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The Cosmic Microwave Background (CMB) refers to the remnants of the first light of the early Universe following the Big Bang. Owing to this, it is believed to hold important clues regarding the origin of our Universe that could pave the way for a quantum theory of gravity and new physics beyond the standard model. It turns out that observing the anisotropies in the CMB polarization could be key to answering one of the biggest questions in cosmology: What mechanism triggered the primordial fluctuations that gave rise to the CMB anisotropies and eventually led to the formation of stars and galaxies?

One of the leading ideas in cosmic inflation theory is that quantum fluctuations, which arise during a period of exponentially fast expansion of the very early Universe, are the source of the primordial fluctuations we aim to measure today. These quantum fluctuations occurred in the fabric of spacetime itself, manifesting as primordial gravitational waves. These primordial gravitational waves, in turn, would have left a characteristic imprint in the polarization of the CMB radiation with a curl component, commonly known as the “B-mode” polarization.

In this paper, an international team of researchers provide an overview of the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection (LiteBIRD) selected by the Japan Aerospace Exploration Agency (JAXA) and to be launched in the late 2020s on its H3 rocket. This is a strategic large-class (L-class) space mission that was established in collaboration with more than 300 researchers across Japan, Europe, and North America.

LiteBIRD will be made to orbit the Sun-Earth Lagrangian point L2, where it will map the CMB polarization over the whole sky for a period of three years. To realize this, LiteBIRD will be equipped with three telescopes spanning 15 frequency bands (34–448 GHz) that will enable it to survey the sky with an unprecedented sensitivity, nearly 30 times more than that achieved in previous full-sky experiments. Since the measurements will be done from space, the equipment will be able to access all frequencies, unhindered by atmospheric emission and fluctuations.

The primary scientific goal of LiteBIRD is the detection of the primordial gravitational wave signature by measuring the B-mode polarization in the CMB power spectra over large angular scales (1°–90°) and multipole range (2–200). If detected, it will be definitive evidence of cosmic inflation. If not, it will at least rule out a large class of inflationary models, shedding light on the nature of physics at the very highest energies. In turn, such insights could provide significant constraints on possible theories of quantum gravity, narrowing down our search for the much-coveted theory of everything.