Is The Universal Matter - Antimatter Asymmetry Fine Tuned?
  The asymmetry between matter and antimatter is key to the existence and
nature of our Universe. A measure of the matter - antimatter asymmetry of the
Universe is provided by the present value of the universal ratio of baryons
(baryons minus antibaryons) to photons (or the ratio of baryons to entropy).
The baryon asymmetry parameter is an important physical and cosmological
parameter. But how fine tuned is it? A "natural" value for this parameter is
zero, corresponding to equal amounts of matter and antimatter. Another, also
possibly natural, choice for this dimensionless parameter would be of order
unity, corresponding to nearly equal amounts (by number) of matter (and
essentially no antimatter) and photons in every comoving volume. However,
observations suggest that in the Universe we inhabit the value of this
parameter is nonzero, but smaller than this natural value by some nine to ten
orders of magnitude. In this contribution we review the evidence that our
Universe does not contain equal amounts of matter and antimatter. Any change in
the magnitude of the baryon asymmetry parameter necessarily leads to a universe
with physical characteristics different from those in our own. The degree of
fine tuning in the baryon asymmetry parameter is determined by the width of the
range over which it can be varied and still allow for the existence of life.
Our results suggest that the baryon asymmetry parameter can be varied over a
very wide range without impacting the prospects for life; this result is not
suggestive of fine tuning. [abridged]
