The Big Bang model.
The big bang model is a theory that describes evolution of the universe from the moment it appeared in extra high temperature and density which occured about 13.8 billion years ago. The big bang theory appeared in 1920s, it’s father is Russian mathematician Alexander Friedmann. Later in 1940s Russian-born American physicist G. Gamov introduced new, modern version of the type of our universe.
The Big Bang Model consists of two assumptions. First assumption will be the famous theory of relativity by Einstein which describes the gravitational interaction of all matter. Second is Cosmological principle which says that an observer’s view of the universe doesn’t depend on the direction in which this observer looks nor on his location. This second assumption relates only to the large-scale universe and it implies that the universe has no edge, which means that the big bang origin occurred not at some particular point in space but rather throughout space at the same time.
From the highly compressed primordial state our universe began to cool down and decrease in density. Few seconds after explosion universe was cooled enough for nuclei to begin to form. 1 million years later formation of atoms began. That time radiation just traveled in space, but since the universe is expanding ( Hubble observed red shift of the distant galaxies which meant there were moving away from us. Red or Blue shift is kind of Doppler effect. It was perfectly described in Hawking’s book ” Brief history of time”. When you observe a car moving toward you the sounds becomes louder and louder, and when the car is moving away it is quite. So the same is with the shifts: when something rapidly moving away from us we see red shift and when moving toward us we observe blue shift). So since the universe is expanding radiation waves become longer and longer which by now we still can observe only as microwaves. These waves are called Microwave Background Radiation or “three degree” ( actually 2.728k). This MBR was discovered accidentally by American scientists Penszias and Wilson.
Source: Science and Technology