<P> Most scientists agree that the universe started in an expansion called the Big Bang . The model for this is called the "hot big bang model". When the universe starts getting bigger, the things inside of it also begin to get cooler . When the universe was first beginning, it was infinitely hot . The temperature of the universe cooled and the things inside the universe began to clump together . </P> <P> Hawking also discusses how the universe could have been . For example, if the universe formed and then collapsed quickly, there would not be enough time for life to form . Another example would be a universe that expanded too quickly . If a universe expanded too quickly, it would become almost empty . The idea of many universes is called the many - worlds interpretation . </P> <P> Inflationary models and the idea of a theory that unifies quantum mechanics and gravity also are discussed in this chapter . </P> <P> Each particle has many histories . This idea is known as Feynman's theory of sum over histories . A theory that unifies quantum mechanics and gravity should have Feynman's theory in it . To find the chance that a particle will pass through a point, the waves of each particle needs to be added up . These waves happen in imaginary time . Imaginary numbers, when multiplied by themselves, make a negative number . For example, 2i X 2i = - 4 . </P>

A brief history of time from the big bang to black holes by stephen hawking pdf