We review the Hubble tension, a discrepancy­ between the Hubble constant values, which quantify the expansion speed of the Universe at the present epoch, derived from distance ladder measurements in the local Universe and those from cosmic microwave background fluctuations. This Hubble tension, if confirmed, poses a serious challenge to the standard cosmology. We discuss how the Hubble tension could be solved. We also emphasize the importance of measurements of the Hubble constant independent of the distance ladder and cosmic microwave background fluctuations for clarifying the origin of the Hubble tension.

A question whether nuclei consisting of four neutrons can exist and how is one of the most important issues to be solved in nuclear physics. In order to resolve this fundamental question, two distinctive experiments were carried out by employing a high-intensity ⁸He secondary beam. The methods of an exothermic double charge exchange reaction on ⁴He nuclei and an α knockout reaction on ⁸He nuclei are introduced and a comparison of the results of the experiments is described. The obtained results exhibit event concentration or a narrow peak just above the four-neutron mass threshold with a broad bump component up to over 60 MeV, which corresponds to continuum states. The observation of the narrow peak provides compelling evidence for the formation of the strongly correlated four-neutron system.

Recently, conducting properties of edge/surface modes of some materials are classified by a phase mismatch of electron wavefunctions across a momentum space. The properties of wavefunctions in terms of continuity, which can induce the phase mismatch, are called topology. Topology has attracted interest since it enables classification of systems without local order parameters.

Recently, the concept of topology has been extended to magnetic quasiparticles, such as magnons and triplons. We measured a dispersion relation of triplon bands in the spin-1/2 dimerized antiferromagnet Ba₂CuSi₂O₆Cl₂ through inelastic neutron scattering experiments. In Ba₂CuSi₂O₆Cl₂ , the dimer forms a rectangular lattice with a small distortion, which yields the bond alternation in magnetic interactions. As a result, an energy gap is induced between the bonding the antibonding triplon modes. A spin model realized in Ba₂CuSi₂O₆Cl₂ is interpreted as a triplon variant of Su–Schrieffer–Heeger (SSH) model, indicating that topologically protected edge states are induced by a bipartite nature of the lattice.

Understanding of atomic-scale liquid dynamics is poorer than that of solid. We demonstrate the experimental determination of Van Hove correlation function―a pair correlation function in real-space and time―of water and molten inorganic salt by using inelastic X-ray and neutron scattering, respectively. This approach makes it possible to depict the distance-dependent dynamics of liquid on the picosecond time-scale. Further advances in fundamental understanding of atomic-scale liquid dynamics is expected.

Interfacial phenomena in crystallization are crucial for understanding the process of phase transformation of materials. Ice crystallization is the most familiar crystallization phenomenon. Microscopic details of interfacial phenomena at ice-air interface have been extensively investigated with the help of abundant and sophisticated experimental techniques. However, interfacial phenomena at ice-water interfaces remain unclear because of the lack of experimental method. Amid this situation, the author has found, by classical in-situ optical microscopy, that unknown waters, which are separated from surrounding water by an interface, macroscopically appear at non-equilibrium interfaces between water and ice grown or melted by (de)pressurization in an anvil cell. The observations revealed varieties in morphology, dynamics and physical properties of the unknown waters. The unknown waters were suggested to have similar local structures with the structures of ices rather than bulk water, implying that the unknown waters can be pre-ordered state of water as mother melt before ice crystallization. Moreover, low-density unknown water and high-density unknown water were suggested to exist, providing implications on liquid polymorphism in a single-component system and the mystery of unique properties of water as liquid.