arXiv subjects

Rafid H. Dejrah

Publications and source records attributed to Rafid H. Dejrah.

6 recordsLinked to original sources

arXiv2026

Gauge Backreaction in Standard Model Warm Inflation

Standard-Model warm inflation (SMWI) makes QCD the microscopic origin of inflationary dissipation, so the real-time response of the gauge plasma enters the cosmological prediction. We show that a slowly relaxing gauge-helicity response cannot, in general, be absorbed into an effective friction coefficient and must instead remain dynamical. We therefore evolve the quark axial densities and a matched hard-helicity mode, with QCD sphalerons and an effective $2\leftrightarrow3$ channel generating both charge transfer and correlated stochastic sources in the full perturbation covariance system. The resulting backreaction is substantial but mainly indirect: gauge-helicity screening can reduce the warm dissipation ratio by an order-one fraction on the strong branch, while the $2\leftrightarrow3$ channel contributes only $0.2\%-0.7\%$ of the zero-density sphaleron friction directly, shifting the amplitude-normalized trajectory in both $n_s$ and $r_{\rm vac}$. The relevant trajectories, however, lack a parametrically broad hard-soft hierarchy and extend beyond strict linear response. The predictions should therefore be understood within the stated local Gaussian transport closure; a parameter-free Standard-Model (SM) result requires finite-affinity real-time $SU(3)$ drift, susceptibility, and noise kernels.

astro-ph.CO↗

arXiv2026

Induced Multi-phase Inflation with Reheating: Leptogenesis and Dark Matter Production in Metric versus Palatini

We study non-minimally coupled scalar-induced multi-phase inflation in metric and Palatini gravity, considering linear, Brans-Dicke-like, and Higgs-like sectors. The scalar spectral index lies in the range $ n_s \simeq 0.93 \ \text{--} \ 0.98 $, consistent with \textit{Planck} and combined \textit{Planck}+ACT data. The tensor-to-scalar ratio can reach $ r \sim 0.03 $ in metric, whereas Palatini models generically predict $ r \lesssim 10^{-5} $. In the Palatini case, field excursions remain sub-Planckian, and the perturbative unitarity cutoff is raised. Reheating proceeds via perturbative inflaton decays into Higgs bosons and fermionic dark matter (DM) through the portal coupling $ λ_{12} $ and Yukawa coupling $ y_χ$. Radiative stability of the inflationary plateau constrains the couplings to $ y_χ, λ_{12} \sim 10^{-7} \ \text{--} \ 10^{-3} $, implying $ 4\,\mathrm{MeV} \lesssim T_{\rm rh} \lesssim 10^{15}\,\mathrm{GeV} $. Palatini realizations require smaller couplings and thus a narrower reheating window. Non-thermal DM production $χ$ from inflaton decays is viable for DM mass $ m_χ\sim \mathrm{keV} \ \text{--} \ \mathrm{PeV} $ with $ y_χ\lesssim 10^{-6} $ over large parameter regions. We estimate the inflaton-right-handed neutrino (RHN) Yukawa coupling $ y_N $ required for successful baryogenesis via non-thermal leptogenesis within a Type-I seesaw framework, for the lightest RHN mass $ M_{N_1} \sim 10^{9} \ \text{--} \ 10^{14}\,\mathrm{GeV} $, provided $ M_{N_1} > T_{\rm max} $, where $ T_{\rm max} $ follows from radiatively consistent reheating. In Palatini scenarios, the lower maximal temperature and tighter stability bounds further restrict the leptogenesis parameter space.

hep-ph↗

arXiv2026

WISPedia -- the WISPs Encyclopedia

The Weakly-Interacting Slim Particle encyclopedia (WISPedia) is a comprehensive reference work dedicated to the systematic compilation of theoretical models, Effective Field Theories, and frameworks involving Weakly Interacting Slim Particles (WISPs): a broad class of light, feebly coupled particles proposed in extensions of the Standard Model. In current times, where the number of models largely surpasses the number of new physics signals, this encyclopedia aims to provide a concise reference of their landscape. The goal is to provide a useful tool to the community to navigate among them. It does not aim to review all the models in detail, but to define their essential characteristics, and point the reader to useful and minimal material such as the original sources, review articles, tools and general compilations of bounds. Hence, the format of this reference resembles the direct style of a model encyclopedia of WISPs.

hep-ph↗

arXiv2026

Grand Unification Higgs-$\mathcal{R}^2$ Inflation: Complementarity between Proton Decay and CMB Observables

We propose a predictive $SO(10)$ Grand Unified Theory (GUT) framework for cosmic inflation in the Palatini $\mathcal{R}^2$ formulation of gravity. In this model, a GUT Higgs field both drives inflation and induces intermediate-scale symmetry breaking, thereby linking primordial cosmology, gauge unification, and topological defect formation. A partial inflationary phase of $N_I \sim 10$--$17$ $e$-folds following monopole formation can dilute magnetic monopoles to abundances $Y_M \sim 10^{-35}$--$10^{-27}$. The model yields Cosmic Microwave Background (CMB) predictions of $0.955 \lesssim n_s \lesssim 0.974$, accommodating the tensions between Planck-BICEP ($n_s \approx 0.965$) and Planck+ACT ($n_s \approx 0.971$) via $ϕ< M$ and $ϕ> M$ branches repectively. The predicted tensor-to-scalar ratio $r \lesssim 8\times10^{-4}$ lies within current observational constraints and is accessible to forthcoming experiments, including the Simons Observatory and LiteBIRD. The resulting correlations between the unification scale $M_U$, the inflationary observables $(n_s, r)$, and proton-decay lifetimes highlight a complementarity between CMB measurements and proton-decay searches, with regions of parameter space testable in forthcoming experiments such as Hyper-Kamiokande and DUNE.

astro-ph.CO↗

arXiv2025

$f(R)$ Gravity: Gravitational Waves Tests

This review explores modified theories of gravity, particularly $f(R)$ gravity, as extensions to General Relativity (GR) that offer alternatives to dark energy for explaining cosmic acceleration. These models generalize the Einstein-Hilbert action to include functions of the Ricci scalar, providing new insights into cosmology and astrophysics. The detection of gravitational waves (GWs) has enabled rigorous tests of $f(R)$ gravity, as deviations in GW propagation, speed, and polarization can signal modifications to GR. Constraints on $f(R)$ models arise from LIGO-Virgo observations of binary mergers, the stochastic gravitational wave background (SGWB), and complementary tests in cosmology and weak-field regimes. Future GW detectors, such as LISA and the Einstein Telescope, will enhance sensitivity to smaller deviations from GR, necessitating advancements in theoretical modeling. Among competing theories -- including scalar-tensor, massive gravity, and Horndeski models -- $f(R)$ gravity remains a pivotal framework for understanding fundamental gravitational physics and cosmology. This review highlights key developments, challenges, and future directions in the field.

gr-qc↗

arXiv2024

Minimally coupled $β$-exponential inflation with an $R^2$ term in the Palatini formulation

We focus on the inflationary predictions of $β$-exponential potential models, in which the inflaton is a representation of the field delineating the size of extra-dimension. Since it offers a well-motivated starting point for the study of physics at very high energies, we incorporate an $R^2$ term in the Palatini gravity. In addition, afterward the inflation, the inflaton oscillates about the minimum of the inflation potential, and reheats the universe. This occurs during the reheating phase, at which the inflaton decays into the standard model particles, which fill the universe. We extend our examination by considering the reheating effects on inflationary observables by employing the different scenarios of the reheat temperature. Supposing the standard thermal history after inflation, we display the inflationary predictions, $n_s, r, \mathrm{d}n_s/\mathrm{d}\ln k$ of $β$-exponential potential with minimal coupling in Palatini $R^2$ gravity. Also, different kinds of constraints from a variety of observations, such as BICEP/Keck, Planck 2018, as well as future possible detectable sensitivities that might be reached by CMB experiments: CMB-S4 and LiteBIRD are taken into account in this work. We indicate that our results are consistent with both the latest data and the future sensitivity forecasts of LiteBIRD/Planck and CMB-S4. Finally, the results in this study highlight the viability of our model even in the case of the existence of more stringent constraints expected from future achievable confidence level limits.

astro-ph.CO↗