We propose a predictive $SO(10)$ grand unified theory (GUT) framework for cosmic inflation within the Palatini ${\mathcal{R}}^2$ formulation of gravity. In this model, a GUT Higgs field simultaneously drives inflation and induces intermediate-scale symmetry breaking, thereby linking primordial cosmology, gauge unification, and formation of topological defects. A subsequent partial inflationary phase of $N_I\sim 10–17$ $e$-folds occurring after monopole formation can dilute magnetic monopoles to abundances $Y_M\sim {10}^{-35}–{10}^{-27}$. The model predicts cosmic microwave background (CMB) observables in the range $0.955\lesssim n_s\lesssim 0.974$, accommodating the mild tension between the Planck–BICEP ($n_s\simeq 0.965$) and Planck$+\mathrm{ACT}$ ($n_s\simeq 0.971$) measurements via the $\varphi <M$ and $\varphi >M$ branches, respectively, where $M$ denotes the renormalization scale entering the Coleman–Weinberg potential. The predicted tensor-to-scalar ratio, $r\lesssim 8\times {10}^{-4}$, is consistent with current observational bounds and may be accessible to forthcoming experiments, including the Simons Observatory and LiteBIRD. Furthermore, correlations among the unification scale $M_U$, the inflationary observables $(n_s,r)$, and proton-decay lifetimes reveal a strong complementarity between CMB measurements and proton-decay searches, with regions of parameter space testable by upcoming experiments, such as Hyper-Kamiokande and DUNE.

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• Cosmology
• Grand unified models
• Inflation
• Phenomenology