Stellar Helium Burning in Other Universes: A solution to the triple alpha fine-tuning problem

Abstract

Motivated by the possible existence of other universes, with different values for the fundamental constants, this paper considers stellar models in universes where ${}^8$Be is stable. Many previous authors have noted that stars in our universe would have difficulty producing carbon and other heavy elements in the absence of the well-known ${}^{12}$C resonance at 7.6 MeV. This resonance is necessary because ${}^8$Be is unstable in our universe, so that carbon must be produced via the triple alpha reaction to achieve the requisite abundance. Although a moderate change in the energy of the resonance (200 -- 300 keV) will indeed affect carbon production, an even smaller change in the binding energy of beryllium ($\sim 100$ keV) would allow ${}^8$Be to be stable. A stable isotope with $A=8$ would obviate the need for the triple alpha process in general, and the ${}^{12}$C resonance in particular, for carbon production. This paper explores the possibility that ${}^8$Be can be stable in other universes. Simple nuclear considerations indicate that bound states can be realized, with binding energy $\sim 0.1-1$ MeV, if the fundamental constants vary by a $\sim \mathrm{few}-10$ percent. In such cases, ${}^8$Be can be synthesized through helium burning, and ${}^{12}$C can be produced later through nuclear burning of beryllium. This paper focuses on stellar models that burn helium into beryllium; once the universe in question has a supply of stable beryllium, carbon production can take place during subsequent evolution in the same star or in later stellar generations. Using both a semi-analytic stellar structure model as well as a state-of-the-art stellar evolution code, we find that viable stellar configurations that produce beryllium exist over a wide range of parameter space. Finally, we demonstrate that carbon can be produced during later evolutionary stages.

arXiv comment48 pages, 9 figures, accepted to Astroparticle Physics

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Cited by in corpus (7)

• The Degree of Fine-Tuning in our Universe -- and Others
• Exploring the astrophysics of dark atoms
• On the Habitability of Universes without Stable Deuterium
• Sensitivity of Carbon and Oxygen Yields to the Triple-Alpha Resonance in Massive Stars
• Effects of Bound Diprotons and Enhanced Nuclear Reaction Rates on Stellar Evolution
• Big Bang Nucleosynthesis with Stable ${}^8$Be and the Primordial Lithium Problem
• Possible Solution to the Triple Alpha Fine-Tuning Problem: Spallation Reactions during Planet Formation