Inflationary paradigms, observable gravitational waves and dark matter in grand unified theories
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Observations of the cosmic microwave background (CMB) have confirmed key predictions of inflation, including a nearly scale-invariant spectrum of primordial perturbations that are predominantly Gaussian and adiabatic. The same mechanism that drives cosmic structure formation may also generate a stochastic background of primordial gravitational waves (PGWs), which imprint a distinctive signature on CMB polarization. A confirmed detection of this tensor signal would yield unprecedented insights into high-energy physics and the dynamics of the inflationary epoch. This work investigates realistic inflationary models within the framework of grand unified theories (GUTs), emphasizing their implications for observable cosmology, gravitational waves, and dark matter. These models naturally resolve the primordial magnetic monopole problem while yielding a scalar spectral index n_s in excellent agreement with recent observations and predicting an observable tensor-to-scalar ratio r—characterizing the amplitude of primordial gravitational waves—potentially within reach of upcoming CMB experiments such as LiteBIRD, CMB-S4, and the Simons Observatory. In addition, these models incorporate a realistic scenario of reheating and non-thermal leptogenesis, offering a compelling explanation for the observed baryon asymmetry of the universe. It is further shown that symmetry breaking in certain GUT scenarios gives rise to metastable cosmic strings, which generate a stochastic gravitational wave background (SGWB) that may explain the recent signal observed by NANOGrav and other pulsar timing array (PTA) experiments. Various possibilities are explored in realistic GUT frameworks where such a metastable cosmic string network naturally emerges at the end of inflation, providing a viable origin for the SGWB. This background also lies within the sensitivity range of future gravitational wave observatories, including SKA, LISA, and DECIGO. In addition, this work explores mechanisms that enhance the primordial curvature power spectrum at small scales, leading to the formation of primordial black holes (PBHs) and scalar-induced secondary gravitational waves (SIGWs). The resulting PBHs may account for all or part of the dark matter abundance, while their associated SIGWs offer an independent probe for gravitational wave observatories such as LISA, SKA, DECIGO, and BBO. Finally, several co-annihilation scenarios are studied where the lightest neutralino serves as a viable dark matter candidate through sbottom-neutralino, stop-neutralino, and gluino-neutralino co-annihilation, consistent with bottom-tau Yukawa unification. These solutions satisfy collider constraints, relic density bounds, and direct and indirect detection limits, with parameter spaces potentially testable at LHC Run-3 and future collider experiments.
Description
Keywords
Stochastic gravitational wave background, Pulsar timing array, Primordial black holes