Constraining Inflation with the BICEP/Keck CMB Polarization Experiments

TL;DR

The BICEP/Keck experiments have progressively constrained the amplitude of primordial gravitational waves through CMB polarization measurements, aiming to improve sensitivity and reach a detection threshold of r around 0.003 by 2027.

Contribution

This paper summarizes the BICEP/Keck program's recent results and details ongoing improvements and techniques to enhance constraints on the tensor-to-scalar ratio r.

Findings

Set the most sensitive constraint on r to date: r_{0.05}<0.036 at 95% C.L.

Achieved a current measurement uncertainty of σ(r)=0.009.

Aiming to reach σ(r) ≲ 0.003 with future data and techniques.

Abstract

The BICEP/$\textit{Keck}$ (BK) series of cosmic microwave background (CMB) polarization experiments has, over the past decade and a half, produced a series of field-leading constraints on cosmic inflation via measurements of the "B-mode" polarization of the CMB. Primordial B modes are directly tied to the amplitude of primordial gravitational waves (PGW), their strength parameterized by the tensor-to-scalar ratio, $r$, and thus the energy scale of inflation. Having set the most sensitive constraints to-date on $r$, $\sigma(r)=0.009$ ($r_{0.05}<0.036, 95\%$ C.L.) using data through the 2018 observing season ("BK18"), the BICEP/$\textit{Keck}$ program has continued to improve its dataset in the years since. We give a brief overview of the BK program and the "BK18" result before discussing the program's ongoing efforts, including the deployment and performance of the $\textit{Keck Array}$'s successor instrument, BICEP Array, improvements to data processing and internal consistency testing, new techniques such as delensing, and how those will ultimately serve to allow BK reach $\sigma(r) \lesssim 0.003$ using data through the 2027 observing season.