One question has always fascinated the scientists ” When the universe began?” . Current Thinking believes that Universe began as a singularity in space and Time. The singularity is called as Big Bang. It is believed at this time everything was condensed into very small space. After that the Universe has been continuously expansion mode.
A Astronomer Edwin Hubble discovered in 1929 the rate of expansion of Universe i.e the rate at which the galaxies are receding from each other. That law is called Hubble Law. As per Law, if the two galaxies are at distance R,Their present relative velocity of separation is given by
Where H is the proportionality Factor called Hubble’s Parameter. The value of H is
H=2.32 X10-18 s-1
We can define a time tH which equals to R/v which corresponds to the time when two galaxies have reached a separation of R, Using Hubble Law
tH=R/v=1/H=1.36 X1010 Years
This Time is called Hubble Time And this is considered to be the fair estimate for the age of the Universe