Lecture 38 - The BIG BANG
The three main evidences that the Big Bang happened are a) the expansion of the universe, b) the Cosmic Microwave Background, and c) the Big Bang Nucleosynthesis.
Yet, "...the standard big bang theory says nothing about what banged, why it banged, or what happened before it banged. The inflationary universe is a theory of the "bang" of the big bang" (Alan Guth 1997).
Around 1970, the following embarrassing questions were staring at us. (1) Why is our universe expanding? (2) How can our universe be so homogeneous and isotropic? (3) Why is the present density of the universe so close to the ‘critical density’ predicted by General Relativity – the density which would imply that the ‘Geometry of the universe must be Euclidean? In 1981, Alan Guth made a great discovery that might have answered all these questions in one go. In this lecture, I shall first elaborate on the three questions mentioned above. I shall then outline Guth’s theory of the ‘exponential expansion’ of the universe, and how it ‘solves’ the three mysteries mentioned above.