Alexander Vilenkin in Inference:
We live in the aftermath of a great explosion—the big bang—that occurred 13.7 billion years ago. At the time of the big bang, the universe was filled with a fireball, a dense mixture of energetic particles and radiation. For nearly a century, physicists have been studying how the fireball expanded and cooled, how particles combined to form atoms, and how galaxies and stars were gradually pulled together by gravity. This story is now understood in great quantitative detail and is supported by abundant observational data.
The question, however, remains whether the big bang was truly the beginning of the universe. A beginning in what? Caused by what? And determined by what, or whom? These questions have prompted physicists to make every attempt to avoid a cosmic beginning.
In this essay, I review where we now stand.
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