Space seems dark to our weak human eyes. Most of the night sky is the blackness between stars. But in this darkness lies an endless number of photons, travelling in all different directions. These photos form background radiation, in three wavelengths in particular. You’ve likely heard of the Cosmic Microwave Background (CMB), it there is also a Cosmic Optical Background (COB) and a Cosmic Infrared Background (CIB).
The COB is explained by the immense number of stars in the Universe. It’s a diffuse glow across the entire sky. The CMB is the leftover radiation from hot plasma that existed when the Universe was only 380,000 years old. But the infrared background remained a mystery. Until now.
A team of researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) have collected light from the faintest sources across the sky, and have used it to show that they account for the entirety of the CIB’s emission. However, there is still some mystery, as 40% of these sources have yet to be identified. The other 60% are faint galaxies, but the yet-unknown sources have no corresponding optical emission, making them difficult to identify.
The next step is zeroing in on the unidentified sources in an attempt to characterize them. Are they several types of objects? or are they all the same thing seen in different directions? We need more science to tell!