That universe was now nearly twenty years old. While observers were trying to measure the two numbers in cosmology—the universe’s current rate of expansion, and how much the expansion was slowing down—theorists were trying to figure out how the expansion itself worked. Like Jim Peebles in his instant classic Physical Cosmology, they wanted to make explicit the connection between the physics of the early universe and the universe we see today. That connection had been implicit from the start, in Lemaître’s invocation of a primeval atom.
