A Universe Without Weak Interactions

TL;DR

This work asks whether a universe entirely devoid of weak interactions can still be habitable. By constructing a Weakless Universe and carefully adjusting Standard Model and cosmological parameters, the authors show that big-bang nucleosynthesis, structure formation, star formation, and long-lived stellar burning are feasible, with chemistry largely preserved. Key mechanisms include an enhanced deuterium fraction enabling deuterium-driven stellar ignition and a neutral Lambda_s0 hyperon population acting as dark matter, together with non-weak channels for heavy-element synthesis up to a broad but finite limit. The study also argues that the cosmological constant problem remains an independent fine-tuning issue, with Planck-scale CC rendering macroscopic structure unlikely, underscoring that electroweak and CC fine-tunings are qualitatively different in the context of habitable universes. Overall, the Weakless Universe provides a concrete counterexample to anthropic explanations for a small electroweak scale, contingent on the details of ultraviolet completions such as string theory.

Abstract

A universe without weak interactions is constructed that undergoes big-bang nucleosynthesis, matter domination, structure formation, and star formation. The stars in this universe are able to burn for billions of years, synthesize elements up to iron, and undergo supernova explosions, dispersing heavy elements into the interstellar medium. These definitive claims are supported by a detailed analysis where this hypothetical "Weakless Universe" is matched to our Universe by simultaneously adjusting Standard Model and cosmological parameters. For instance, chemistry and nuclear physics are essentially unchanged. The apparent habitability of the Weakless Universe suggests that the anthropic principle does not determine the scale of electroweak breaking, or even require that it be smaller than the Planck scale, so long as technically natural parameters may be suitably adjusted. Whether the multi-parameter adjustment is realized or probable is dependent on the ultraviolet completion, such as the string landscape. Considering a similar analysis for the cosmological constant, however, we argue that no adjustments of other parameters are able to allow the cosmological constant to raise up even remotely close to the Planck scale while obtaining macroscopic structure. The fine-tuning problems associated with the electroweak breaking scale and the cosmological constant therefore appear to be qualitatively different from the perspective of obtaining a habitable universe.

Figures (5)

• Figure 1: Elemental abundance mass fractions as a function of the visible baryon-to-photon ratio assuming $n_p = n_n \gg n_{\Lambda_s^0}$.
• Figure 2: Same as Fig. \ref{['bbn1-fig']} except $n_p = 9 n_n \gg n_{\Lambda_s^0}$.
• Figure 3: Same as Fig. \ref{['bbn1-fig']} except $n_p = n_n/9 \gg n_{\Lambda_s^0}$.
• Figure 4: The expected isotopic stability table in the Weakless Universe. Only isotopes with $A \le 20$ and $Z \le 8$ are shown. Reliable knowledge of neutron-rich isotopes is not extensive for most low $Z$ elements; those not shown on the right-hand side are not necessarily unstable to neutron emission. The proton-rich isotopes not shown on the left-hand side are unstable to proton emission. The crossed-out elements are unstable to proton, neutron, or $\alpha$ emission.
• Figure 5: A schematic plot of the stellar luminosity (black) and nuclear burning rate (red) on a log scale as a function to the stellar core temperature. The phase of decreasing luminosity is the Hayashi phase, when convection is dominant. Stars will remain in a steady state when the luminosity and nuclear rate are equal. The lifetime of the star in steady state is the ratio of burnable energy to the steady state luminosity. In the Weakless Universe one can get long lives stars if the primordial deuterium abundance is higher than in our universe, increasing the amount of burnable fuel. The longest living stars, i.e. with low luminosity, are of order 0.01-0.02 $M_\odot$.