Image from Physics of the UniverseQuantum mechanics tells us that the vacuum of space is not empty; instead, it crackles with energy. It also tells us that, sooner or later, any given universe will decay spontaneously into another one with lower energy. Indeed, most cosmologists envisage our big bang as precisely such an event, during which the vacuum we live in emerged from a higher-energy vacuum that constituted a universe before ours. What matters here, though, is that there are a plethora of possible universes that can be produced in this way - each one with its own probability. By adding up these probabilities, Bousso was able to work out the various probabilities of the observer ending up in a universe with a particular set of characteristics.
A brief chronological listing of some of the most important discoveries in cosmology, astronomy and physics, from ancient Babylon, India and Greece, right up to the 20th Century. http://www.physicsoftheuniverse.com/dates.html