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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.
