Under an Iron Sky: On the Entropy at the Start of the Universe

TL;DR

This paper explores the low entropy initial state of the universe, emphasizing the role of nuclear processes and matter-antimatter asymmetry in shaping its evolution and energy distribution.

Contribution

It provides detailed astrophysical analysis of how early nucleosynthesis conditions influence the universe's entropy and composition, highlighting the significance of matter-antimatter asymmetry.

Findings

Universe's low entropy state linked to nuclear processes

Early nucleosynthesis leaves hydrogen for star formation

Matter-antimatter asymmetry influences entropy evolution

Abstract

Curiously, our Universe was born in a low entropy state, with abundant free energy to power stars and life. The form that this free energy takes is usually thought to be gravitational: the Universe is almost perfectly smooth, and so can produce sources of energy as matter collapses under gravity. It has recently been argued that a more important source of low-entropy energy is nuclear: the Universe expands too fast to remain in nuclear statistical equilibrium (NSE), effectively shutting off nucleosynthesis in the first few minutes, providing leftover hydrogen as fuel for stars. Here, we fill in the astrophysical details of this scenario, and seek the conditions under which a Universe will emerge from early nucleosynthesis as almost-purely iron. In so doing, we identify a hitherto-overlooked character in the story of the origin of the second law: matter-antimatter asymmetry.