LiteBIRD Mission Overview after Mission Reformation

Technical Summary: LiteBIRD Mission Overview after Mission Reformation

Problem Statement
The primary scientific challenge addressed by the LiteBIRD mission is the detection of primordial gravitational waves through the measurement of B-mode polarization in the Cosmic Microwave Background (CMB). While scalar perturbations have been precisely measured via CMB temperature anisotropies and E-mode polarization, tensor perturbations remain undetected. Ground-based experiments (e.g., BICEP/Keck, SPT, ACT) have improved sensitivity but are limited by atmospheric fluctuations and restricted sky coverage, particularly at the largest angular scales where the primordial B-mode signal is expected to peak. Furthermore, separating the faint primordial signal from Galactic foregrounds (synchrotron and thermal dust) requires broad frequency coverage and stable, all-sky observations that are difficult to achieve from the ground.

Methodology and Mission Architecture
Following a Mission Definition Review (MDR) in 2024 and subsequent reformation activities, the LiteBIRD collaboration has updated the mission concept to simplify the payload while preserving core scientific objectives. The methodology involves a space-based observatory operating from a Lissajous orbit around the Sun–Earth L2 point.

• Payload Configuration: The mission has transitioned from a previous three-telescope concept to a single-telescope architecture. The instrument is a cross-Dragone reflector with a 500-mm aperture diameter, cooled to approximately 5 K.
• Frequency Coverage: The single telescope covers 12 distinct frequency bands with centers spanning 40 to 402 GHz (optical coverage of 34–448 GHz). This broad coverage is designed to separate CMB polarization from synchrotron, thermal dust, and anomalous microwave emission.
• Detectors and Cryogenics: The focal plane utilizes approximately 4,000 transition-edge-sensor (TES) bolometer arrays operated at 0.1 K. These are dichroic pixels sensing two orthogonal polarizations, coupled via feedhorns and orthomode transducers (OMTs). The cryogenic chain employs passive V-groove radiators, two-stage Stirling coolers, Joule-Thomson (JT) coolers, and a sub-Kelvin continuous adiabatic demagnetization refrigerator (ADR).
• Modulation: The baseline design includes a cryogenic polarization modulation unit (PMU) featuring a continuously rotating metamaterial-based transmissive half-wave plate (HWP) to modulate polarization signals above the 1/f noise knee. A non-PMU configuration remains as a backup.
• Observation Strategy: The spacecraft will conduct a nominal 3-year all-sky survey. The scan strategy utilizes a spin rate of 0.05 rpm (with PMU) and a precession angle of 37.5°, optimized for thermal stability and cross-linking to control systematic errors.

Key Contributions and Results
The paper outlines the reformed mission parameters and requirements established as of the June 2026 MDR2 completion:

• Scientific Targets: The primary objective is to constrain the tensor-to-scalar ratio ($r$) with a total uncertainty of $\delta r < 0.002$ (68% C.L.), assuming $r=0$. This includes contributions from statistical noise, foreground residuals, instrumental systematics, and contingency margins. The statistical uncertainty target is $\sigma_r < 0.001$.
• Noise Requirements: The mission defines combined map-noise levels (in $\mu K_{CMB} \cdot \text{arcmin}$) across three frequency ranges (Low: 40–100 GHz, Mid: 100–200 GHz, High: 200–402 GHz) and two multipole ranges (Reionization: $2 \le \ell \le 29$; Recombination: $30 \le \ell \le 200$). For example, the required noise for the reionization range in the low-frequency band is $< 8.0$.
• System Specifications: The mission is scheduled for launch in the Japanese fiscal year 2036 via the JAXA H3 rocket. The satellite will spin at 0.05 rpm with a precession period of 1 to 10 hours. The angular resolution ranges from 53 arcmin (at 40 GHz) to 6 arcmin (at 402 GHz).
• Ground Segment: The mission includes an Operational Ground Segment (OGS) for spacecraft operations and a Science Ground Segment (SGS) responsible for the end-to-end processing pipeline, including map-making, component separation, and systematic error characterization.

Significance and Claims
The paper asserts that the reformed LiteBIRD mission represents a simplified yet robust approach to probing primordial inflation. By achieving the specified sensitivity ($\delta r < 0.002$), LiteBIRD aims to test representative inflationary models, including Starobinsky-like and Higgs-inflation scenarios, at meaningful statistical significance.

Beyond the primary goal of detecting primordial B-modes, the mission claims to provide a "legacy all-sky, multi-frequency polarization data set." This dataset is intended to support a broad range of scientific investigations, including:

• Measurements of the optical depth to reionization.
• Constraints on the sum of neutrino masses.
• Probes of cosmic birefringence, CMB lensing, and the Sunyaev–Zeldovich effect.
• Studies of primordial magnetic fields, non-Gaussianities, and Galactic science (dust and magnetic field orientations).

The paper emphasizes that LiteBIRD is designed to complement ground-based experiments; while ground surveys offer high angular resolution on small scales, LiteBIRD's space-based platform provides the necessary stability and full-sky coverage for large angular scales and foreground characterization. The collaboration intends to release public data and produce scientific results in conjunction with other surveys (e.g., Euclid, Rubin-LSST, SKA) to probe structure formation and dark energy. The reformation activity is presented as a successful consolidation of technical responsibility and mission architecture, ensuring the feasibility of the science goals within the constraints of the H3 launch vehicle and international partnership capabilities.