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Grand unification Higgs-R2 Inflation: Complementarity between proton decay and CMB observables

Nilay Bostan1,†, Rafid H. Dejrah2,‡, and Anish Ghoshal3,§

  • †Contact author: nilay.bostan@marmara.edu.tr
  • ‡Contact author: rafid.dejrah@gmail.com
  • §Contact author: anish.ghoshal@fuw.edu.pl
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Phys. Rev. D 113, 055042 – Published 25 March, 2026

DOI: https://doi.org/10.1103/nx7w-xstd

Abstract

We propose a predictive SO(10) grand unified theory (GUT) framework for cosmic inflation within the Palatini R2 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 NI∼10–17 e-folds occurring after monopole formation can dilute magnetic monopoles to abundances YM∼10−35–10−27. The model predicts cosmic microwave background (CMB) observables in the range 0.955≲ns≲0.974, accommodating the mild tension between the Planck–BICEP (ns≃0.965) and Planck+ACT (ns≃0.971) measurements via the ϕ<M and ϕ>M branches, respectively, where M denotes the renormalization scale entering the Coleman–Weinberg potential. The predicted tensor-to-scalar ratio, r≲8×10−4, 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 MU, the inflationary observables (ns,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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Phys. Rev. D 113, 055042– Published 25 March, 2026
  • Received 17 November 2025
  • Accepted 11 February 2026
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