To unravel the history of the evolution of the universe, from its inception up until the present, it is important to find early objects in the universe, and study how and when they formed and evolved into the stars and galaxies which exist today. In particular, research on the first stars and black holes *1 is one of the most important topics in astrophysics and astromony. These early objects are thought to have shone brightly in ultraviolet light. In the present, that light is expected to be observed as near-infrared light, due to the cosmological redshift*2 associated with the expansion of the universe which began with the Big Bang. The most common method for the kind of research above is to individually observe distant objects in the near-infrared. Many researchers are working on this. However, early objects are too dark to be observed individually, even with huge telescopes.

To solve this problem, the team has been conducting research using a unique method: observing cosmic infrared background radiation *3, which is a combination of light from early objects and distant galaxies. As a result of conducting observations via Japanese and U.S. satellites (COBE, IRTS, AKARI) and the CIBER experiment, the team has clearly shown that the observed cosmic infrared background radiation is not bright enough to explain the observed brightness, even if the contribution from known sources, such as galaxies, is taken into account. (Figure 1) This means that there are still objects in the universe which are unknown. Theoretical interpretations regarding the identity of these unknown objects have been announced, such as the brightness being an afterglow from the first stars in the universe or primordial black holes, and this has become a hot topic among researchers.

    However, CIBER observation data suggests that most of the unknown objects are likely to be in nearby space, such as old stars hidden in galactic haloes, and that there is not much of a contribution from the early universe. For CIBER-2, using a telescope ten times more sensitive than the one used for CIBER, the team observed cosmic infrared background radiation and aimed to detect a small amount of the radiation component from the early universe.

     For the experiment’s new objective, in order to boost focusing power and resolution, the team developed a 30cm reflecting telescope three times the diameter of the CIBER telescope, as well as a 4-megapixel infrared camera with a field of view double that of the CIBER camera, which was installed at the back of the telescope (Figure 2). The equipment is cooled down to -200 degrees Celsius using liquid nitrogen to prevent noise from the infrared radiation emitted by the equipment itself. Every part of the telescope is made of aluminum alloy, in order to prevent it from losing focus due to thermal contraction caused by cooling. The shape of the telescope is designed to contract in a similar way.

For CIBER-2, six photometric filters were used to separate the wavelength ranges of 0.5 to 2 μm, and the equipment took pictures of four areas in space with no bright stars at each wavelength. In addition, the equipment had spectroscopic functionality in the same wavelength ranges, and the team measured the cosmic infrared background radiation in detail. The team achieved this functionality by introducing light from one telescope into three infrared cameras, separated by wavelength, and attaching a two-wavelength filter and a spectroscopic filter to each sensor. Development of the observation equipment was shared among the countries in the international group which collaborated on the experiment. The team on the Japanese side was in charge of the development of the core components: the telescope, the optics, and the cooling system. Kwansei Gakuin University graduate and undergraduate students, not just researchers, served as the main work force for the Japanese team. They went to the American hub universities (the California Institute of Technology and Rochester Institute of Technology), led by the representatives of NASA rocket experiments, as well as to the NASA test site, to conduct various experiments until the equipment was completed (Figures 3 and 4).

CIBER-2 covers a wider range of wavelengths than CIBER did, including visible light (Figure 5). With the addition of visible light observations conducted for the first time, we will be able to clarify how much early objects contributed to the cosmic infrared background radiation, based on the Lyman-break feature *4, which is unique to early objects. After the launch, when the rocket falls, the equipment will open a parachute and land on the ground. Just the name implies, White Sands is a vast desert. If the impact from the fall is relatively minor, the instrument will be mostly undamaged, and can be reused for the next experiment. The team proposed to NASA that the CIBER-2 experiment should be done a total of four times, and that implementation was authorized. Although nothing can be said about the scientific results of this launch without a full analysis of the data, it is necessary to improve the accuracy through repetition, as each experiment only lasts for a short time. The equipment recovered by parachute will be transported to the Rochester Institute of Technology for internal verification.

The Japanese team was not able to witness the launch due to the impact of COVID-19, but hopes to visit the site in the near future to conduct verification and operation tests, as many of the devices were manufactured in Japan. If there are no major problems with the observation equipment, the plan in the future is to move forward with conducting the experiment once a year. Analysis of the observation data gained from this experiment will be divided up to some extent among the international team, and conducted concurrently.

Although the project is not at a stage where anything can be said about the possibility of detecting early objects, the team has finally been able to stand at the starting line for their goal. The hope is that CIBER-2 will continue to produce amazing scientific results in the future. Furthermore, after the CIBER-2 experiment has concluded, the team will send probes out into interplanetary space and move forward with planning for the Interplanetary Space Telescope (IPST), in order to make it possible to conduct long-term observations in a better environment than a rocket experiment. As a first step, the team is considering a plan to install infrared observation equipment, such as that developed for CIBER-2, on the transformer spacecraft and solar sail spacecraft which are being developed at the Institute of Space and Astronautical Science of JAXA. In the future, precise observations of cosmic infrared background radiation could open new doors for exploration of the early universe.