3. In 1929, Edwin Hubble found the rst observational evidence of the fact that our Universe is expanding. Specically, he found that galaxies are receding from us at a velocity given by: 'U =HDD (1) where v is the velocity, D is the distance to the galaxy from Earth, and H0 is Hubble's constant. However, if the Universe is expanding, it implies that at any time in the past it must have been smaller, and if we continue extrapolating backwards, we should reach a point when all galaxies were almost next to each other. We can consider this as the point when expansion began. So, the time that galaxies have taken to reach their present separation gives us an estimate of the age of the Universe. We can obtain this time easily from equation 1 if we assume that 1: remains constant over the history of the universe. Report your answer in units of Gyr (109year) (Ho = 70 km/s/Mpc. You might want to convert this funny looking unit to SI unit which is just 31.) Do you think we made a reasonable assumption? Explain your answer briey