Historically radio astronomy held a rather modest place. Around 40-50 years ago it was thought that only a few objects in space would emit radio waves strong enough to be detected by receivers on Earth, thereby making it not profitable to build radio telescopes at all.
This conclusion was incorrect not because astronomers made mistakes with computing the numbers, but because the majority of sources had not yet been discovered. Now we know that all galaxies emit radio waves at some level, and that certain types of galaxies are powerful sources, the so-called radio galaxies. The strong radio emission is thought to be produced as a result of the violent process of material in-falling into the center onto supermassive black holes.
Even apart from the fascinating radio galaxies, radio astronomy is interesting because this particular color of light has the longest wavelengths, from millimeters to kilometers in size. The great benefit of such long wavelengths is that other telescopes that operate at the same wavelengths can be used contemporaneously to give an effective telescope size equal to the distance between the two telescopes!
This trick of ‘phasing’ the light can only work for radio telescopes, and allows us to see fine details of distant objects such as the accretion disks of the most powerful supermassive black holes in the centers of radio galaxies.
At the same time, there is the cost that the collecting area of the two telescopes involved is equal only to the meager sum of the area of the two dish receivers. This drawback allows only the strongest of radio signals to be detected. How can we discover the vast majority of the galaxies which are too faint to detect with current technology?
There is a massive project underway to build the “Square Kilometer Array” (SKA). SKA uses the trick of phasing the wavelengths detected by multiple radio receivers all observing the same object at the same. It gets past the problem that radio emitters in space tend to be faint by assembling a great many receivers into its array.
The SKA has 16 telescopes all working collectively at present, and by 2030 will consist of thousands of radio antennas spread out over a large distance extending from South Africa to Australia. With it, we should be able to detect the atomic element hydrogen present at the time the first stars were made in the universe.