Hubble's law

Hubble'

s law states that "the speed of recession of a galaxy is proportional to its distance from us."

   Hence, If v is the speed with which a galaxy recedes and r is its distance from the earth,

  Then, 

                                        v∝r

                       or, v = H0r

where, H0 is called Hubble's constant. It's value is 2.3*10^-18 s^-1.

Significance of Hubble's law:

      It is used to predict the age of the universe.

    According to Hubble's law, matter at a distance r away from us is travelling with speed v=H0r.

The time t needed to travel a distance r is given by, t=r/v

    so, t=r/H0r

            =1/H0

            = 4.3 * 10^17 s

            = 1.4 *10^10 years

Hence we can conclude that the universe is 1.4 *10^10 years old.