Weakless Universe

The Weakless Universe is a hypothetical universe that contains no weak interactions, but is otherwise very similar to our own universe. In particular, the Weakless Universe is constructed to have nuclear physics and chemistry identical to standard nuclear physics and chemistry. The dynamics of the Weakless Universe includes a period of big bang nuclear synthesis, star formation, stars that burn for billions of years, stellar nuclear synthesis of heavy elements and also supernovae that distribute the heavy elements into the interstellar medium.

Motivation and Anthropics

The strength of the weak interaction is an outstanding problem in modern particle physics. A theory should ideally explain why the weak interaction is 32 orders of magnitude stronger than gravity; this is known as the hierarchy problem.

There are various models that address the hierarchy problem in a dynamical and natural way, for example, supersymmetry, technicolor, warped extra dimensions, etc. An alternative approach to explaining the hierarchy problem is to invoke the anthropic principle. Within this approach one assumes that there are many other patches of the Universe (or Multiverse) in which physics is very different. In particular one can assume that the "landscape" of universes contains ones where the weak force has a different strength compared to our own. In such a scenario observers would presumably evolve wherever they can. If the observed strength of the weak force is then vital for the emergence of observers, this would explain why the weak force is indeed observed with this strength. It was indeed argued by Barr and others that if one only allows the electroweak symmetry breaking scale to vary between universes, keeping all other parameters fixed, atomic physics would change in ways that would not allow life as we know it.Anthropic arguments have recently been boosted by the realization that string theory has many solutions, or vacua, dubbed the string landscape, and by Weinberg's prediction of the cosmological constant by anthropic reasoningFact|date=January 2008.

The hypothetical weak interaction-less universe is meant to serve as a counter example to the anthropic approach to the hierarchy problem. In the weakless universe other parameters are varied as the electroweak breaking scale is changed. Indeed, string theory implies that the landscape is very big and diverse. The seeming habitability of the Weakless Universe implies that one cannot explain the hierarchy problem by anthropic reasoning alone, and that one must make strong assumptions about the available vacua in the landscape.

Weakless stars

Perhaps the biggest obstacle for a habitable Weakless Universe is the necessary existence of stars. Our sun works through burning two protons to deuterium, which proceeds through weak interactions. In the Weakless Universe of Harnik, et al. (see below) this is overcome by ensuring a high primordial deuterium/hydrogen ratio during Big Bang Nucleosynthesis (BBN). This permits long lived stars fueled by direct deuterium-proton burning to helium, which proceeds through strong interactions. The high initial deuterium/hydrogen ratio (~1/3 by mass) is arranged by simply reducing the overall baryon/photon ratio, which allows the BBN deuterium to be produced at a lower temperature, where the Coulomb barrier then protects it from immediate fusion into 4He.

Another potential problem for a Weakless Universe is that supernova explosions are necessarily neutrinoless. The resulting efficiency of production and dispersion of heavy elements (in particular, oxygen) into the interstellar medium for subsequent incorporation into habitable planets has been questioned by Clavelli and White (see below).

References

* [http://arxiv.org/abs/hep-ph/0604027 "A Universe without Weak interactions"] - Roni Harnik, Graham Kribs, Gilad Perez - Phys. Rev.D74:035006,2006.
* [http://arxiv.org/abs/hep-ph/0609050 "Problems in a weakless universe"] - L. Clavelli, R. E. White III