arXiv subjects

Nadir Ijaz

Publications and source records attributed to Nadir Ijaz.

6 recordsLinked to original sources

arXiv2026

Revisiting primordial black hole dark matter from axion inflation

We revisit the production of primordial black holes (PBHs) by a U(1) gauge field with a pseudo-scalar coupling to the inflaton. We improve upon the existing literature by working in the homogeneous backreaction regime with numerically computed gauge mode functions, adopting state-of-the-art PBH abundance calculations, and incorporating the uncertainty in the statistics of $δρ$. We find that PBHs can account for all of the dark matter in the asteroidal mass range, even when the inflaton gradient energy density is highly subdominant ($10^{-4}$--$10^{-3}$ of the kinetic energy), supporting the validity of the backreaction scheme. This mechanism also unavoidably generates a stochastic gravitational wave background with an amplitude that will be measured at LISA and that will allow to indirectly discriminate between different statistics of $δρ$.

astro-ph.CO↗

arXiv2026

Shifted Hybrid Realization of Non-Minimal Higgs Inflation in Light of ACT DR6 and Planck Data

We investigate non-minimal Higgs inflation in a no-scale-inspired supergravity framework and confront its predictions with the latest CMB constraints from ACT DR6 and \emph{Planck}. Working within a shifted hybrid inflation scenario, we construct an effective single-field description in which the GUT Higgs direction serves as the inflaton after stabilization of the orthogonal scalar fields. We show that the inclusion of the leading nonrenormalizable operator in the superpotential induces a controlled deformation of the Starobinsky attractor, allowing the scalar spectral index to be shifted into the range favored by recent ACT and related CMB datasets while maintaining a small tensor-to-scalar ratio, $r \sim 10^{-3}-10^{-2}$. The resulting inflationary dynamics remain theoretically consistent, with controlled supergravity corrections and sub-Planckian inflaton field values. We perform a detailed numerical analysis of the model parameter space, including reheating and nonthermal leptogenesis, and identify regions that simultaneously satisfy current observational constraints and yield a viable post-inflationary cosmological history.

hep-ph↗

arXiv2025

Axion inflation in the regime of homogeneous backreaction

We investigate the Stochastic Gravitational Wave Background (SGWB) produced in models of axion inflation coupled to gauge fields. Achieving a detectable signal at Pulsar Timing Array, astrometry, or interferometer frequencies requires a sufficiently strong amplification of the gauge fields, at a level that induces significant backreaction on the inflaton background dynamics. Numerical studies based on the approximation of homogeneous backreaction (i.e., neglecting inhomogeneities of the inflaton field) exhibit oscillations in the inflaton velocity, with corresponding peaks in the SGWB spectrum. The most recent lattice simulations have questioned the validity of this regime, showing examples characterized by a rapid increase in the inflaton gradient energy and the breakdown of homogeneous backreaction. We compute this energy density perturbatively within the assumption of homogeneous backreaction, obtaining examples with a detectable SGWB and with an inflaton gradient energy that remains subdominant to the inflaton zero-mode kinetic energy throughout inflation.

astro-ph.CO↗

arXiv2025

Exploring primordial black holes and gravitational waves with R-symmetric GUT Higgs inflation

This study investigates the realization of R-symmetric Higgs inflation within the framework of no-scale-like supergravity, aiming to elucidate the formation of primordial black holes and observable gravitational waves within a class of GUT models. We explore the possibility of an ultra-slow-roll phase in a hybrid inflation framework, where the GUT Higgs field primarily takes on the role of the inflaton. The amplification of the scalar power spectrum gives rise to scalar-induced gravitational waves and the generation of primordial black holes. The predicted stochastic gravitational wave background falls within the sensitivity range of existing and upcoming gravitational wave detectors, while primordial black holes hold the potential to explain the abundance of dark matter. Furthermore, we highlight the significance of the leading-order nonrenormalizable term in the superpotential of achieving inflationary observables consistent with the latest experimental data. Additionally, the predicted range of the tensor-to-scalar ratio, a key measure of primordial gravitational waves, lies within the observational window of future experiments searching for B-mode polarization patterns in cosmic microwave background data.

astro-ph.CO↗

arXiv2024

The Stochastic Gravitational-Wave Background from Primordial Black Holes in R-Symmetric $SU(5)$ Inflation

This study explores the realization of nonminimally coupled Higgs inflation in the context of no-scale supergravity, investigates the formation of primordial black holes, and examines the potential for observable proton decay within the framework of the R-symmetric $SU(5)$ model. For inflation, both single and multifield scenarios are investigated. The prediction of the single-field model for the tensor-to-scalar ratio, $r$, is approximately $10^{-3}$, and the scalar spectral index falls within Plank's 1$σ$ range. The running of the scalar spectral index, $-{dn_{s}}/{d\ln{k}}$, is approximately $10^{-4}$. A realistic scenario of reheating and non-thermal leptogenesis is employed with reheat temperature $T_r\sim10^9$ GeV. In the multifield case, we mainly focus on Primordial Black Holes (PBHs) and Gravitational Waves (GWs). In this inflationary framework, we demonstrate how a suitable choice of parameters can result in an enhanced scalar power spectrum, leading to the production of primordial black holes (PBHs) capable of fully accounting for dark matter. We also show that this scenario leads to Scalar Induced Gravitational Waves (SIGW) which can be detected in current and future GW detectors. We explore different proton decay channels to look for observable predictions for the next-generation proton decay experiments Hyper-K and DUNE consistent with gauge coupling unification and cosmological bounds.

astro-ph.CO↗

arXiv2023

New Inflation in Waterfall Region

We introduce a class of new inflation models within the waterfall region of a generalized hybrid inflation framework. The initial conditions are generated in the valley of hybrid preinflation. Both single-field and multi-field inflationary scenarios have been identified within this context. A supersymmetric realization of this scenario can successfully be achieved within the tribrid inflation framework. To assess the model's viability, we calculate the predictions of inflationary observables using the $δN$ formalism, demonstrating excellent agreement with the most recent Planck data. Furthermore, this model facilitates successful reheating and nonthermal leptogenesis, with the matter-field component of the inflaton identified as a sneutrino.

hep-ph↗