arXiv subjects

Norma Sidik Risdianto

Publications and source records attributed to Norma Sidik Risdianto.

6 recordsLinked to original sources

arXiv2026

The Preheating Stage on The Starobinsky Inflation after ACT

Recent ACT results favor larger e-fold numbers, which tighten the reheating requirements in Starobinsky inflation. We show that efficient reheating generically requires an additional spectator-assisted preheating stage. The preheating dynamics and reheating temperature must be significantly modified to accommodate the higher reheating temperature. We present viable parameter sets compatible with PBH formation and the preferred reheating mechanism in this model. Furthermore, the momentum spectrum evolves from infrared-dominated modes at the early stage of preheating to higher-momentum modes at later times, supporting our scenario.

gr-qc↗

arXiv2026

Minimal Extensions of the $α$-Starobinsky Model: Reconciling ACT DR6 and Reheating Constraints

The latest combined data from the Atacama Cosmology Telescope (ACT) DR6, Planck, and DESI yields a scalar spectral index $n_s = 0.9743 \pm 0.0034$, which lies approximately $2σ$ above the prediction of the standard $α$-Starobinsky inflation model. To address this tension, we propose two minimal extensions that preserve the model's plateau structure and attractor properties: a multiplicative exponential modification and an additive polynomial deformation, both governed by a single small perturbative parameter $δ>0$. We analytically derive the slow-roll parameters and inflationary observables up to second order in $δ$ and integrate them with reheating dynamics via the consistency equation. It is shown that the $δ$ term effectively shifts $n_s$ into the $1σ$ confidence region of the joint P-ACT-LB-BK18 dataset without violating the tensor-to-scalar ratio bound ($r < 0.038$). The viable parameter space at $1σ$ requires $α\lesssim 35$ with $δ\sim \mathcal{O}(10^{-2})$ for the exponential model, while the additive model requires $δ\sim \mathcal{O}(10^{-3})$ for $p=1$ and $δ\sim \mathcal{O}(10^{-4})$ for $p=2$. For the e-folding range $N_k \in [50, 65]$, the relevant reheating equation of state is $0<ω_{\mathrm{re}}\le1$. All viable scenarios yield a reheating temperature $T_{\mathrm{re}} \sim 10^9$ GeV, which is safely above the Big Bang Nucleosynthesis (BBN) bound and below the gravitino overproduction limit.

astro-ph.CO↗

arXiv2025

On Inflation and Reheating Features in the Higgs-$R^2$ Model

We investigated inflation in the Higgs-$R^2$ model and assumed the trajectory to follow a single-field approximation called minimal two-field mode. Using this approximation, we tried to constrain the Higgs' non-minimal coupling $ξ$. During inflation, we investigated the effect of $ξ$ on the non-gaussianity. We found that $ξ$ could not provide the large non-gaussianity. In this paper, we divided the preheating stage to be quadratic regime and quartic regime. During the quadratic regime, the gauge bosons' production is the most dominant. However, it could not drain the whole inflaton's energy. Thus, we introduced a dark matter candidate with a large coupling that could drain the whole inflaton's energy. We also found that if $ξ<4.2$, the oscillation on the quadratic regime continued to the quartic regime. The reheating temperature obtained by this mode can be reached to be $\sim 10^{10}$ GeV and strongly depends on the remaining inflaton's energy density. On contrary, if $ξ>4.2$, the oscillation only happens at the quadratic regime. As we assumed the Pauli exclusion principle does exist during the quadratic regime, the instant decay of gauge bosons to fermions is restricted. Hence we expected, the reheating temperature to be low for about $\sim 10^9$ GeV.

astro-ph.CO↗

arXiv2025

Preventing Curvature Singularities in $f(R)$ Dark Energy Models

The curvature singularity problem in $f(R)$ dark energy models poses a significant challenge to their viability as alternatives to the $Λ$CDM paradigm. In this work, we investigate the possibility of resolving this issue by incorporating higher-order corrections that are compatible with the inflationary phase. We analyze the effects of adding $R^2$, $R^{\frac{m+2}{m+1}}$ and $α$-attractor representation in $f(R)$ gravity terms to types of dark energy $f(R)$ models, focusing on their ability to prevent singularities in high-density environments. Our results demonstrate that these corrections can effectively stabilize the models, ensuring their consistency across both inflationary and late-time cosmological scales.

gr-qc↗

arXiv2025

The Second-Level Preheating

This paper proposes two levels of preheating for the inflaton that is non-minimally coupled with gravity. The first level, later named by the quadratic regime, corresponds to the matter-dominated era and is responsible for draining the inflaton's energy density. The second level, which we will call the quartic regime, corresponds to the radiation-dominated era and is responsible for the reheating of the universe. We investigate the behavior of non-renormalizable higher dimension operators in both the quadratic and quartic regimes. In the quadratic regime, the preheating is also efficient, even though it is less than the lower dimension. On the other hand, the non-renormalizable higher dimension operators in the quartic regime are extremely inefficient. In our work, we also introduce a simple mechanism controlled by the characteristic momentum $α$ to suppress the particle production during preheating. Additionally, we emphasized the significance of the small momentum of the particles produced during preheating for the abundance of primordial black holes. This result supports the efficient preheating in the quadratic regime. Finally, we evaluate two modes of the reheating temperature, which differ based on the preheating efficiency during the quadratic regime.

gr-qc↗

arXiv2021

Inflation connected to the origin of $CP$ violation

We consider a simple extension of the standard model, which could give a solution for its $CP$ issues such as the origin of both CKM and PMNS phases and the strong $CP$ problem. The model is extended with singlet scalars which could cause spontaneous $CP$ violation to result in these phases at low energy regions. The singlet scalars could give a good inflaton candidate if they have a suitable nonminimal coupling with the Ricci scalar. $CP$ issues and inflation could be closely related through these singlet scalars in a natural way. In a case where inflaton is a mixture of the singlet scalars, we study reheating and leptogenesis as notable phenomena affected by the fields introduced in this extension.

hep-ph↗