arXiv subjects

Amare Abebe

Publications and source records attributed to Amare Abebe.

At least 37 records · Page 2Linked to original sources

arXiv2026

Study of dynamical systems and large-scale structure

In this study, we employ dynamical systems methods to analyse the large-scale structure by considering two distinct interaction models (linear and non-linear) within the dark sector, associated with a specific dynamical dark energy model inspired by the Veneziano ghost theory in quantum chromodynamics (QCD). In these models, the dark energy density ($ρ_{DE}$) varies with the Hubble parameter ($H$), expressed as $ρ_{DE} = αH + βH^2$. After defining the dimensionless parameters, we present autonomous equations that allow us to find the trace $\text{Tr}(J)$ and the determinant $D(J)$. With these solutions, we demonstrate the presence of unstable, saddle, and stable fixed points, corresponding to the radiation-, matter-, and dark-energy-dominated eras, respectively. Our results suggest that these models are theoretically viable for representing the interaction between dark sector fluids.

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arXiv2026

Late-time Background Constraints on Linear and Non-linear Interacting Dark Energy after DESI DR2

In this study, we present observational constraints on a class of phenomenological interacting dark energy (IDE) models that admit analytical solutions for the Hubble parameter $H(z)$. We consider a set of five linear and three non-linear IDE scenarios, encompassing both interactions proportional to the dark matter and/or dark energy densities, as well as non-linear combinations of the two. For all eight IDE models, we find a better fit than $Λ$CDM from a $Δχ^2$ analysis for both combinations of datasets considered. When using the Akaike Information Criterion ($Δ$AIC), we find a similarly improved fit in all cases, except for one dataset combination in $Q=3Hδρ_{\rm de}$. Our analysis also shows a preference for sign-switching interactions, with energy transfer from dark energy to dark matter at low redshift, reversing direction at higher redshift. These results should be interpreted with caution, as the latter direction of energy transfer is accompanied by negative dark energy densities in the past, which may be unphysical. Models that do not allow sign-changing behaviour instead show a preference for energy flow from dark matter to dark energy, and hence negative dark energy densities. The only exceptions are $Q=3Hδρ_{\rm de}$ and $Q=3Hδ\left(\tfrac{ρ_{\rm de}^2}{ρ_{\rm dm}+ρ_{\rm de}}\right)$, which exhibit energy flow in the opposite direction. Furthermore, for all interactions considered, we find a phantom-divide crossing in the effective dark energy equation of state $w^{\rm eff}_{\rm de}$, with the dark energy density decreasing ($w^{\rm eff}_{\rm de}>-1$) at present and at low redshift, while increasing ($w^{\rm eff}_{\rm de}<-1$) in the past at high redshift. These results highlight the promising, but problematic, nature of dark sector interactions, as well as the need to extend the analysis using early-time physics and datasets.

astro-ph.CO↗

arXiv2025

Challenging $Λ$CDM with Higher-Order GUP Corrections

We study quantum corrections to the $Λ$CDM model model arising from a minimum measurable length in Heisenberg's uncertainty principle. We focus on a higher-order Generalized Uncertainty Principle, beyond the quadratic form. This generalized GUP introduces two free parameters, and we determine the modified Friedmann equation. This framework leads to a perturbative cosmological model that naturally reduces to $Λ$CDM in an appropriate limiting case of the deformation parameters. We construct the modified cosmological scenario, analyze its deviations from the standard case, and examine it as a mechanism for the description of dynamical dark energy. To constrain the model, we employ Cosmic Chronometers, the latest Baryon Acoustic Oscillations from the DESI DR2 release, and Supernova data from the PantheonPlus and Union3 catalogues. Our analysis indicates that the modified GUP model is statistically competitive with the $Λ$CDM scenario, providing comparable or even improved fits to some of the combined datasets. Moreover, the data constrain the deformation parameter of the GUP model, with the preferred value found to be negative, which corresponds to a phantom regime in the effective dynamical dark energy description.

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arXiv2025

Implementing F (T ) Gravity in Boltzmann Codes: A Framework for Power Spectrum Computation

This work investigates the nonlinearity of the power-law model of F(T) gravity, highlighting the inability of the Boltzmann solver CLASS to handle nonlinear models. As a workaround, a second-order Taylor expansion is applied to the nonlinear field equations, under the assumption that the extra degree of freedom n, which quantifies deviations from the currently favored cosmological model (ΛCDM), remains sufficiently small to preserve the key properties of the ΛCDM model. The validity of the Taylor expansion is supported by supernova data indicating n \leq 0.05, for which the power spectrum can be accurately computed within CLASS with a negligible truncation error.

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arXiv2025

Observational constraints of diffusive dark-fluid cosmology

In this manuscript, we investigate late-time cosmology and the evolution of cosmic structures using an interacting dark fluid model in which dark matter (DM) and dark energy (DE) interact through a diffusive mechanism. To provide a comprehensive understanding, we derive the background evolution and perturbation equations within this model and obtain cosmological parameters through MCMC simulations. We use recent measurements for statistical analysis and constrain the parameters $H_0$ in km/s/Mpc, $Ω_m$, $r_d$, $M$, $σ_8$, $S_8$, and the interaction term $Q_{dm}$. From the constrained values of $Q_{dm}$, we show that the diffusive model is a promising alternative DE model, capable of driving late-time cosmic acceleration due to energy exchange from DM to DE. State-finder diagnostics indicate that the model behaves like a Chaplygin gas when energy transfers from DM to DE during the Universe's expansion. We also investigate the growth of density contrast, finding $δ_m(z)\ggδ_{de}(z)$, which highlights the dominant role of DM in structure formation. Redshift space distortion and growth rate analysis show that minor deviations from $Λ$CDM at low redshifts, with larger differences at higher redshifts, indicate the impact of energy diffusion on early structure growth. Finally, we perform a detailed statistical analysis, including ${\mathcal{L}(\hatΘ|data)}$, $χ^2$, $\rm{AIC}$, and $\rm{BIC}$, which strongly supports the proposed diffusive dark-fluid model.

astro-ph.CO↗

arXiv2025

III. Interacting Dark Energy: Summary of Models, Pathologies, and Constraints

We present an overview of the main results from our two companion papers that are relevant for observational constraints on interacting dark energy (IDE) models. We provide analytical solutions for the dark matter and dark energy densities, $ρ_{\rm dm}$ and $ρ_{\rm de}$, as well as the normalized Hubble function $h(z)$, for eight IDE models. These include five linear IDE models, namely $Q=3H(δ_{\rm dm} ρ_{\rm dm} + δ_{\rm de} ρ_{\rm de})$ and four special cases: $Q=3Hδ(ρ_{\rm dm}+ρ_{\rm de})$, $Q=3Hδ(ρ_{\rm dm}-ρ_{\rm de})$, $Q=3Hδρ_{\rm dm}$, and $Q=3Hδρ_{\rm de}$, together with three non-linear IDE models: $Q=3Hδ\left( \tfrac{ρ_{\rm dm} ρ_{\rm de}}{ρ_{\rm dm}+ρ_{\rm de}} \right)$, $Q=3Hδ\left( \tfrac{ρ_{\rm dm}^2}{ρ_{\rm dm}+ρ_{\rm de}} \right)$, and $Q=3Hδ\left( \tfrac{ρ_{\rm de}^2}{ρ_{\rm dm}+ρ_{\rm de}} \right)$. For these eight models, we present conditions to avoid imaginary, undefined, and negative energy densities. In seven of the eight cases, negative densities arise if energy flows from DM to DE, implying a strong theoretical preference for energy transfer from DE to DM. We also provide conditions to avoid future big rip singularities and evaluate how each model addresses the coincidence problem in both the past and the future. Finally, we propose a set of approaches and simplifying assumptions that can be used when constraining IDE models, by defining regimes that restrict the parameter space according to the behavior researchers are willing to tolerate.

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arXiv2025

Compartmentalization in the Dark Sector of the Universe after DESI DR2 BAO data

We consider a non-linear interaction between the dark matter and dark energy components of the universe. In particular, within the FLRW geometry, where dark matter is described by a dust fluid and dark energy by an ideal gas with a constant equation of state parameter, we introduce energy transfer between the two fluids. The effective cosmological fluid leads to a unified dynamical dark energy model with the feature that the Hubble function admits an analytic expression. We study this model using the DESI DR2 Baryonic Acoustic Oscillations data and the Supernova data from Pantheon+. The interacting model fits the data better than the $Λ$CDM model, with $χ_{\text{model}}^{2}-χ_{Λ\text{CDM}}^{2}=-5$. Using the Akaike Information Criterion to compare the two models, we derive $\text{AIC}_{\operatorname{model}}-\text{AIC}_{Λ\text{CDM}}=-1$, from which we conclude that the interacting model is marginally better supported by the data than the $Λ$CDM, but the difference is not statistically significant.

astro-ph.CO↗

arXiv2025

Geodesics and Light Deflection in Schwarzschild-like Spacetime from Cosmology-Inspired Modified Gravity

We investigate cosmology-driven modifications to Schwarzschild-like black hole spacetimes and analyze their impact on photon propagation, gravitational lensing, and shadow observation. The gravitational deflection angle is computed using the Rindler-Ishak method, which incorporates finite-distance corrections and provides a consistent framework for non-asym-ptotically flat spacetimes. The effective potential for null geodesics exhibits a single unstable maximum corresponding to the photon sphere, and we study photon orbits classified according to the critical impact parameter into capture, escape, and unstable circular trajectories. Our analysis shows that the deflection angle decreases with increasing model parameter $(α)$, resulting in weaker light bending compared to the Schwarzschild case. In addition, we examine the angular diameter of the black hole shadow as measured by a static observer, highlighting its dependence on the cosmological modification parameters. These results suggest that high-precision astrometric and lensing observations can place meaningful constraints on cosmology-inspired modifications to gravity, thereby linking astrophysical black holes with cosmic expansion and offering a novel probe of gravitational physics in strong-field regimes.

gr-qc↗

arXiv2025

I. Linear Interacting Dark Energy: Analytical Solutions and Theoretical Pathologies

Interacting dark energy (IDE) models, in which dark matter (DM) and dark energy (DE) exchange energy through a non-gravitational interaction, have long been proposed as candidates to address key challenges in modern cosmology. These include the coincidence problem, the $H_0$ and $S_8$ tensions, and, more recently, the hints of dynamical dark energy reported by the DESI collaboration. Given the renewed interest in IDE models, it is crucial to fully understand their parameter space when constraining them observationally, especially with regard to the often-neglected issues of negative energy densities and future big rip singularities. In this work, we present a comparative study of the general linear interaction $Q=3H(δ_{\rm dm}ρ_{\rm dm} + δ_{\rm de}ρ_{\rm de})$ and four special cases: $Q=3Hδ(ρ_{\rm dm}+ρ_{\rm de})$, $Q=3Hδ(ρ_{\rm dm}-ρ_{\rm de})$, $Q=3Hδρ_{\rm dm}$, and $Q=3Hδρ_{\rm de}$. For these five models, we perform a dynamical system analysis and derive new conditions that ensure positive, real, and well-defined energy densities throughout cosmic evolution, as well as criteria to avoid future big rip singularities. We obtain exact analytical solutions for $ρ_{\rm{dm}}$, $ρ_{\rm{de}}$, the effective equations of state ($w_{\mathrm{eff}}^{\rm{dm}}$, $w_{\mathrm{eff}}^{\rm{de}}$, $w_{\mathrm{eff}}^{\rm{tot}}$), and a reconstructed dynamical DE equation of state $\tilde{w}$. Using these results, we examine phantom crossings, address the coincidence problem, and apply the statefinder diagnostic to distinguish between models. We show that energy transfer from DM to DE inevitably produces negative energy densities and make future singularities more likely, while transfer from DE to DM avoids these pathologies and is thus theoretically favored.

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arXiv2025

II. Non-Linear Interacting Dark Energy: Analytical Solutions and Theoretical Pathologies

We investigate interacting dark energy (IDE) models with phenomenological, non-linear interaction kernels $Q$, specifically $Q_{1}=3Hδ\left(\frac{ρ_{\rm dm}ρ_{\rm de}}{ρ_{\rm dm}+ρ_{\rm de}}\right)$, $Q_{2}=3Hδ\left(\frac{ρ_{\rm dm}^2}{ρ_{\rm dm}+ρ_{\rm de}}\right)$, and $Q_{3}=3Hδ\left(\frac{ρ_{\rm de}^2}{ρ_{\rm dm}+ρ_{\rm de}}\right)$. Using dynamical system techniques developed in our companion paper on linear kernels, we derive new conditions that ensure positive and well-defined energy densities, as well as criteria to avoid future big rip singularities. We find that for $Q_{1}$, all densities remain positive, while for $Q_{2}$ and $Q_{3}$ negative values of either DM or DE are unavoidable if energy flows from DM to DE. We also show that for $Q_{1}$ and $Q_{2}$ a big rip singularity always arises in the phantom regime $w<-1$, whereas for $Q_{3}$ this fate may be avoided if energy flows from DE to DM. In addition, we provide new exact analytical solutions for $ρ_{\rm dm}$ and $ρ_{\rm de}$ in the cases of $Q_{2}$ and $Q_{3}$, and obtain new expressions for the effective equations of state of DM, DE, the total fluid, and the reconstructed dynamical DE equation of state ($w_{\rm dm}^{\rm eff}$, $w_{\rm de}^{\rm eff}$, $w_{\rm tot}^{\rm eff}$, and $\tilde{w}$). Using these results, we discuss phantom crossings, evaluate how each kernel addresses the coincidence problem, and apply statefinder diagnostics to compare the models. These findings extend the theoretical understanding of non-linear IDE models and provide analytical tools for future observational constraints.

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arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Constraining viscous fluid models in $f(Q)$ gravity with data

We investigate the impact of bulk viscosity on the accelerating expansion and large-scale structure formation of a Universe in which the underlying gravitational interaction is described by $f(Q)$ gravity. Various paradigmatic choices of the $f(Q)$ gravity theory, including power-law, exponential, and logarithmic models, are considered. To test the cosmological viability of these $f(Q)$ gravity models, we use {the Baryon Acoustic Oscillations ($BAO$) measurements from the Dark Energy Spectroscopic Instrument (DESI) Survey, cosmic chronometers ($CC$) from Hubble measurements, the SNIa distance moduli measurements from the PantheonP + SH0ES, growth rate ($f$-data), and redshift-space distortions ($fσ_8$) datasets, the latter two once the linear cosmological perturbations, growth rate $f(z)$, and redshift-space distortion $fσ_8(z)$ are studied. Thus, we perform the combined analyses for: PantheonP + SH0ES, PantheonP + SH0ES + f, and PantheonP + SH0ES + $fσ_8$. We compute the best-fit values $Ω_m$, $H_0\,\mathrm{(km/s/Mpc)}$, $r_d$, $M_{abs}$, $γ$, $σ_8$, $n$, $p$ and $Γ$ including the bulk viscosity coefficient $ζ$. Through a detailed statistical analysis, based on the Akaike Information Criterion (AIC) and Bayesian / Schwartz Information Criterion (BIC), a statistical comparison of the $f(Q)$ gravity models with $Λ$CDM is made. Among the three $f(Q)$ models, only the non-viscous $f(Q)$ power-law model yields robust parameter estimates and substantial observational support without any outright rejections. In contrast, both exponential and logarithmic $f(Q)$ models (with or without bulk viscosity) are rejected by multiple model selection criteria.

gr-qc↗

arXiv2025

Stellar isotropic model in the symmetric teleparallel equivalent of general relativity theory

Recently, the theory of symmetric teleparallel equivalent of general relativity (STEGR) has gained much interest in the cosmology and astrophysics community. Within this theory, we discuss the method of deriving a stellar isotropic model. In this respect, we implement the equations of motion of STEGR theory to a spacetime that is symmetric in a spherical manner, resulting in a set of nonlinear differential equations with more unknowns than equations. To solve this issue, we assume a special form of $g_{tt}$, and suppose a null value of the anisotropy to obtain the form of $g_{rr}$. We then investigate the possibility of obtaining an isotropic stellar model consistent with observational data. To test the stability of our model, we apply the adiabatic index and the Tolman-Oppenheimer-Volkoff equation. Furthermore, we examine our model using different observed values of radii and masses of pulsars, showing that all of them fit in a consistent way.

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arXiv2025

Null Geodesics, Thermodynamics, Weak Gravitational Lensing, and Black Hole Shadow Characteristics of a Frolov Regular Black Hole with Constraints from EHT Observations

In this paper, we investigate the properties of null geodesics, thermodynamics, gravitational lensing, and black hole shadows in the vicinity of a static regular Frolov black hole. By analyzing the trajectories of null geodesics, we investigate the bending of light in weak field regimes. The black hole shadow is studied in detail, with constraints on its parameters derived from observational data of the EHT collaboration. Further, we examine shadow images under a spherically symmetric accretion flow and compute the energy emission rate to understand the black hole radiation characteristics. The obtained results demonstrate how the Frolov black hole differs from well-known black hole solutions, such as Schwarzschild, Reissner-Nordström, and Hayward black holes. This study provides new insights into the impact of modified non-rotating regular black hole metrics on observational signatures. The findings have significant implications for future astrophysical observations and the testing of alternative gravity theories.

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arXiv2025

Structure growth in $f(Q)$ cosmology

We take into account redshift-space distortion measurements to investigate the growth of cosmological large-scale structures within the framework of modified symmetric teleparallel $f(Q)$ gravity. After comparing the predictions of the $f(Q)$-gravity expansion history with OHD and SNIa Pantheon+ sample datasets and constraining the pertinent cosmological parameters $Ω_{m}$ and $H_0$, together with the exponent $n$ for $f(Q)$ power-law models, we derive the full system of equations governing linear cosmological perturbations to study matter fluctuations using the $1 + 3$ covariant formalism when applied to $f(Q)$ gravity. Thus, we resort to both the usual redshift-space distortion data $fσ_8$ and some recent separate measurements of the growth rate $f$ and the amplitude of matter fluctuations $σ_8$ from the VIPERS and SDSS collaborations to find the best-fit cosmological parameters $Ω_m$, $σ_{8}$ and $n$. We also apply a collective analysis of such growth-structure data together with the aforementioned cosmic expansion measurements, to restrict these parameters through Monte Carlo Markov Chain simulations. determining the statistical significance for the best-fit parameter values through the AIC and BIC Bayesian selection criteria.

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arXiv2024

Constraining the modified symmetric teleparallel gravity using cosmological data

This paper examines the late-time accelerating Universe and the formation of large-scale structures within the modified symmetric teleparallel gravity framework, specifically using the $f(Q)$-gravity model, in light of recent cosmological data. After reviewing the background history of the Universe, and the linear cosmological perturbations, we consider the toy model $F(Q) = α\sqrt{Q}+β$ ( where $Q$ represents nonmetricity, $α$ and $β$ are model parameters) for further analysis. To evaluate the cosmological viability of this model, we utilize 57 Observational Hubble Data (OHD) points, 1048 supernovae distance modulus measurements (SNIa), their combined analysis (OHD+SNIa), 14 growth rate data points (f-data), and 30 redshift-space distortions (f$σ_8$) datasets. Through a detailed statistical analysis, the comparison between our model and $Λ$CDM has been conducted after we compute the best-fit values through the Markov Chain Monte Carlo (MCMC) simulations. Based on the results, we obtain the Hubble parameter, $H_0 = 69.20^{+4.40}_{{-}2.10}$ and the amplitude of the matter power spectrum normalization $σ_8 = 0.827^{+0.03}_{{-}0.01}$. These values suggest that our model holds significant promise in addressing the cosmological tensions.

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arXiv2024

Interacting dark energy models

This work focuses on two linear interaction models between dark matter and dark energy, which are proposed as key factors in explaining cosmic history, specifically the latetime accelerating expansion of the universe. Both models are constrained using a Markov chain Monte Carlo analysis (MCMC) using different sets of observational data. The analysis was composed using the Pantheon data set, consisting of 1048 points of SNIa distance moduli measurements from the Pantheon analysis and the Observed Hubble Parameter (OHD) data set using Baryon acoustic Oscillation (BAO), consisting of 57 data points using distance and expansion rate measurement. Both models showed promising results with the OHD data (BAO), with a interaction that results in a higher dark matter content of 56% and 44%, and a Hubble parameter of 65.7+-3km/ s/Mpc and 65.8+-3km/ s/Mpc for the interaction dependent on dark matter and dark energy respectively. The pantheon data set however predicted a reverse interaction for both models which does not follow initial assumptions that were made. The pantheon data measured a dark matter content of 18% and 20% with a Hubble parameter of 72.1 +- 0.003km/ s/Mpc and 72.3 +- 0.004km /s/Mpc. The constrained results are used to revisit the coincidence problem and other problems in standard cosmology. The analysis provided a discrepancy between the different data sets with one having a large error margin.

astro-ph.CO↗

arXiv2024

Constraints of Cosmic Expansion Using an MSF

In this paper, we propose a modified scale factor (MSF) that allows us to explore the accelerating expansion of the universe without invoking the traditional dark-energy model, as described in the Lambda cold dark matter ($Λ$CDM) model. Instead, the MSF model introduces parameters that encapsulate the effects traditionally attributed to dark energy. To test the viability of this MSF, we constrained the model using the observational Hubble parameter (OHD), distance modulus measurements (SNIa), and their combined datasets (OHD + SNIa). We implement a Monte Carlo Markov Chain (MCMC) simulation to find the best-fit values of the model parameters. The MSF model produced best-fit values for the parameter $p$ associated with the power law of the matter-dominated era and $β$, the exponential parameter for the darkenergy-dominated era. For our MSF, these values are $p$ = 0.28 and $β$ = 0.52 when using SNIa data, $p$ = 0.63 and $β$ = 0.30 for OHD data and $p$ = 0.45 and $β$ = 0.53 for a combination of datasets (OHD + SNIa). The numerical results and plots of the deceleration parameter, fractional energy density, Hubble parameter, and luminosity distance are presented which are the key parameters for studying the accelerated expansion of the universe. We compare the results of our model with that of the $Λ$CDM model and reconcile them with astronomical observational data. Our results indicate that the MSF model shows promise, demonstrating good compatibility with current astronomical observations and performing comparably to the $Λ$CDM model across various datasets, particularly in predicting the accelerating expansion of the universe, while providing a unified framework that incorporates the simultaneous influence of matter and dark energy components.

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