The Universe Is Fine Tuned To Permit Humans To Exist Somewhere In It.The laws of physics are "fine tuned" to permit intelligent life to exist somewhere in the universe. We have no idea why this is, and I say that that is the greatest unsolved scientific problem.
The universe around us began 13.7 billion years ago; we call this beginning the Big Bang. We have no idea how or why. We do know that it began very small, very condensed, and very hot. We have a lot of reason to suspect that this initial state was very very simple. It rapidly expanded, and thanks to that expansion it cooled. As it cooled, the complexity of present-day physics soon emerged: the four interactions, the up and down quarks that would in due course form all nuclei, the electrons enabling nearly all properties of matter including all chemistry, and the neutrino and photon bath all around us. We have no idea why the Big Bang resulted in equal amounts of positive and negative charge, namely two up quarks per down quark, and as many electrons as down quarks. We are still struggling to understand why the Big Bang gave rise to a slight excess of matter over antimatter, which is why the baryonic universe about us is made of matter but not antimatter.
The expansion of the universe was rapid enough to prevent a recollapse due to gravity. It was not so rapid as to prevent galaxies and stars from forming. We have no idea why the dynamics of our universe happen to be just right in this way.
Large stars end as supernovas, which are the pressure cookers in which all elements other than hydrogen and helium are made. Astrophysicists call these elements "metals." Without supernovas, rocky planets, water, and biochemistry are impossible. This nucleosynthesis requires that the fundamental laws and constants of physics be just right.
Most regions of our galaxy have too many energetic photons, or are poor in metals. Hence it is likely that life, especially life more complicated than microbes, is rare in the universe. But the amazing thing is that intelligent macroscopic life is at all possible anywhere in our universe. We still have no idea how life on Earth began, and how it apparently began within a few hundred million years of the cessation of the asteroid bombardment that took place 3.8 billion years ago. Some viruses are "coded" by RNA. All other living things are coded by DNA strands. We have no idea of how and why nature chose the coding system it did.
I date the start of the intellectual adventure this post touches on, with Newton's work in the 1660s. Much of our awareness of the perplexities posed by our fine tuned universe we owe to Dickey and Wheeler at Princeton, and to Dennis Sciama's students in the UK. This awareness is barely 40 years old.
The gentlest path to what this post has touched on is the popular books of Martin Rees. More challenging is
Davies, Paul (2006) The Goldilocks Enigma. Penguin. http://en.wikipedia.org/wiki/Cosmic_Jackpot
Much harder yet, but also more rewarding, is
Barrow, John, and Tipler, F J (1986) The Anthropic Cosmological Principle. Oxford University Press.