Expert: Patrick Weiler - 12/23/2011

Question
Patrick,
I can appreciate the "Goldilocks" zones we exist in on many levels, from the planetary to the cosmic, but is it really true that the rate of expansion after the big bang had to be within one-quintillionth of what it actually was in order for us to have a universe, as was stated on the episode of "Universe" I just watched?
Thanks,
Paul

Answer
Hello Paul...and thanks for using AllExperts
I saw that same fascinating program too. I can't personally verify the information presented, but I've read science papers from many cosmologists with the same figures and results given.

It's not just fine-tuning of the cosmic expansion parameters that are critical either. The formation of stable  atomic nuclei depends on the ratio of the strong and electromagnetic forces - the protons in a nucleus repel each other, but the strong force overcomes this repulsion. A miniscule change in their relative strengths could allow the electromagnetic force to overcome the strong force, and atoms could not exist. If electrons were any more massive, then electrons and protons couldn't form neutrons, thus disrupting the formation of heavy elements. The strength of gravity is also important: if it were any stronger, stellar matter would bind more strongly and stars would use their nuclear fuel much faster, thus negating the possibility of the evolution of life. If gravity were any weaker, matter might not "clump together" to form larger structures, thereby preventing the formation of stars in the first place. The answer to the question of why the forces are the way they are may yet come with string theory.

We live in a very precisely fine-tuned universe.