LiteBIRD: an all-sky cosmic microwave background probe of inflation

The Litebird mission will map polarized fluctuations in the cosmic microwave background (CMB) to search for the signature of gravitational waves from inflation, potentially opening a window on the Universe a fraction of a second after the Big Bang. CMB measurements from space give access to the largest angular scales and the full frequency range to constrain Galactic foregrounds, and Litebird has been designed to take best advantage of the unique window of space. Litebird will have a powerful ability to separate Galactic foreground emission from the CMB due to its 15 frequency bands spaced between 40 and 402 GHz and sensitive 100-mK bolometers. Litebird will provide stringent control of systematic errors due to the benign thermal environment at the second Lagrange point, L2, 20-K rapidly rotating half-wave plates on each telescope, and the ability to crosscheck its results by measuring both the reionization and recombination peaks in the B-mode power spectrum. Litebird would be the next step in the series of CMB space missions, COBE, WMAP, and Planck, each of which has given landmark scientific discoveries. The 4,736 detectors are distributed between three 5-K cooled telescopes, called the Low-, Medium-, and High-frequency telescopes (LFT, MFT, and HFT), with 31 arc-min resolution at 140 GHz. Litebird will map 20 times deeper than Planck, with a total error of δr < 0.001, conservatively assuming equal contributions of statistical error, systematic error, and margin. Litebird will be designed to discover or disfavor the best motivated inflation models – singlefield models that naturally explain the observed value of the spectral index of primordial density perturbations, with a characteristic scale of the potential comparable to or larger than the Planck scale. Litebird will also measure the optical depth to reionization to cosmicvariance-limited error, enabling ground-based high-resolution CMB experiments to measure the sum of neutrino masses. The proposed mission will be a partnership. Japan Aerospace Exploration Agency (JAXA) will provide the launch, spacecraft, Joule-Thomson coolers, LFT and its wave-plate. Europe will build the MFT and HFT, their waveplates, and the 100-mK cooler. Canada will contribute the 300-K detector readout electronics. The U.S. will build the detector arrays, cold readout electronics, and the 1.8-K cooler likely through a NASA mission of opportunity cost capped at $75M. In May 2019, JAXA selected Litebird as a “strategic L-class” mission for launch in early 2028. The total mission cost is estimated to be approximately $500M, and therefore the U.S. contribution is highly leveraged. Finally, Litebird technologies have been tested or will be tested in the near future on ground-based experiments. Litebird’s ability to measure the entire sky at the largest angular scales with 15 frequency bands is complementary to that of ground-based experiments such as South Pole Observatory, Simons Observatory, and CMB-S4, which will focus on deep observations of low-foreground sky. Litebird can provide valuable foreground information for ground-based experiments and ground-based experiments can improve Litebird’s observations with high-resolution lensing data.