In both stars and in the early universe, the production of deuterium is the first step on the way to producing heavier nuclei. If the strong force were slightly weaker, deuterium would not be stable, and many authors have noted that nuclesynthesis would be compromised so that helium production could not proceed through standard reaction chains. Motivated by the possibility that other regions of space-time could have different values for the fundamental constants, this paper considers stellar evolution in universes without stable deuterium and argues that such universes can remain habitable. Even in universes with no stellar nucleosynthesis, stars can form and will generate energy through gravitational contraction. We show that such stars can be sufficiently luminous and long-lived to support life. Stars with initial masses that exceed the Chandrasekhar mass cannot be supported by degeneracy pressure and explode at the end of their contraction phase. The resulting explosive nucleosynthesis can provide the universe with some heavy elements. We also explore the possibility that helium can be produced in stellar cores through a triple-nucleon reaction (roughly analogous to the triple-alpha reaction). Next we show that with even trace amounts of heavy elements --- produced through the triple-nucleon process or by explosive nucleosynthesis --- the CNO cycle can operate and allow stars to function. Finally, we consider Big Bang Nucleosynthesis without stable deuterium and find that only trace amounts of helium are produced, with even smaller abundances of other nuclei. With stars evolving through gravitational contraction, explosive nucleosynthesis, the triple-nucleon reaction, and the CNO cycle, universes with no stable deuterium are thus potentially habitable, contrary to many previous claims.
The habitability of the Universe through 13 billion years of cosmic time
天体生物学领域在模拟银河系尺度的宜居带方面取得了巨大的进展，这些宜居带为生命的形成和复杂的进化提供了稳定的环境。最近，这个想法已经扩展到宇宙学尺度，通过对局部宇宙的可居住性进行整体建模的研究（例如Dayal et al. 2015; Li & Zhang 2015）。然而，所有这些研究都只集中在估计II型超新星（SNII）或伽马射线暴（GRB）的潜在有害影响，忽略了Ia型超新星（SNIa）和活动星系核（AGN）的贡献。在这项研究中，我们遵循两种不同的方法，基于（i）有害辐射的幅度和（ii）有害辐射照射的总行星托管体积。我们同时跟踪整个宇宙的关键天体物理源（SNII，SNIa，AGN和GRB）的贡献，以确定其在138亿年宇宙时间内的可居住性。我们发现SNII在任何宇宙时期的总辐射收支和有害辐射照射量中占主导地位，紧随其后的是SNIa（贡献为SNII的一半），GRB和AGN占微不足道的部分（<1%）。其次，由于恒星的总质量（或行星的总数）随着时间的推移而缓慢增加，而总的有害辐射密度和受影响的体积在最初的30亿年后下降，我们发现宇宙的可居住性在宇宙时间内稳步增加。我们发现，根据确切的模型假设，与40亿年前生命首次出现在地球上时相比，今天的宇宙可居住性是2.5到20倍。我们发现，这种可居住性的增加将持续到最后的恒星在接下来的数千亿年内消亡。
