The cosmic microwave background (CMB) is the remnant, in the form of electromagnetic radiation (light), from the Big Bang. This light permeates the universe in all directions, and it allows scientists to study accurately the evolution and the structure of the cosmos at its largest scale.
Because of the expansion of the universe, the photons forming the CMB have lost a lot of energy, and they belong to the microwave region (hence its name) of the electromagnetic spectrum. It is for this reason that we aren’t able to see this remnant of the Big Bang, even if powerful radio telescopes can capture this signal.
The primordial universe and the first light
During the first 380,000 years after the Big Bang, the universe was a very hot soup of particles and photons. During this period, the temperature was very high and the numerous collisions between particles and photons didn’t allow atoms to form. All these collisions also mean that the universe in that epoch was opaque, because photons couldn’t travel long distances.
After 380,000 years of the Big Bang, the universe became cold and big enough to allow stable atoms to form (mainly hydrogen and helium). This event is called recombination. With the absence of free, particles, photons were able to travel freely through the cosmos, which became transparent and full of this radiation produced in the moment of recombination.
This light (the cosmic microwave background) can be detected using radio telescopes situated both on Earth and in space. As recombination happened in all the universe, we see the CMB coming from all directions.
Study of the CMB: an image of the universe
The cosmic microwave background is very useful for scientists as it allows them to study the evolution of the universe and other aspects of the formation of stars and galaxies. The temperature of this cosmic background is incredibly uniform: the regions with the more pronounced temperature variations differ in one part of 100,000.
The images of this radiation that permeates space have also allowed scientists to study the structure of the cosmos at large scales. As can be observed in the above image, the hottest regions (represented by the red colour) correspond to regions of great density, where there are lots of galaxies. However, the coldest regions have a much lower density of matter.
The great uniformity of the CMB also helps us to understand a very important aspect of the primordial universe. Two opposite points in space shouldn’t have almost the same temperature, because in the moment of recombination they weren’t close one to another, and light wouldn’t have had time to take the information from one point to the other.
The most accepted explanation for this phenomenon is inflation. According to this theory, a fraction of a second after the Big Bang, quantum fluctuations made the universe expand at a gigantic rate. So, very distant points of the universe were very close at the beginning, which makes it possible for them to have the same temperature.