arXiv subjects

Saddam Hussain

Publications and source records attributed to Saddam Hussain.

At least 19 recordsLinked to original sources

arXiv2026

Observational constraints on a damped harmonic oscillator model of dark energy

We constrain a damped harmonic oscillator (DHO) dark-energy equation of state using the full cosmic microwave background (CMB) likelihoods in combination with DESI BAO and three distinct Type Ia supernova compilations: Pantheon+, DES-Dovekie, and Union3. The equation of state obeys a second-order damped oscillator equation in number of $e$-folds, so that its frequency $f$, damping rate $b$, and equilibrium value $w_{\rm m}$ fully specify the late-time dynamics. The model exhibits oscillatory behavior only at low redshifts, around the equilibrium value $w=-1$, with distinct characteristics for the different supernova compilations: an underdamped solution for DES-Dovekie and Union3, and an overdamped solution for Pantheon+. At higher redshifts, the model closely mimics $\Lambda$CDM and deviates significantly only at $z<0.6$, with the magnitude of the deviation depending on the supernova compilation. We further identify a region of the $(f,b)$ parameter space, corresponding to rapid variation of the equation of state at low redshift, in which the perturbation equations become numerically stiff and cannot be integrated with a canonical dark-energy sound speed i.e., $c_s^2 =1$. We show that reducing the rest-frame sound speed removes this obstruction while leaving the observables unchanged at the $10^{-3}$ level, and therefore treat it as a numerical prescription rather than a physical modification of the model. The model yields $H_0 = 67.53^{+1.22}_{-1.18}$ km/s/Mpc for Pantheon+, $H_0 = 69.08^{+1.23}_{-1.16}$ km/s/Mpc for DES-Dovekie, and $H_0 = 70.67^{+1.89}_{-1.87}$ km/s/Mpc for Union3. The present-day equation-of-state parameter is constrained to $w_0 = -0.521^{+0.891}_{-0.414}$, $-3.01^{+1.14}_{-1.21}$, and $-3.16^{+1.11}_{-1.16}$ for Pantheon+, DES-Dovekie, and Union3, respectively.

astro-ph.CO↗

arXiv2026

Does DESI prefer Damped Oscillating Dark Energy over Cosmological constant?

We investigate a dark-energy equation of state governed by a damped harmonic oscillator equation, admitting underdamped, critically damped, and overdamped solutions. Confronting the model with Planck CMB distance priors, DESI BAO, BBN, cosmic chronometers, and three Type~Ia supernova compilations, we find that the data select an underdamped solution yielding $H_0 = 70.9 \pm 1.1$ km/s/Mpc with DES-Dovekie and $H_0 = 72.0^{+1.4}_{-2.1}$ km/s/Mpc with Union3, without any local $H_0$ prior. These higher values of $H_0$ arise along the $\Omega_{\rm m}$--$H_0$ degeneracy direction while the sound horizon remains nearly unchanged at $r_{\rm d} \simeq 145$~Mpc, indicating that the enhancement of the late-time expansion rate is a geometrical effect that does not address the early-time calibration of $r_{\rm d}$. In contrast, the Pantheon+ compilation selects a near-critically damped solution with a prior-limited positive $w_0$ and $H_0 = 66.23 \pm 0.85$ km/s/Mpc, highlighting the sensitivity of the model to the low-redshift distance information encoded in the different supernova compilations. The Bayesian evidence relative to $\Lambda$CDM is inconclusive for the DES-Dovekie and Union3 combinations, whereas Pantheon+ shows a strong preference for the damped-oscillator model, driven by the departure from $w=-1$ at $z\lesssim0.1$.

astro-ph.CO↗

arXiv2026

Interacting $k$-essence field with non-pressureless Dark Matter: Cosmological Dynamics and Observational Constraints

We investigate a class of interacting dark energy and dark matter (DM) models, where dark energy is modeled as a $k$-essence scalar field with an inverse-square potential. Two general forms of interaction are considered: one proportional to the Hubble parameter, and another independent of the Hubble parameter, depending instead on combinations of the energy densities and pressures of the dark sectors. {The cosmological evolution is reformulated in terms of an autonomous system of equations, which provides a convenient phase-space parametrization for the numerical integration of the background dynamics and for confronting the models with observations.} The models are tested against a wide range of observational datasets, including cosmic chronometers (CC), BAO measurements from DESI DR2, compressed Planck data (PLA), Pantheon+ (PP), DES supernovae, Big Bang Nucleosynthesis (BBN), and strong lensing data from H0LiCOW (HCW). The analysis shows that the models consistently reproduce all major cosmological epochs and yield statistically competitive results compared to the flat $Λ$CDM model. The models exhibit late-time de-Sitter solutions, ensuring ghost-free evolution, with the Hubble constant in the range $H_0 \sim 67$--$70$ km/s/Mpc.

astro-ph.CO↗

arXiv2026

Interacting Scalar Fields as Dark Energy and Dark Matter in Einstein scalar Gauss Bonnet Gravity

A Gauss-Bonnet (GB) coupled scalar field $ϕ$, responsible for the late-time cosmic acceleration and interacting with a coherent scalar field $ψ$ through an interaction potential $W(ϕ,ψ)$, is considered from the point of view of particle physics for two different models. The non-minimal coupling between the GB curvature term and the field $ϕ$ leads to a time-dependent speed of gravitational waves (GWs), which is fixed to unity in order to be consistent with current GW observations, rendering the GB coupling function model-independent. We investigate the dynamical stability of the system by formulating it as an autonomous system, and provide a detailed discussion on the choice of initial conditions required to obtain stable background evolution of the models. We constrain the model parameters using various sets of observational data, including both early- and late-time probes. We incorporate the improved Dark Energy Survey (DES) 5-year Type Ia supernova sample (DES-SN5YR), referred to as DES-Dovekie, which exhibits substantially lower tension with the Pantheon+ supernova sample. We find that both models are physically viable and closely follow the $Λ$CDM trend for the Pantheon+ and DES samples. However, upon including the Roman mock data, a significant departure is observed at higher redshifts, yielding statistically strong preference over the flat $Λ$CDM model.

gr-qc↗

arXiv2026

Interacting bosonic dark energy and fermionic dark matter in Einstein scalar Gauss-Bonnet gravity

We explore a cosmological framework in which a Gauss-Bonnet (GB) coupled scalar field, acting as dark energy, interacts with a fermionic dark matter field through a coupling obtained from the point of view of particle physics. This setup is inspired by string/M-theory, and two representative scalar field potentials are investigated: exponential and power-law. A distinctive feature of the GB-coupled models is their potential to alter the propagation speed of gravitational waves (GWs), a property with significant implications in light of recent multi-messenger astrophysical observations. To account for this, we analyze models under two scenarios: one where the GW speed differs from that of light and the other where they are equal, but all consistent with current observational constraints. The dynamical evolution of the system is investigated by reformulating the field equations into an autonomous dynamical system, enabling a detailed analysis of the Universe's long-term behavior, including the radiation-, matter- and dark energy-dominated epochs. We constrain the model parameters using a broad set of recent observational data, including mock high-redshift measurements from the Roman Space Telescope. Our findings indicate that both potentials yield cosmologies that are in excellent agreement with current data, closely tracking the expansion history predicted by the standard $Λ$CDM model, while still allowing room for subtle deviations that could be tested by future observations.

astro-ph.CO↗

arXiv2026

Probing Kerr black hole in a uniform Bertotti-Robinson magnetic field through astrophysical quasi-periodic oscillations

In this study, the behavior of high-frequency quasi-periodic oscillations (QPOs) is investigated around a Kerr black hole immersed in a uniform Bertotti-Robinson magnetic field. The motion of the test particle is analyzed by determining the geodesic equations and evaluating the corresponding orbital, radial, and vertical epicyclic frequencies. These fundamental frequencies are used to construct the theoretical framework of QPO models based on parametric and forced resonance mechanisms. Observational data obtained from several black hole X-ray binaries (GRO J1655-40, XTE J1550-564, XTE J1859+226, GRS 1915+105, H1743-322, M82~X-1, and Sgr~A$^{*}$) are used to constrain the black hole parameters through Bayesian inference and Markov Chain Monte Carlo (MCMC) analyses. For the X-ray binaries GRO J1655-40, GRS 1915+105, H1743-322, and M82~X-1, nonzero values of the dimensionless parameter $b=Bm$ are obtained at the $68\%$ confidence level within the framework of the parametric resonance model, while only upper bounds at the $90\%$ confidence level are obtained for the remaining sources. In contrast, in the case of the forced resonance model, only an upper bound at the $90\%$ confidence interval is obtained for the magnetic field parameter for all considered X-ray binary sources. The analysis indicates that the value of the magnetic field parameter is small but not negligible, producing minor modifications to particle dynamics and epicyclic frequencies. The influence of the magnetic field is further examined through the properties of the innermost stable circular orbit and the radiative properties of the thin accretion disk, including the energy flux and temperature profiles, within the allowed parameter range inferred from the MCMC analysis.

astro-ph.HE↗

arXiv2026

Dynamical systems approach to Cold and Warm Inflation within slow-roll and beyond

In this work, we systematically present a new dynamical systems approach to standard inflationary processes and their variants as constant-roll inflation. Using the techniques presented in our work one can in general investigate the attractor nature of the inflationary models in the phase space. We have compactified the phase space coordinates, wherever necessary, and regulated the nonlinear differential equations, constituting the autonomous system of equations defining the dynamical system, at the cost of a new redefined time variable which is a monotonic increasing function of the standard time coordinate. We have shown that in most of the relevant cases the program is executable although the two time coordinates may show different durations of cosmological events. If one wishes one can revert back to the cosmological time via an inverse transformation. The present work establishes a standard norm for studying dynamical as well as stability issues in any new inflationary system.

gr-qc↗

arXiv2025

Large scale structure constraints and matter power spectrum in $f (Q,\mathcal{L}_{m})$ gravity

In the present work, we take into account the dynamical system analysis to investigate the matter power spectrum within the framework of the $f(Q,\mathcal{L}_{m})$ gravitational theory. After obtaining autonomous dynamical system variables for two different particular pedagogical choices of $f(Q,\mathcal{L}_{m})$ models (A and B), we derive the full system of perturbation equations using the $1+3$ covariant formalism to study the matter fluctuations. We present and solve the energy density perturbation equations to obtain the energy density contrast, which decays with redshift for both models for a particular choice of model parameters. After obtaining the numerical results of the density contrast, we computed the matter spectra for each model and conducted a comparative analysis with the $Λ$CDM. Furthermore, by employing the Markov Chain Monte Carlo (MCMC) analysis,the model parameters were constrained using a combination of different observational data sets to improve the robustness and accuracy of the parameter estimation. Our results indicate that only model A can be compatible with the considered observational data sets.

gr-qc↗

arXiv2025

Probing the Dynamics of Gaussian Dark Energy Equation of State Using DESI BAO

We present an updated reconstruction of the DE equation of state (EoS), $w(a)$, employing the newly released DESI DR2 Baryon Acoustic Oscillation data. This analysis constrains the cosmological scenarios influenced by different models through the joint examination of a range of recently available cosmological probes, specifically the Pantheon+ sample and the DESY5 sample of Type Ia Supernovae, baryon acoustic oscillations, Hubble parameter measurements derived from cosmic chronometers, and cosmic microwave background distance priors based on the Planck 2018 data. Furthermore, we provide a concise perspective on the dynamical evolution of all models (CPL, PADE, GEDE, GDE, BellDE) and their interrelations. A Bayesian inference procedure is adopted to estimate the models parameters that yield the best fit to the data. The EoS remains within the phantom regime at higher redshifts, while favoring the quintessence regime in the current epoch. In this context, we propose a new Gaussian-like form of EoS, termed BellDE, which avoids phantom behavior ($w \geq -1$) at higher redshifts while remaining precisely calibrated at lower redshifts. Interestingly, BellDE exhibits a transient phantom nature ($w < -1$) around the transition redshift $z \sim 0.5$, subsequently evolving into a quintessential regime ($w > -1$). In particular, the BellDE model provides competitive statistical preference while offering greater flexibility in the redshift regime $z \sim 0.5-1$, where DE is observationally significant.

astro-ph.CO↗

arXiv2025

Exploring the accelerating black holes from the observations of quasi-periodic oscillations in X-ray binaries

Black holes in dense astrophysical environments, such as globular clusters or in the vicinity of other massive objects, may possess accelerations. Such acceleration would modulate the characteristics of the quasi-periodic oscillations (QPOs) observed in X-ray black hole binaries. In this paper, we explore the influence of spin-aligned acceleration of a black hole on QPOs observed in X-ray binaries. For this purpose, we compute the fundamental frequencies arising from the motion of test particles around an accelerating (spin-aligned) black hole and apply the relativistic precession, parametric resonance, and forced resonance models to establish their correspondence with several observed QPOs of X-ray binaries (GRO J1655-40, XTE J1550-564, XTE J1859+226, GRS 1915+105, H1743-322, M82~X-1, and Sgr~A$^{*}$). We then employ the Bayesian Markov-Chain Monte Carlo method to constrain the black hole parameters. Our results show no evidence for spin-aligned acceleration in any of the analyzed sources, suggesting that most of these X-ray binaries reside in isolated environments and therefore experience only small perturbations to the background spacetime geometries.

astro-ph.HE↗

arXiv2025

Yano-Schrödinger Hyperfluid: Cosmological Implications

Perfect cosmological hyperfluids generalize the concept of a perfect fluid within the framework of metric affine gravity. These hyperfluids encode the microstructure of matter including shear, dilation, and spin via the hypermomentum tensor. In this paper, we focus on the observational constraints of the recently introduced Yano-Schrödinger hyperfluid, which sources a special type of nonmetricity, that preserves the lengths of vectors under autoparallel transport. We propose a model in which the effective nonmetricity contributions to pressure and matter density are related linearly as $p_{\text{eff}} = ωρ_{\text{eff}}$. This assumption allows for a straightforward parameterization of deviations from standard cosmological behavior while maintaining analytical tractability. To constrain the effective equation of state parameter $ω$, we perform a Bayesian parameter estimation using Nested Sampling, implemented via the \texttt{PyPolyChord} library. We use Baryon Acoustic Oscillation measurements from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2), along with Type Ia supernova and Cosmic Chronometer data. In our analysis, we treat $r_d$ as a free parameter, enabling late-time data to extract posterior distributions for the Hubble constant ($ H_0$) and the sound horizon ($r_d$), along with the corresponding model parameters. Our results yield $H_0 = 67.4 \pm 4.0 km s ^{-1} Mpc ^{-1}$ and $r_d = 148.8 \pm 7.4$ Mpc, with $ω= -0.488$ . Finally, we use the logarithm of the Bayes factor to compare different Yano-Schrödinger model against the $Λ$CDM model. We find that the LESC model provides a better fit to the data, suggesting that modifications to metric-affine gravity could offer viable alternatives to the standard cosmological paradigm.

gr-qc↗

arXiv2025

Constraints on extra charges in dyonic Kerr-Newman-Kasuya-Taub-NUT black hole from the observations of quasi-periodic oscillations

This paper investigates the influence of the dimensionless electric charge ($Q/M$), magnetic charge ($P/M$), and Taub-NUT parameter ($n/M$) of a dyonic Kerr-Newman-Kasuya-Taub-NUT black hole on quasi-periodic oscillations (QPOs) observed in X-ray binaries. Using the relativistic precession model, we calculate the three fundamental frequencies arising from particle motion in the accretion disk around the black hole. These theoretical predictions are then confronted with observational QPO data from five X-ray binaries (GRO J1655-40, XTE J1859$+$226, XTE J1550-564, GRS 1915$+$105, and H1743-322), and the Markov Chain Monte Carlo technique is used to constrain the black hole parameters. Our analysis reveals no significant evidence for nonzero values of $Q/M$ and $P/M$ across all sources, thereby allowing us to place several stringent upper limits on electric charge ($Q/M$) and magnetic charge ($P/M$) of the black hole in these systems. Similarly, no compelling indication of a nonzero Taub-NUT parameter is found in QPOs from GRO J1655-40, XTE J1859$+$226, XTE J1550-564, and H1743-322. In contrast, the posterior distribution derived from GRS 1915$+$105 data suggests a nonzero Taub-NUT parameter, i.e., gravitomagnetic monopole moment. This result indicates a potential deviation from the Kerr metric in this astrophysical black hole.

astro-ph.HE↗

arXiv2025

Particle Production Scenario in an Algebraically Coupled Quintessence Field with a Dark Matter Fluid

We investigate the dynamics of an algebraically coupled quintessence field with a dark matter fluid, focusing on particle production through the action principle via a modified interaction Lagrangian. The interaction parameter serves as the source of dark matter particle production and entropy generation. As particle creation occurs due to the interaction between the field and fluid sectors, the system exhibits additional pressure. Our analysis includes studying the system's dynamics by considering an exponential type of interaction corresponding to the field's exponential potential. We assess the system's background dynamics using the dynamical system stability technique to derive the constraints on the model parameters. Additionally, we determine the best-fit values of the model parameters against two combinations of data sets: (i) CC+Pantheon+SH0ES, and (ii) CC+Pantheon+SH0ES+SDSS BAO+ DESI BAO. By employing a comprehensive data analysis technique, we compare the evidence of our models to flat $Λ$CDM. Based on the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC), one of the models emerges as a robust alternative to $Λ$CDM when considering the joint data sets.

gr-qc↗

arXiv2025

Nonminimally Coupled Quintessence with Double Exponential Potential: Observational Evidence Against Big Crunch Singularity

We investigate a class of scalar field dark energy models non-minimally coupled to gravity, characterized by a double exponential potential and parameterized coupling $\xi$. We study the cosmological dynamics for a recently proposed descending dark energy model, namely, Q-SC-CDM. Initially, we choose distinct values of coupling parameter. For some values of $\xi$, the evolution of the universe is split up into three different phases: {\it decelerated expansion (early time), accelerated expansion (late-time) and slow-contraction (future era)}, and provide Big Crunch Singularity at distant future. In other scenario, the phase of slow-contraction vanishes, cosmic acceleration is obtained at current epoch, and the universe gets de-Sitter expansion at distant future. We confront the model with various datasets, including Cosmic Chronometers, Type Ia Supernovae (Pantheon+, DES, and Union 3), and Baryon Acoustic Oscillation measurements from DESI. Our analysis reveals that observational constraints naturally favor regions of parameter space in which the model avoids future singularities and slow-contraction phases, even for relatively small values of $\xi \simeq 0.12$. The preferred solutions yield values of $\Omega_{m}$, $H_0$, and $r_d$ that are consistent with those of $\Lambda$CDM at the $68\%$ confidence level. In contrast, the models with fixed couplings $\xi=0.3$ and $\xi=0.5$, which predict $H_0>70$ km/s/Mpc, are strongly disfavored relative to $\Lambda$CDM by the current datasets. Finally, the phase-space analysis confirms that the observationally constrained model with $\xi\simeq 0.12$ evolves toward a stable de Sitter attractor.

gr-qc↗

arXiv2025

Comprehensive Study of $k$-essence Model: Dynamical System Analysis and Observational Constraints from Latest Type Ia Supernova and BAO Observations

We constrain the parameters of the $k$-essence scalar field model with inverse square and exponential potentials using data sets including Pantheon+SHOES and the Dark Energy Survey (DES) of Type Ia supernovae, Baryon Acoustic Oscillation (BAO) data from SDSS and DESI surveys, and direct measurements of the Hubble parameter and redshift obtained from the differential age method (CC). We also provide a brief perspective on the dynamical evolution of both models and derive stability constraints on the model parameters, which are then used to set appropriate priors. We adopt a Bayesian inference procedure to estimate the model parameters that best fit the data. A comprehensive analysis in light of observational data shows that the $k$-essence model fits well across all data combinations. However, according to the BIC criterion, the $Λ$CDM model provides a slightly better fit compared to the $k$-essence model.

astro-ph.CO↗

arXiv2024

Interacting Models of Dark Energy and Dark Matter in Einstein scalar Gauss Bonnet Gravity

We study the dynamics of the interacting models between the Gauss-Bonnet (GB) coupled scalar field and the dark matter fluid in a homogeneous and isotropic background. A key feature of GB coupling models is the varying speed of gravitational waves (GWs). We utilize recent constraints on the GW speed and conduct our analysis in two primary scenarios: model-dependent and model-independent. In the model-dependent scenario, where determining the GW speed requires a specific GB coupling functional form, we choose an exponential GB coupling. We adopt a dynamical system analysis to obtain the necessary constraints on the model parameters that describe different phases of the universe and produce a stable late-time accelerating solution following the GW constraint, and find that to satisfy all these constraints, fine-tuning of the free parameters involved in the models is often needed. In the model-independent scenario, the GW speed is fixed to one, and we construct the autonomous system to identify the late-time stable accelerating critical points. Furthermore, we adopt a Bayesian inference method using late-time observational data sets, including 31 data points from cosmic chronometer data (Hubble data) and 1701 data points from Pantheon+ and find that all the observational constraints can be satisfied without fine-tuning. In addition, we also utilize simulated binned Roman and LSST data to study the evolution of the universe in the model-independent scenario. We find that the model shows significant deviation at higher redshifts from $Λ$CDM and fits the current data much better than $Λ$CDM within the error bars.

gr-qc↗

arXiv2024

Hybridized Projected Differential Transform Method For collisional-breakage equation

The non-linear collision induced fragmentation plays a crucial role in modeling several engineering and physical problems. In contrast to linear breakage, it has not been thoroughly investigated in the existing literature. This study introduces an innovative method that leverages the Elzaki integral transform as a preparatory step to enhance the accuracy and convergence of domain decomposition, used alongside the projected differential transform method to obtain closed-form or series approximations of solutions for the collisional breakage equation (CBE). A significant advantages of this technique is its capability to directly address both linear and nonlinear differential equations without the need for discretization or linearization. The mathematical framework is reinforced by a thorough convergence analysis, applying fixed point theory within an adequately defined Banach space. Additionally, error estimates for the approximated solutions are derived, offering more profound insights into the accuracy and dependability of the proposed method. The validity of this approach is demonstrated by comparing the obtained results with exact or finite volume approximated solutions considering several physical examples. Interestingly, the proposed algorithm yields accurate approximations for the number density functions as well as moments with fewer terms and maintains higher precision over extended time periods.

cs.CE↗

arXiv2024

Stability analysis of a dark energy model in Rastall gravity

We study a cosmological model in Rastall's theory of gravity in the framework of the flat FLRW metric. We formulate the value of the Hubble parameter, which contains two model parameters, $ α$ and $ j $. Employing the Markov Chain Monte Carlo (MCMC) sampling technique, we determine the values of these model parameters along with their uncertainties. Moreover, we derive the equation of state (EoS) parameter, which converges around the quintessence region. We perform a dynamical system analysis using the linearization technique to validate the results independently. Also, we discuss various physical attributes of the model, highlighting the transition to acceleration and the violation of the strong energy condition (SEC) in the late stages of evolution. In conclusion, our model mimics the behavior of a dark matter fluid during the past epoch and transitions into a quintessence dark energy model in the future epoch.

gr-qc↗