We have several methods of calculating the distance of other galaxies from the earth (see Cepeid variables and type 1a supernova). From Hubble’s observations we know that the ratio of velocity to distance is constant and in this calculation we assume that the universe has been expanding since the big bang with all the constituents moving at constant velocities relative to one another. (That is the big assumption we are making we are assuming the velocity is constant and so the rate of expansion has been constant)

From the ratio of velocity to distance from Earth we measure Hubble's Constant.

Calculating Hubble's Constant

Using the value calculated we get from now  a large expanding universe with galaxies moving apart at high speed.

Age of the universe calculation

To the very distant past when , at the start of everything, the Big Bang when all matter was concentrated at a tiny point.

Other pages of notes and video on astronomy which may be useful are:

Units of distance notes and video    Measuring distance by parallax/triangulation notes and video     Life cycle of stars    Geostationary and polar satellites notes and video     Big Bang theory and evidence     Development of the Universe after the Big Bang      Real and apparent magnitude     Hubble's Law and measuring distance notes and video         Using Hertzsprung Russell diagrams notes and video     Cepheid variable stars      Type 1A supernova