Weakless universe

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A weakless universe is a hypothetical universe that contains no weak interactions, but is otherwise very similar to our own universe.

In particular, a weakless universe is constructed to have

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

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
, and so on.

An alternative approach to explaining the hierarchy problem is to invoke 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 landscape”, and by Steven Weinberg's prediction of the cosmological constant by anthropic reasoning.[citation needed
]

The hypothetical universe without the weak interaction is meant to serve as a counter-example to the anthropic approach to the hierarchy problem. For this “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 ostensible habitability of the weakless universe implies that anthropic reasoning alone cannot explain the hierarchy problem, unless the available vacua in the landscape are severely restricted for some other reason.

Obstacles

Weakless stars

One of the biggest obstacles for a habitable weakless universe is the necessary existence of stars. Main sequence stars work through fusing two protons to

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 to photon ratio, which allows the BBN deuterium to be produced at a lower temperature where the Coulomb barrier protects deuterium from immediate fusion into 4
He
.

Oxygen abundance

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.[2]

Baryogenesis

Baryogenesis and leptogenesis within the Standard Model rely on the weak interaction: For matter not to be wiped off by anti-matter at the very early universe, the universe must either have to start with a different amount of each (i.e. initial non-zero baryon number), or admit Sakharov's conditions to baryogenesis. In the latter case, there are two options:

  • Baryon number conservation is broken
    exotic particles
    that are also created abundantly in the universe and interact in peculiar ways with the baryonic matter, or very weak, or both. If the particles interacting with baryons are not strongly (and/or electromagnetically) interacting themselves, the strong interaction (and/or electromagnetic interaction) has to be part of a larger, spontaneously broken, gauge symmetry.
  • Baryon number conservation is broken non-perturbatively, i.e. by quantum anomaly. At least one such mechanism is chiral anomaly, which requires the existence of the weak interaction, or at least something very similar to it: [3]
    • There has to be a chiral gauge interaction, where the fermions are in its fundamental representation.
    • In order not to be anomalous itself (as gauge interaction anomaly leads to inconsistency), the gauge group is highly restricted, with SU(2) symmetry being the only option among SU(N) groups.
    • Mass terms break chiral symmetry, so in order for baryon masses to be possible, the chiral gauge interaction has to be spontaneously broken, leading to a Higgs mechanism.
    • Since the electromagnetic and the strong gauge groups also need to be non-anomalous, this leads to additional constraints. For example, if the sum of electromagnetic charges of all quark types is positive (more generally, non-zero), then there have to be additional, negatively charged particles, coupled to the chiral gauge group, which will also be created during baryogenesis - namely, the leptons.

Harnik, Kribs, and Perez argue that the Standard Model does not explain the observed size of the baryon asymmetry either, and that their weakless universe model only focuses on the time where the asymmetry already exists.[1]

References

  1. ^
    S2CID 14340180
    .
  2. .
  3. ^ Peskin, M. (2018). An introduction to quantum field theory. CRC press.

External links