Noninflationary solution to the monopole problem

Magnetic monopoles are a long-standing prediction of Grand Unified Theories, yet their efficient production in early universe phase transitions would lead to a monopole abundance that far exceeds observational limits. The standard solution of the problem invokes inflation occurring after monopole production, diluting their density to undetectable levels and eliminating any possibility of present-day observation. Here, we propose an alternative solution based on the breaking, in the early universe prior to Big Bang Nucleosynthesis, of the Weyl conformal symmetry of the gauge kinetic sector of the Lagrangian. This mechanism enhances monopole annihilation, thereby reducing their abundance to acceptable levels without requiring inflation. This scenario also predicts a residual flux of GUT monopoles potentially within the sensitivity of current and upcoming cosmic ray detectors, making their discovery possible in the near future.

💡 Research Summary

The paper addresses the long‑standing “monopole problem” that arises in Grand Unified Theories (GUTs): topological magnetic monopoles produced during a high‑temperature phase transition would, according to the standard Kibble estimate, populate the universe with roughly one monopole per Hubble volume. For a GUT‑scale critical temperature (T_c ≳ 10¹¹ GeV) this would overclose the universe, in clear conflict with observations. The conventional solution is to invoke a period of inflation after monopole production, which dilutes their density to negligible levels. The authors propose an alternative that does not rely on inflation but instead modifies the gauge kinetic sector of the theory by breaking Weyl (conformal) symmetry through a time‑dependent prefactor I(t) in front of the usual F_{μν}F^{μν} term.

In the proposed I²FF framework the Lagrangian contains a term –(I²/4)F_{μν}F^{μν}, where I(t) > 0 is a function of cosmic time (or equivalently of the scale factor a). The authors assume a simple monotonic evolution: I = (a_con / a)^s for a ≤ a_con and I = 1 for a ≥ a_con, with s a positive integer and a_con chosen so that the transition occurs before Big‑Bang Nucleosynthesis (T_con > 1 MeV). When I ≫ 1 in the early universe the effective gauge coupling \tilde{e}=e/I is very small, the gauge boson mass M_V ∝ \tilde{e}v = e v / I is suppressed, and the Hubble rate H exceeds M_V. In this regime the gauge sector is effectively decoupled; the monopoles that form at the scalar symmetry‑breaking scale v behave as global monopoles (they carry only a topological charge associated with the scalar field, not magnetic charge). Global monopoles have a linearly divergent energy with radius, leading to a distance‑independent attractive force F ≈ 4πv² between a monopole–antimonopole pair. Numerical simulations (cited as Ref.