyzing B-mode polarization in the cosmic microwave background.
• To reduce uncertainty in the tensor-to-scalar ratio r by combining BICEP/Keck data with Planck and BAO data.
• To test the viability of inflationary models, particularly natural and monomial inflation, using updated observational limits.
• To assess the impact of foregrounds, especially dust polarization, on the interpretation of B-mode signals.
• To project future sensitivity improvements through the Bicep Array and delensing techniques.

Proposed method

• Conducted a joint likelihood analysis of BICEP/Keck 2015–2018 data with Planck temperature and polarization data, and BAO measurements.
• Used a baseline model in CosmoMC to marginalize over foreground parameters, including dust amplitude (A_d) and spectral index (β_d), with updated priors.
• Applied the Hamimeche-Lewis (HL) likelihood method to evaluate posterior distributions for cosmological parameters.
• Incorporated delensing using overlapping SPT-3G maps to reduce lensing B-mode contamination and improve sensitivity to primordial r.
• Projected future sensitivity using forecasts based on achieved detector performance and scaling laws for Bicep Array.
• Evaluated the impact of dust decorrelation on delensing performance, adjusting sensitivity forecasts accordingly.

Experimental results

Research questions

• RQ1 What is the current upper limit on the tensor-to-scalar ratio r at 95% confidence after including the latest BICEP/Keck 2018 data?
• RQ2 How do the updated constraints affect the viability of natural and monomial inflation models?
• RQ3 To what extent do B-mode measurements alone, rather than temperature or other data, drive improvements in r constraints?
• RQ4 What is the projected sensitivity of the Bicep Array to r, and how does delensing enhance this sensitivity?
• RQ5 How does dust decorrelation affect the achievable uncertainty in r when using delensing techniques?

Key findings

• The 95% upper limit on the tensor-to-scalar ratio r₀.₀₅ is reduced to 0.036, down from 0.07 in the BK15 analysis.
• The posterior constraint on r₀.₀₅ tightens to 0.014⁺⁰.⁰¹⁰₋₀.⁰¹¹, with a 95% confidence interval of r₀.₀₅ < 0.036.
• The dust amplitude A_d is constrained to 4.4⁺⁰.⁸₋₀.⁷ μK², reflecting improved foreground modeling with additional 220 GHz data.
• Natural inflation and monomial inflation models are now excluded at the 95% confidence level due to tighter r constraints.
• The improvement in r constraints is now driven entirely by B-mode measurements, as adding Planck temperature and other data only marginally improves the upper limit.
• Delensing using SPT-3G maps is projected to achieve σ(r) ≲ 0.003 by 2027, significantly enhancing sensitivity beyond current limits.