The effect of hypothetical diproton stability on the universe

Abstract

By calculation of the proton-proton capture cross-section, it is shown that the existence of a bound diproton state would not lead to significant production of diprotons during big bang nucleosynthesis, contrary to popular belief. In typical stellar interiors, the stability of diprotons would lead to a reaction pathway for converting protons to deuterons perhaps ∼ 10 10 times faster than the usual weak capture reaction. This would prevent stars of the familiar hot, dense type from occurring in the universe. However, if diproton stability is achieved by an increase in the low-energy strong coupling, g s , then stars with temperatures and densities sufficiently reduced so as to offset the faster reaction pathway to deuterium will appear to meet elementary stability criteria. The claim that there is a fine-tuned, anthropic upper bound to the strong force which ensures diproton instability therefore appears to be unfounded.

Life at the interface of particle physics and string theory

“…If the di-proton were stable, the deuteron production rate could be ten orders of magnitude larger than in our universe, with unknown consequences (Bradford, 2009). So the di-proton stability line -if it exists at all -marks the end of our region and the beginning of terra incognita.…”

“…There are no "anthropic" tunings to be amazed about. Perhaps, as argued by Bradford (2011), fine tuning is an inevitable consequence of complexity and hence any complexity-based life will observe a fine-tuned environment. But this just strengthens the argument that we live in a special place in the space of all quantum field theories, unless one drops the link between complexity and life.…”

“…Claims about the important impact of di-proton stability on BBN, in much of the literature on anthropic tuning, are probably exaggerated, as they incorrectly assume that the di-proton production cross-section would be comparable to that of deuterium (Bradford, 2009;MacDonald and Mullan, 2009). …”

The weak scale from BBN

“…estimates [28] suggest that photodissociation of diprotons is important until after diproton production freezes out, resulting in a negligible diproton abundance during BBN. In either case, the existence of the diproton bound state is unlikely to qualitatively affect our conclusions, since it will not deplete the mass fraction of helium formed during BBN.…”

The Inevitability of Fine Tuning in a Complex Universe

“…However, this argument is just wrong. The reason is that, even if the diproton were stable, the rate of its formation via p + p → 2 2 He is too slow for any significant number of diprotons to be formed during BBN, [12]. It is true that the nuclear physics of stars would subsequently be very different, but there is no obvious reason why biophilic stars would not be stable, [12].…”