arXiv subjects

Imtiyaz Ahmad Bhat

Publications and source records attributed to Imtiyaz Ahmad Bhat.

11 recordsLinked to original sources

arXiv2026

Constraining Quintessential Inflation with ACT: A Gauss-Bonnet Gateway

Recent results from the Atacama Cosmology Telescope (ACT), indicating a higher and more tightly constrained scalar spectral index, $n_s = 0.9743 \pm 0.0034$, place several inflationary models under tension, with quintessential inflation pushed close to or beyond the $2σ$ boundary in the $r$--$n_s$ plane. In this work, we revisit quintessential inflation within the framework of Einstein--Gauss--Bonnet (EGB) gravity, where a scalar field non-minimally coupled to the Gauss--Bonnet invariant modifies the inflationary dynamics. We consider three representative coupling functions -- exponential, hyperbolic secant, and hyperbolic tangent -- and show that the exponential and sech-type couplings can shift the predicted values of $r$ and $n_s$ into the $1σ$ region allowed by ACT, thereby restoring consistency with observations. In contrast, the tanh-type coupling remains disfavored, underscoring the sensitivity of inflationary observables to the coupling structure. We further investigate the reheating phase using a model-independent parametrization and demonstrate that viable thermal histories can be realized even in the absence of a potential minimum, with reheating temperatures consistent with Big Bang nucleosynthesis bounds. Overall, our analysis shows that EGB corrections provide a viable and robust extension that reconciles quintessential inflation with current precision cosmological data, and we identify the corresponding allowed parameter space.

astro-ph.CO↗

arXiv2025

Inflationary Dynamics in Einstein-Gauss-Bonnet Gravity Using New Slow-Roll Approximations considering Generalised Reheating

The Cosmic Microwave Background (CMB) observations results reported by {\it Planck} satellite suggest a plateau characteristic in the flat potential of the inflaton field is favored to drive the Universe's early acceleration. On that note, Power Law Plateau (PLP) potential has gained a lot of attention. In this study, we demonstrate that implementing this inflationary model in an Einstein-Gauss-Bonnet(EGB) gravity background makes the model compatible with observations while avoiding late-stage thermal inflation which is otherwise unavoidable in a standard scenario. More importantly, we have taken the PLP model as a case study to check the new slow-roll approximations method proposed in \cite{Pozdeeva:2024ihc}. Then we take one step further to check the consequence of generalized reheating in this scenario. Thus opening a new promising window to study inflationary dynamics in EGB gravity.

astro-ph.CO↗

arXiv2025

Reconciling Fractional Power Potential and EGB Gravity in the light of ACT

Recent results from the ACT collaboration indicate a higher value for the scalar spectral index, with $n_s = 0.9743 \pm 0.0034$, which sets tighter constraints on inflationary models, and these shifts are not in favor of many pre existing scenarios, including the widely studied and accepted standard Starobinsky model. In this paper, we examine the fractional power scalar potential within the framework of Einstein Gauss Bonnet (EGB) gravity, incorporating standard slow roll approximation. The EGB theory, motivated by higher dimensional models, introduces quadratic curvature corrections and a coupling between the scalar field and the Gauss Bonnet term, thereby modifying the cosmological dynamics. The results show good agreement with observational data, placing the predictions within the $1 σ$ region of the ACT $r-n_s$ constraint plot. Furthermore, incorporating the running of the scalar spectral index reinforces the models consistency with observational bounds. We also explore the parameter space of the EGB couplings and identify the range of free parameters for which the results of $n_s$ and $r$ values remain within the $1 σ$ region of the ACT constraints. Finally, we also investigate the reheating phase, demonstrating that the model not only agrees with ACT data but also satisfies the lower bound on the reheating temperature, thereby ensuring a consistent and viable cosmological scenario.

astro-ph.CO↗

arXiv2025

Inflationary Dynamics of Mutated Hilltop Inflation in Einstein-Gauss-Bonnet Gravity Under New Slow-Roll Approximations with Generalised Reheating

The advancement in the observational cosmology of the early universe such as Cosmic Microwave Background (CMB) observations, puts severe constraints on the inflationary models. Many inflationary models have been ruled out by CMB, nevertheless the models ruled out in standard cold inflationary scenarios can be resurrected in modified gravity models. In this regard we examine the dynamics of inflation within the framework of Einstein-Gauss-Bonnet (EGB) Gravity using the new slow-roll approximation methods proposed in Pozdeeva et al. (2024). We consider the Mutated Hilltop inflation model (Pal et al., 2010; Pinhero and Pal, 2019) due to its origin from super-gravity, a naturally perfect choice to study the impact of EGB on inflationary observables such as tensor-to-scalar ratio ($r$) and scalar spectral index ($n_s$). The period of reheating following the inflationary phase is also examined, and for the {\it Planck'18} permitted values of $n_s$, constraints on the reheating temperature ($T_{re}$) are computed for various equations of states during reheating ($ω_{re}$).

astro-ph.CO↗

arXiv2025

Holographic Consequences of Heterotic String Theory beyond its Supergravity Approximation

In this work, we study the effects of stringy corrections on the low energy effective action derived from heterotic string theory beyond its supergravity approximation. Compactifying the ten dimensional theory with these stringy corrections produces an effective action for a scalar field whose higher derivative term is governed by a single coefficient that depends on the internal volume, average curvature, and flux of the compactification manifold. The higher derivative coupling imported from compactification shifts the Breitenlohner Freedman stability bound by an amount set by the relative strengths of internal flux and curvature, relaxing it in flux dominated vacua and tightening it in curvature dominated ones. Furthermore, we analyze how the stringy corrections shift the scaling dimensions of the dual operators, track the resulting renormalization group flow, and investigate the higher derivative term using a holographic Lee Wick regulator. In holographic superconductors, stringy corrections lower the effective bulk mass and raise the critical temperature when flux dominates, but have the opposite effect when curvature dominates.

hep-th↗

arXiv2023

Nonthermal acceleration radiation of atoms near a black hole in presence of dark energy

We investigate how dark energy affects atom-field interaction. To this end, we consider acceleration radiation of a freely falling atom close to a Schwarzschild black hole (BH) in the presence of dark energy characterized by a positive cosmological constant $Λ$. The resulting spacetime is endowed with a BH and a cosmological (or de Sitter) horizon. Our consideration is a \textit{nonextremal} $(1+1)$-dimensional geometry with horizons far apart, giving rise to a flat Minkowski-like region in between the two horizons. Assuming a scalar ($\text{spin}-0$) field in a Boulware-like vacuum state, and by using a basic quantum optics approach, we numerically achieve excitation probabilities for the atom to detect a photon as it falls toward the BH horizon. It turns out that the nature of the emitted radiation deeply drives its origin from the magnitude of $Λ$. In particular, radiation emission is enhanced due to dilation of the BH horizon by dark energy. Also, we report an oscillatory nonthermal spectrum in the presence of $Λ$, and these oscillations, in a varying degree, also depend on BH mass and atomic excitation frequency. We conjecture that such a hoedown may be a natural consequence of a constrained motion due to the bifurcate Killing horizon of the given spacetime. The situation is akin to the Parikh-Wilzcek tunneling approach to Hawking radiation where the presence of extra contributions to the Boltzmann factor deforms the thermality of flux. It apparently hints at field satisfying a modified energy-momentum dispersion relation within classical regime of general relativity arising as an effective low energy consequence of an underlying quantum gravity theory. Our findings may signal new ways of conceiving the subtleties surrounding the physics of dark energy.

gr-qc↗

arXiv2022

Gamma Ray Bursters and Black Holes in the Gravity's Rainbow

In this letter, we analyze the modification to the thermodynamics of a Schwarzchild black hole and a Kerr black hole due to gravity's rainbow. The metric for these black holes will be made energy dependent. This will be done by using rainbow functions motivated from the hard spectra from gamma-ray bursters at cosmological distances. This modification of the metric by these rainbow functions will in turn modify the temperature and entropy of these black holes. We will also discuss how this effects the formation of virtual black holes.

gr-qc↗

arXiv2022

The effect of loop quantum gravitational rainbow functions on the formation of naked singularities

In this paper, we will investigate the consequences of loop quantum gravitational modifications on the formation of naked singularities. The loop quantum gravitational effects will be incorporated in the collapsing system using gravity's rainbow. This will be done by using rainbow functions, which are constructed from loop quantum gravitational modifications to the energy momentum dispersion. It will be observed that this modification will prevent the formation of naked singularity. Thus, such a modification can ensures that the weak cosmic censorship hypothesis will hold for any collapsing system.

gr-qc↗

arXiv2022

Anomalous Magnetic Moment and Higgs Coupling of the Muon in a Sequential U(1) Gauge Model with Dark Matter

We study an Abelian gauge extension of the standard model with fermion families having non-universal gauge charges. The gauge charges and scalar content are chosen in such an anomaly-free way that only the third generation fermions receive Dirac masses via renormalisable couplings with the Higgs boson. Incorporating additional vector like fermions and scalars with appropriate $U(1)$ charges can lead to radiative Dirac masses of first two generations with neutral fermions going in the loop being dark matter candidates. Focusing on radiative muon mass, we constrain the model from the requirement of satisfying muon mass, recently measured muon anomalous magnetic moment by the E989 experiment at Fermilab along with other experimental bounds including the large hadron collider (LHC) limits. The anomalous Higgs coupling to muon is constrained from the LHC measurements of Higgs to dimuon decay. The singlet fermion dark matter phenomenology is discussed showing the importance of both annihilation and coannihilation effects. Incorporating all bounds lead to a constrained parameter space which can be probed at different experiments.

hep-ph↗

arXiv2020

Inflation, reheating, leptogenesis and bounds on soft supersymmetry breaking parameters

In the no-scale supergravity with Type-I Seesaw model of Non-minimal supersymmetric standard model (NMSSM), we have analysed inflation, reheating and leptogenesis. A no-scale supergravity realization of Starobinsky model of inflation in simple Wess-Zumino model have been shown earlier by Ellis et al. Here we show a no-scale supergravity realization of Starobinsky model of inflation in Type-I Seesaw framework of NMSSM. In this framework an appropriate choice of no-scale Kähler potential results in Starobinsky like plateau inflation along a Higgs-sneutrino $D$-flat direction consistent with the CMB observations. In leptogenesis, the soft-breaking trilinear and bilinear terms play important role. Using conditions for non-thermal contribution to $CP$ asymmetry and successful leptogenesis together with the appropriate reheating at the end of inflation, we have obtained important constraints on the soft supersymmetry breaking parameters.

hep-ph↗

arXiv2020

Dark matter mass from relic abundance, an extra $U(1)$ gauge boson, and active-sterile neutrino mixing

In a model with an extra $U(1)$ gauge to SM gauge group, we have shown the allowed region of masses of extra gauge boson and the dark matter which is the lightest one among other right-handed Majorana fermions present in the model. To obtain this region, we have used bounds coming from constraints on active-sterile neutrino masses and mixing from various oscillation experiments, constraint on dark matter relic density obtained by PLANCK together with the constraint on the extra gauge boson mass and its gauge coupling recently obtained by ATLAS Collaboration at LHC. From the allowed regions, it is possible to get some lower bounds on the masses of the extra gauge boson and the dark matter and considering those values it is possible to infer what could be the spontaneous symmetry breaking scale of an extra $U(1)$ gauge symmetry.

hep-ph↗