Although the Big Bang theory has been well tested based on many observations, there are critical problems that remain unanswered. The leading hypothesis to resolve these problems is cosmological inflation, or the theory that our universe went through a period of accelerated expansion at a very early stage before ordinary (i.e. decelerating) expansion started. The inflation theory predicts that primordial gravitational waves were created during the inflationary era. Measuring the polarization of cosmic microwave background (CMB) radiation is thought to be the best probe to detect the primordial gravitational waves. Since the expected signal is very faint, state-of-the-art low-temperature detector arrays with superconducting sensors are indispensible for future measurements. In this article, first, we describe the scientific goals of the CMB polarization measurements. This is followed by an explanation of the current status and future plans in Japan, including a discussion of experimental requirements for very-low-temperature detectors and cryogenics.
As discussed in the last lecture, since Ohm's law does not hold for superconductors, another principle is needed to describe the electric current flow in superconductors. This principle is the London equation in the Meissner-Ochsenfeld state, and the force-balance equation used in the critical state model in the mixed state. These equations are derived from the first principle; that is, the variation principle for corresponding free energy. In particular, the force-balance equation derived from this principle for an isolated flux line system is generalized to a non-isolated flux line system by introducing virtual displacement. In addition, the pinning force derived from the pinning energy is of a reversible nature, while the practical pinning force is irreversible. This apparent discrepancy can be resolved using the statistical summation theory that shows continuous variation of pinning force from a reversible state to an irreversible state according to the displacement of flux lines. In fact, reversible electromagnetic phenomena are observed within a small range of displacement of flux lines inside pinning potential. These theoretical treatments provide a theoretical foundation for the critical state model.