Noninflationary solution to the monopole problem

TL;DR

The paper addresses the monopole overabundance problem from early-universe phase transitions and proposes a non-inflationary solution based on breaking Weyl conformal symmetry in the gauge-kinetic sector with a time-dependent $I^2FF$ term before BBN. By treating monopoles first as global defects and then dressing them with magnetic charge as conformal invariance is restored, the model enhances monopole annihilation and suppresses their present-day density, allowing symmetry-breaking scales up to near the Planck mass. This mechanism yields a residual flux of GUT monopoles potentially detectable by current or upcoming experiments, and even allows monopoles to constitute a dark matter component in some parameter regions. The approach broadens viable cosmological histories for monopoles without invoking inflation and motivates targeted experimental searches for GUT-scale magnetic monopoles.

Abstract

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.

Figures (2)

• Figure 1: Timeline of the model considered in this work. We denote by $t_{\rm i}$ the onset of radiation domination, $t_{\rm global}$ the time of global symmetry breaking, $t_{\rm local}$ the time of local symmetry breaking, $t_{\rm con}$ the restoration of conformal symmetry in the gauge sector, and $t_{\rm BBN}$ the epoch of BBN. The red region evidences the time interval during which the monopoles act as global topological defects.
• Figure 2: Parameter space of the relic monopole energy density as a function of the vacuum expectation value $v$ and the temperature $T_{\rm con}$ at which conformal symmetry in the gauge kinetic sector is restored. Results are shown for two different values of the positive integer $s$ defined in Eq. \ref{['power-law']} ((a): $s=2$, (b): $s=4$). The gray region is excluded, as monopoles would overclose the Universe ($\rho_{\rm M} > \rho_{\rm crit}$). In the red region, monopoles are produced directly as local and their abundance is determined by Eq. \ref{['eq:rho_local']}. In the blue region, monopoles are initially produced as global and their abundance is given by Eq. \ref{['eq:rho_global']}. White dashed contours indicate the expected present-day monopole energy density in units of the critical density. The solid black line marks the parameter combinations for which the monopole density equals the critical density, while the solid black line marks those for which only the standard computation in Eq. \ref{['eq:rho_local']} equals the critical density. The solid purple line corresponds to $T_{\rm local} = v$. Here, before BBN, we assume $g_{*(s)} = 10$.