Abstract
Recent developments of transition-edge sensors (TESs), based on extensive experience in ground-based experiments, have been making the sensor techniques mature enough for their application on future satellite cosmic microwave background (CMB) polarization experiments. LiteBIRD is in the most advanced phase among such future satellites, targeting its launch in Japanese Fiscal Year 2027 (2027FY) with JAXA’s H3 rocket. It will accommodate more than 4000 TESs in focal planes of reflective low-frequency and refractive medium-and-high-frequency telescopes in order to detect a signature imprinted on the CMB by the primordial gravitational waves predicted in cosmic inflation. The total wide frequency coverage between 34 and 448 GHz enables us to extract such weak spiral polarization patterns through the precise subtraction of our Galaxy’s foreground emission by using spectral differences among CMB and foreground signals. Telescopes are cooled down to 5 K for suppressing thermal noise and contain polarization modulators with transmissive half-wave plates at individual apertures for separating sky polarization signals from artificial polarization and for mitigating from instrumental 1/f noise. Passive cooling by using V-grooves supports active cooling with mechanical coolers as well as adiabatic demagnetization refrigerators. Sky observations from the second Sun–Earth Lagrangian point, L2, are planned for 3 years. An international collaboration between Japan, the USA, Canada, and Europe is sharing various roles. In May 2019, the Institute of Space and Astronautical Science, JAXA, selected LiteBIRD as the strategic large mission No. 2.
Highlights
It has been suggested since the 1980s [1,2,3] that inflation occurred in the very early, high energy Universe, to resolve remaining issues of the Big Bang theory, such as uniformity, flatness, and monopole problems
The sizes of expected spiral patterns in the sky are characterized by Hubble lengths at the electron scattering eras of the Cosmic Microwave Background (CMB), since primordial gravitational waves that enter the horizon in these eras most effectively produce tensor anisotropies
LiteBIRD is focused on this point: targeting [6] both the recombination era with the multipole moment l between 11 and 200 and the reionization era with l between 2 and 10, optimizing the angular resolution
Summary
It has been suggested since the 1980s [1,2,3] that inflation occurred in the very early, high energy Universe, to resolve remaining issues of the Big Bang theory, such as uniformity, flatness, and monopole problems. Advantages of measurements from space are being free from atmospheric effects, providing high sensitivity, stability with less systematic uncertainties [e.g., 7], and no restrictions on observing band selection. The Sun–Earth L2 point has been selected, since the Sun, the Earth, and the Moon are all located in almost the same direction, which makes it easier to avoid facing them in terms of optical and thermal aspects. On cosmic ray effects [8] because the satellite is more directly exposed to them. A scanning strategy with a combination of boresight spin angle of 50° around the satellite axis and its precession-like rotation around the anti-Sun direction of 45° is used (Fig. 1), since this combination provides a fairly uniform sky coverage and the minimization of instrumental systematic uncertainties in polarization measurements
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