Quantum many-body problems are ubiquitous in modern physics across a wide range of research fields. One of the most important challenges is the elucidation of dense matter properties relevant to neutron star physics. While numerous attempts have been made to address this problem within nuclear many-body theories, we pursue an alternative approach by exploiting an analogy with ultracold atomic systems. In this talk, we discuss a microscopic mechanism of the hadron-quark crossover that occurs in dense matter, drawing an analogy with the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover established in ultracold atomic experiments. Moreover, we present our recent theoretical progress on a tunable three-body force in ultracold atoms, in analogy with the Fujita Miyazawa three-nucleon force.

https://www-conf.kek.jp/kincha/

Recently, there has been growing attention to applying quantum field-theoretic techniques, such as scattering amplitudes, to problems of classical gravity. It has led to the highest precision calculations of gravitational two-body systems, refined connections between classical and quantum physics, and a novel perspective on black holes. In this talk, we push forward this program to understand the physics of black holes, especially their horizons, at both classical and quantum levels. Central to this is the mass-changing amplitude, where a black hole transitions to a different mass state by absorption or emission. We discuss that this amplitude correctly describes mergers of two black holes, the horizon absorption of gravitational waves, and even the Hawking radiation, and serves as the building block for computing various observables of black holes.