Cosmic Microwave Background (CMB)

 

Cosmic Microwave Background (CMB) is the remnant of the radiations that followed until 38,000 years after the Big Bang. The same photons that were present as visible light in the childhood of our universe are now microwaves in their early adulthood. 

Similarly, another 50 billion years and these photons will become radiowaves and form the Cosmic Radiowave Background. How?

After the Big Bang, what followed majorly was the expansion of the universe. The same energy was spread in an increasingly larger area. And so, its concentration and the overall temperature of the cosmic particle soup decreased. As the temperature dropped below 3000 Kelvins, electrons started slowing down. Eventually, their speeds were not enough to overcome attraction by the passing protons, and these two building blocks came together to bring atoms into the universe, for the first time ever.

Similarly, the photons that once rendered the universe with only light and nothing else, started to cool down too. And as they cooled down, they lost their energies and shifted from the visible spectrum of light to the infrared region.

Building on this concept of expansion followed by energy decrease followed by spectrum shift, the CMB comes up. The universe has expanded by around 1000 times since the photons were liberated. As a result, the photons that we discussed have 1/1000^th of the energy they earlier used to have. This loss of energy made them shift from the infrared to the microwave region of light. 

And that is how, we now have the CMB, a map of everything that happened in the years that followed the Big Bang. The study of CMB is a look at the time of the creation of everything we know, the sizes, the temperatures, the energy distribution, even how strong gravity was at that time.