In Aug. 10, 2011, the headline "AKARI detected mysterious far-infrared radiation from space!" (*1) attracted attention on the top page of the ISAS web. Some may wondered if that was really serious research, but was a scientifically valuable achievement. This article is to explain the topic in a little more detail.
Infrared cosmic background radiation
Key point of this news is that an unexpected bright cosmic infrared background radiation was discovered while AKARI, an infrared astronomical satellite, was making space survey in far-infrared. The cosmic infrared background radiation is an extended diffuse light that comes from distant universe. Background radiation means that it exists in the background of the known celestial bodies.
The most famous cosmic background radiation is Cosmic Microwave Background (CMB), which is an afterglow of light emitted by the scorching universe just after the Big Bang. CMB is observed to be literally an electromagnetic wave of millimeter-wave to microwave (1 to 10 mm) on the ground to some extent. CMB dominates most of background radiation energy from the all wavelength electromagnetic waves spreading in the universe (Fig. 1). It is unbelievably lucky for researchers exploring the origin of the universe that the first light in the universe is most bright in this world.
The second brightest one is an infrared cosmic background, which is a little shorter in wavelength (1 to 100 micrometer (µm)) than CMB. As discussed later, this should be explored in detail in order to elucidate the evolution of the universe. However, brightness and origin of the infrared cosmic background are not so clear compared to the CMB. One of the reasons is that, since all warm celestial bodies in the foreground such as the solar system, our Galaxy, and galaxies outside our Galaxy emit strong infrared, it is very difficult to distinguish the infrared cosmic background from the foreground infrared emission. Besides, we cannot observe the infrared cosmic background on the ground, which limits observation opportunities to hinder our understanding of the background. Since the atmosphere absorbs all the infrared coming from the universe, there is no choice but to go to space to measure the brightness accurately. Then, our AKARI comes on.