Why the Universe is Just So

Abstract

Some properties of the world are fixed by physics derived from mathematical symmetries, while others are selected from an ensemble of possibilities. Several successes and failures of ``anthropic'' reasoning in this context are reviewed in the light of recent developments in astrobiology, cosmology and unification physics. Specific issues raised include our spacetime location (including the reason for the present age of the universe), the timescale of biological evolution, the tuning of global cosmological parameters, and the origin of the Large Numbers of astrophysics and the parameters of the Standard Model. Out of the twenty parameters of the Standard Model, the basic behavior and structures of the world (nucleons, nuclei, atoms, molecules, planets, stars, galaxies) depend mainly on five of them: $m_e,m_u,m_d,α,$ and $α_G$ (where $m_{proton}$ and $α_{QCD}$ are taken as defined quantities). Three of these appear to be independent in the context of Grand Unified Theories (that is, not fixed by any known symmetry) and at the same time have values within a very narrow window which provides for stable nucleons and nuclei and abundant carbon. The conjecture is made that the two light quark masses and one coupling constant are ultimately determined even in the ``Final Theory'' by a choice from a large or continuous ensemble, and the prediction is offered that the correct unification scheme will not allow calculation of $(m_d-m_u)/m_{proton}$ from first principles alone.