The Degree of Fine-Tuning in our Universe and Others

The fundamental constants of nature must fall within a range
of values in order for the universe to develop structure and support
life. This talk outlines the current constraints on these quantities
in order to assess the degree of tuning required for the universe to
be viable. In the realm of particle physics, the relevant parameters
are the strengths of the fundamental forces and the particle masses.
Additional astrophysical parameters include the cosmic energy density,
the cosmological constant, the abundances of baryons and dark matter,
and the amplitude of primordial density fluctuations. These quantities
are constrained by the need for the universe to live for a long time,
emerge from its early epochs with an acceptable chemical composition,
and successfully produce galaxies. On smaller scales, stars and
planets must be able to form and function. The stars must also have
sufficiently long lifetimes and hot surface temperatures.  We also
consider potential fine-tuning related to the triple alpha reaction
that produces carbon, the case of unstable deuterium, and the
possibility of stable diprotons. For all of these issues, the goal is
to delineate the range of parameter space for which universes can
remain habitable. In spite of its biophilic properties, our universe
is not optimized for the emergence of life, in that favorable
variations could result in more galaxies, more stars, and potentially
more habitable planets.