Hubble's Velocity-Distance relationship
v = H.destablished that the Universe is expanding. Edwin Hubble made a plot of recession velocity vs distance for galaxies that he had observed he found a straight line relationship. The slope of this line, known as the Hubble Parameter, H 
20 (km/s)/million l.y., has units of 1/time. It is easy to see that, if the Universe is expanding at the present time, then at some point in the past, all matter was once together. Thus, 1/H, called the Hubble Time is an estimate of the Age of the Universe, about 15 billion years.
If we rewind the motion picture representing the history of the Universe, we can understand a great deal about its early state, just after the Big Bang. In its early stages the Universe was simpler than it has ever been. It was very hot and in a state of Thermal Equilibrium, that is its temperature determined all its other properties. Just after the Big Bang, temperatures were so high that particle pairs could be created purely out of the heat energy present. For example, a pair of thermal photons - which would be gamma-rays at these temperatures - might react to form an electron/positron pair:
+ 
e+ + e- During its early phases the Universe was radiation dominated, that is the photons dominated the energy and pressure of the Universe. As the Universe expanded, it cooled, T 
1/R, where R is some measure of the "scale of the Universe".
The Big Bang Model
Let's review the observational evidence:
- Distance/velocity relationship: distant galaxies are moving away from us, with speeds which increase linearly with distance
- Chemistry: the universe is almost entirely hydrogen and helium, in a mixture of roughly 12 H atoms to 1 He atom
- Cosmic Microwave Background: no matter where we look in the universe, we see radio waves which look like those radiated by a blackbody at about 2.7 degrees above absolute zero. There are tiny (one part in 10,000) variations in the brightness of this radiation on scales of a degree or so
Is there any way to tie all these pieces of data together? Yes! One model which can explain them all is called the Big Bang model. The name was coined by a scientist who didn't like the theory and tried to make it sound silly.
