The vast empty space around us that consists of stars, solar system, galaxies etc is called universe. The Universe contains billions of galaxies, each containing millions or billions of stars. It is supposed that space time, matter and space were formed after the formation of universe. Before the birth of the universe, time, space and matter did not exist.
Big bang theory
The big bang theory explains the origin of the universe. According to this theory, the mass of the entire universe was concentrated in a single place. Due to its huge mass the gravity of the mass was so high that the size was very small like a proton. Due to some reason a huge explosion took place which is known as the big bang. By the release of energy during this process the masses spread in every possible direction from which stars, galaxies and planets were eventually formed. There are many facts to support this theory such as the red shift, Hubble’s law etc.
During the big bang different particles spread with different velocities in all directions. The velocity of the spreading particle is different. All the masses were at a single point but now they are far apart. This suggests that the mass which is far from us has been receding from us with greater velocity. This was suggested by Hubble. According to him the velocity any heavenly body ‘v’ is directly proportional to the distance ‘r’ from us is:
V ∝ r
Or, v = Hor,
Where Ho is known as Hubble’s constant and its value is 2.3×10-8 s-1.
Age of universe
According to Hubble’ law, every heavenly body is receding with the velocity, v = Hor where ‘r’ is the distance from us. So time taken by it to move distance r with velocity v is:
Or, T =r/Hor
Or, T =1/Ho
∴T = 14 billion years
So, earth was formed 14 billion years ago.