Editing A Brief History of Time (section)

=== Chapter 8: The Origin and Fate of the Universe ===
[[File:Cosmic History 020622 b.jpg|thumb|right|400px|The Big Bang and the evolution of the universe]]

Hawking recalls a conference on cosmology at the Vatican, where he was given an audience with [[Pope John Paul II]]. The Pope said it was fine to study the early universe, but scientists should not study the Big Bang itself, as that was the moment of Creation and the work of God. Hawking writes: "I was glad then that he did not know the subject of the talk I had just given at the conference -- the possibility that space-time was finite but had no boundary, which means that it had no beginning, no moment of Creation. I had no desire to share the fate of Galileo, with whom I feel a strong sense of identity, partly because of the coincidence of having been born exactly 300 years after his death!"{{Sfnp| Hawking| 1996|  p=145}}

At the Big Bang, the universe had an extremely high temperature, which prevented the formation of complex structures like stars, or even very simple ones like atoms. [[George Gamow]] predicted that radiation from the Big Bang should still fill the present universe. This was the cosmic microwave background discovered by Penzias and Wilson. The Big Bang created hydrogen and helium, and heavier elements were [[stellar nucleosynthesis| forged in stars]].

The Big Bang model was supported by the redshift of galaxies, the cosmic microwave background and the relative abundance of hydrogen and helium. But mysteries remained: Why is the universe isotropic? Why is the cosmic microwave background so homogenous? Widely separated parts of the universe have the same temperature, but there would not have been time for these regions to have come into contact. [[Alan Guth]]'s model of [[cosmic Inflation]] provided an answer to this [[horizon problem]]. Inflation explains other characteristics of the universe that had previously greatly confused researchers. After inflation, the universe continued to expand at a slower pace. It became much colder, eventually allowing for the formation of such stars.

Hawking discusses how the universe might have appeared if it had expanded slower or faster than it actually has. If the universe expanded too slowly, it would collapse, and there would not be enough time for [[Abiogenesis|life to form]]. If the universe expanded too quickly, it would have become almost empty. He discusses the [[anthropic principle]], which states that the universe has laws of physics that allow for the evolution of life because, if it didn't, we wouldn't be here.
Hawking suggests the [[no boundary proposal]]: that the universe is finite but has no beginning in [[imaginary time]]. It might merely exist.