[Paper Review] Probing Cosmic Inflation with the LiteBIRD Cosmic Microwave Background Polarization Survey

LiteBIRD Collaboration, Erwan Allys+189 more|arXiv (Cornell University)|Feb 6, 2022

Dark Matter and Cosmic Phenomena36 citations

TL;DR

LiteBIRD proposes a space-based cosmic microwave background (CMB) polarization survey to probe primordial gravitational waves from cosmic inflation via B-mode detection. Using a suite of 15 frequency bands and continuous polarization modulation with achromatic half-wave plates, it aims to achieve a sensitivity of 2.2 µK-arcmin over 3 years, targeting a tensor-to-scalar ratio precision of δr < 0.001 for multipoles 2 ≤ ℓ ≤ 200.

ABSTRACT

LiteBIRD, the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. The Japan Aerospace Exploration Agency (JAXA) selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with an expected launch in the late 2020s using JAXA's H3 rocket. LiteBIRD is planned to orbit the Sun-Earth Lagrangian point L2, where it will map the cosmic microwave background (CMB) polarization over the entire sky for three years, with three telescopes in 15 frequency bands between 34 and 448 GHz, to achieve an unprecedented total sensitivity of 2.2$\mu$K-arcmin, with a typical angular resolution of 0.5$^\circ$ at 100 GHz. The primary scientific objective of LiteBIRD is to search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. We provide an overview of the LiteBIRD project, including scientific objectives, mission and system requirements, operation concept, spacecraft and payload module design, expected scientific outcomes, potential design extensions and synergies with other projects.

Motivation & Objective

• To measure the primordial B-mode polarization of the cosmic microwave background (CMB) with unprecedented sensitivity to detect a stochastic background of primordial gravitational waves.
• To test the inflationary paradigm by constraining the tensor-to-scalar ratio r, a key prediction of slow-roll inflation models.
• To achieve a sensitivity of δr < 0.001 for multipoles 2 ≤ ℓ ≤ 200, enabling distinction between competing inflationary models.
• To mitigate systematics through continuous polarization modulation using achromatic half-wave plates and low-noise superconducting detectors.
• To conduct a full-sky survey from the Sun-Earth L2 Lagrange point with a 3-year observation baseline.

Proposed method

• Employing a three-telescope system (LFT, MFT, HFT) with crossed-Dragone and refractive optics to cover 34–448 GHz across 15 frequency bands.
• Using continuously rotating polarization modulator units (PMUs) with achromatic half-wave plates (AHWPs) to modulate polarization at 46/39/61 rpm for LFT/MFT/HFT, respectively.
• Equipping each telescope with superconducting transition-edge sensor (TES) bolometer arrays cooled to 0.1 K via a three-stage cryogenic chain (ST/JT/ADR).
• Implementing digital frequency-domain multiplexing (DfMux) and low-noise SQUID readout electronics to process signals from 10,000+ detectors.
• Applying a Lissajous orbit at the Sun-Earth L2 point to enable stable, all-sky observations with 20-minute spin and 3.2-hour precession.
• Using a 19.1 Hz sampling rate and time-ordered data (TOD) processing with the TOAST software framework to model and remove systematics.

Experimental results

Research questions

• RQ1 Can LiteBIRD achieve the required sensitivity of 2.2 µK-arcmin in CMB polarization over 3 years to detect a primordial B-mode signal at r ≈ 0.001?
• RQ2 What is the maximum tensor-to-scalar ratio r that LiteBIRD can constrain with 95% confidence, given its frequency coverage and angular resolution?
• RQ3 How effectively can LiteBIRD suppress systematics such as instrumental polarization and beam asymmetries through continuous modulation and calibration?
• RQ4 What level of foreground contamination (e.g., synchrotron, dust, CIB) can be mitigated using multi-frequency observation and component separation techniques?
• RQ5 Can the mission achieve its science goal of δr < 0.001 for 2 ≤ ℓ ≤ 200 with a 3-year all-sky survey and a 2.6-ton spacecraft with 3.0 kW power?

Key findings

• LiteBIRD is designed to achieve a polarization sensitivity of 2.2 µK-arcmin after 3 years of observation, meeting the primary science requirement.
• The mission targets a tensor-to-scalar ratio sensitivity of δr < 0.001 for multipoles 2 ≤ ℓ ≤ 200, enabling discrimination between many inflationary models.
• The 15-frequency band system (34–448 GHz) allows for effective separation of CMB B-modes from astrophysical foregrounds such as dust and synchrotron emission.
• The use of continuously rotating achromatic half-wave plates (AHWPs) in the PMUs enables efficient modulation of polarization, reducing low-frequency noise and systematics.
• The 0.1 K cooling chain using a combination of Joule-Thomson and adiabatic demagnetization refrigerators (ADR) ensures stable, low-noise operation of the TES detectors.
• The mission is planned for launch in 2029 on the JAXA H3 rocket, with a 3-year all-sky survey from the L2 Lissajous orbit.