  Motivated by landscape models in string theory, cosmic nuclear evolution is
analyzed allowing the Standard Model Higgs expectation value w to take values
different from that in our world (w=1), while holding the Yukawa couplings
fixed. Thresholds are estimated, and astrophysical consequences are described,
for several sensitive dependences of nuclear behavior on w. The dependence of
the neutron-proton mass difference on w is estimated based on recent
calculations of strong isospin symmetry breaking, and is used to derive the
threshold of neutron-stable worlds, w ~ 0.6+/- 0.2. The effect of a stable
neutron on nuclear evolution in the Big Bang and stars is shown to lead to
radical differences from our world, such as a predominance of heavy r-process
and s-process nuclei and a lack of normal galaxies, stars and planets. Rough
estimates are reviewed of w thresholds for deuteron stability and the pp and
pep reactions dominant in many stars. A simple model of nuclear resonances is
used to estimate the w dependence of overall carbon and oxygen production
during normal stellar nucleosynthesis; carbon production is estimated to change
by a fraction ~15(1-w). Radical changes in astrophysical behavior seem to
require changes in w of more than a few percent, even for the most sensitive
phenomena.
