arXiv subjects

Amare Abebe

Publications and source records attributed to Amare Abebe.

At least 55 records · Page 3Linked to original sources

arXiv2024

Compartmentalization and Coexistence in the Dark Sector of the Universe

We revise the cosmological interaction between dark energy and dark matter. More precisely, we focus on models that support compartmentalization or co-existence in the dark sector of the universe. Within the framework of a homogeneous and isotropic, spatially flat Friedmann--Lemaître--Robertson--Walker geometry, we analyse the asymptotic behaviour of the physical parameters for two interacting models, where dark energy and dark matter have constant equations of state parameters, in the presence of dark radiation, when dark energy is described by a quintessence scalar field. For each model, we determine the asymptotic solutions and attempt to understand how the interaction affects the cosmological evolution and history.

gr-qc↗

arXiv2024

The cosmology of $f(R, L_m)$ gravity: constraining the background and perturbed dynamics

This paper delves into the late-time accelerated expansion of the universe and the evolution of cosmic structures within the context of a specific $ f(R, L_m) $ gravity model, formulated as $ f(R, L_m) = λR + βL_m^α+ η$. To study the cosmological viability of the model, we employed the latest cosmic measurement datasets: i) 57 observational Hubble parameter data points (\texttt{OHD}); ii) 1048 distance moduli data points (\texttt{SNIa}); iii) a combined dataset (\texttt{OHD+SNIa}); and large scale structure datasets, including iv) 14 growth rate data points (\texttt{f}); and v) 30 redshift space distortion data points (\texttt{f}$σ_8$). These datasets facilitated the constraint of the $ f(R, L_m) $-gravity model via MCMC simulations, followed by a comparative analysis with the $Λ$CDM model. A comprehensive statistical analysis has been conducted to evaluate the $ f(R, L_m) $-gravity model's efficacy in explaining both the accelerated expansion of the universe and the growth of cosmic structures.

astro-ph.CO↗

arXiv2024

A dynamical systems formulation for inhomogeneous LRS-II spacetimes

We present a dynamical system formulation for inhomogeneous LRS-II spacetimes using the covariant 1+1+2 decomposition approach. Our approach describes the LRS-II dynamics from the point of view of a comoving observer. Promoting the covariant radial derivatives of the covariant dynamical quantities to new dynamical variables and utilizing the commutation relation between the covariant temporal and radial derivatives, we were able to construct an autonomous system of first-order ordinary differential equations along with some purely algebraic constraints. Using our dynamical system formulation we found several interesting features in the LRS-II phase space with dust, one of them being that the homogeneous solutions constitute an invariant submanifold. For the particular case of LTB, we were also able to recover the previously known result that an expanding LTB tends to Milne in the absence of a cosmological constant, providing a potential validation of our formalism.

gr-qc↗

arXiv2024

Interacting dark energy: clarifying the cosmological implications and viability conditions

In this study, cosmological models are considered, where dark matter and dark energy are coupled and may exchange energy through non-gravitational interactions with one other. These interacting dark energy (IDE) models have previously been introduced to address problems with the standard $Λ$CDM model of cosmology (which include the coincidence problem, Hubble tension and $S_8$ discrepancy). However, conditions ensuring positive energy densities have often been overlooked. Assuming two different linear dark energy couplings, $Q = δH ρ_{\rm{de}}$ and $Q = δH ρ_{\rm{dm}}$, we find that negative energy densities are inevitable if energy flows from dark matter to dark energy (iDMDE regime) and that consequently, we should only seriously consider models where energy flows from dark energy to dark matter (iDEDM regime). To additionally ensure that these models are free from early time instabilities, we need to require that dark energy is in the `phantom' ($ω<-1$) regime. This has the consequence that model $Q=δH ρ_{\rm{dm}}$ will end with a future big rip singularity, while $Q = δH ρ_{\rm{de}}$ may avoid this fate with the right choice of cosmological parameters.

gr-qc↗

arXiv2024

Cross-correlation between the thermal Sunyaev-Zeldovich effect and the Integrated Sachs-Wolfe effect

We present a joint cosmological analysis of the power spectra measurement of the Planck Compton parameter and the integrated Sachs-Wolfe (ISW) maps. We detect the statistical correlation between the Planck Thermal Sunyaev-Zeldovich (tSZ) map and ISW data with a significance of a $3.6σ$ confidence level~(CL), with the autocorrelation of the Planck tSZ data being measured at a $25 σ$ CL. The joint auto- and cross-power spectra constrain the matter density to be $Ω_{\rm m}= 0.317^{+0.040}_{-0.031}$, the Hubble constant $H_{0}=66.5^{+2.0}_{-1.9}\,{\rm km}\,{\rm s}^{-1}\,{\rm Mpc}^{-1}$ and the rms matter density fluctuations to be $σ_{8}=0.730^{+0.040}_{-0.037}$ at the 68% CL. The derived large-scale structure $S_{8}$ parameter is $S_8 \equiv σ_{8}(Ω_{\rm m}/0.3)^{0.5} = 0.755\pm{0.060} $. If using only the diagonal blocks of covariance matrices, the Hubble constant becomes $H_{0}=69.7^{+2.0}_{-1.5}\,{\rm km}\,{\rm s}^{-1}\,{\rm Mpc}^{-1}$. In addition, we obtain the constraint of the product of the gas bias, gas temperature, and density as $b_{\rm gas} \left(T_{\rm e}/(0.1\,{\rm keV}) \right ) \left(\bar{n}_{\rm e}/1\,{\rm m}^{-3} \right) = 3.09^{+0.320}_{-0.380}$. We find that this constraint leads to an estimate on the electron temperature today as $T_{\rm e}=(2.40^{+0.250}_{-0.300}) \times 10^{6} \,{\rm K}$, consistent with the expected temperature of the warm-hot intergalactic medium. Our studies show that the ISW-tSZ cross-correlation is capable of probing the properties of the large-scale diffuse gas.

astro-ph.CO↗

arXiv2023

Constraining the cosmological model of modified $f(Q)$ gravity: Phantom dark energy and observational insights

Despite the significant accomplishments of general relativity, numerous unresolved issues persist in our understanding of the cosmos. One of the most perplexing challenges is the ongoing accelerated expansion of the Universe, which continues to elude a complete explanation. Consequently, scientists have proposed various alternative theories to GR in pursuit of a deeper understanding. In our analysis, we delve into the recently proposed modified $f(Q)$ gravity, where $Q$ represents the non-metricity scalar responsible for gravitational effects. Specifically, we investigate a cosmological model characterized by the functional form $f(Q) = Q+αQ^n$, where $α$ (with $α\neq 0$) and $n$ serve as free parameters. Utilizing this functional form, we construct our Hubble rate, incorporating a specific equation of state to describe the cosmic fluid. Furthermore, we leverage a dataset consisting of 31 data points from Hubble measurements and an additional 1048 data points from the Pantheon dataset. These data serve as crucial constraints for our model parameters, and we employ the Markov Chain Monte Carlo (MCMC) method to explore the parameter space and derive meaningful results. With our parameter values constrained, our analysis yields several noteworthy findings. The deceleration parameter suggests a recent accelerated phase in the cosmic expansion. In addition, the EoS parameter paints a portrait of dark energy exhibiting phantom-like characteristics. Furthermore, we delve into the application of cosmological diagnostic tools, specifically the statefinder and the $Om(z)$ diagnostics. Both of these tools align with our previous conclusions, confirming the phantom-like behavior exhibited by our cosmological model. These results collectively contribute to our understanding of the dynamic interplay between gravity, dark energy, and the expanding cosmos.

astro-ph.CO↗

arXiv2023

Confronting the Chaplygin gas with data: background and perturbed cosmic dynamics

In this paper, we undertake a unified study of background dynamics and cosmological perturbations in the presence of the Chaplygin gas. This is done by first constraining the background cosmological parameters of different Chaplygin gas models with SNIa and $H(z)$ data for detailed statistical analysis of the CG models. Based on the statistical criteria we followed, none of the models has substantial observational support, but we show that the so-called `original' and `extended/generalised' Chaplygin gas models have {\it some observational support} and {\it less observational support}, respectively, whereas the `modified and `modified generalised Chaplygin gas models miss out on the category {\it less observational support}, but cannot be ruled out. The so-called `generalised cosmic Chaplygin gas model, on the other hand, falls under the {\it no observational support} category of the statistical criterion and can be ruled out. The models which are statistically accepted are considered for perturbation level in both theoretical and observational aspects. We also apply the $1+3$ covariant formalism of perturbation theory and derive the evolution equations of the fluctuations in the matter density contrast of the matter-Chaplygin gas system for the models with some or less statistical support. The solutions to these coupled systems of equations are then computed in both short-wavelength and long-wavelength modes. Then feed these observationally restricted parameters into the analysis of cosmological perturbations {to address the growth of density contrast through redshift}. Using the most recent linear growth of the data $f_{σ8}$, CG models are considered to study the linear growth of the structure.

gr-qc↗

arXiv2023

Constrained evolution of effective equation of state parameter in non-linear $f(R, L_m)$ dark energy model: Insights from Bayesian analysis of cosmic chronometers and Pantheon samples

We conduct a Bayesian analysis of recent observational datasets, specifically the Cosmic Chronometers (CC) dataset and Pantheon samples, to investigate the evolution of the EoS parameter in dark energy models. Our study focused on the effective EoS parameter, which is described by the parametric form $ω_{eff}=-\frac{1}{1+m(1+z)^n}$, where $m$ and $n$ are model parameters. This parametric form is applicable within the framework of $f(R,L_m)$ gravity, where $R$ represents the Ricci scalar and $L_m$ is the matter Lagrangian. Here, we examine a non-linear $f(R,L_m)$ model characterized by the functional form $f(R,L_m)=\frac{R}{2}+L_m^α$, where $α$ is the free parameter of the model. We examine the evolution of several cosmological parameters, including the effective EoS parameter $ω_{eff}$, the deceleration parameter $q$, the density parameter $ρ$, the pressure $p$, and the statefinder parameters. Our analysis revealed that the constrained current value of the effective EoS parameter, $ω_{eff}^{0}=-0.68\pm0.06$ for both the CC and Pantheon datasets, points towards a quintessence phase. Moreover, at redshift $z=0$, the deceleration parameter, $q_0 = -0.61^{+0.01}_{-0.01}$, indicates that the present Universe is undergoing accelerated expansion.

astro-ph.CO↗

arXiv2023

CoLFI: Cosmological Likelihood-free Inference with Neural Density Estimators

In previous works, we proposed to estimate cosmological parameters with the artificial neural network (ANN) and the mixture density network (MDN). In this work, we propose an improved method called the mixture neural network (MNN) to achieve parameter estimation by combining ANN and MDN, which can overcome shortcomings of the ANN and MDN methods. Besides, we propose sampling parameters in a hyper-ellipsoid for the generation of the training set, which makes the parameter estimation more efficient. A high-fidelity posterior distribution can be obtained using $\mathcal{O}(10^2)$ forward simulation samples. In addition, we develop a code-named CoLFI for parameter estimation, which incorporates the advantages of MNN, ANN, and MDN, and is suitable for any parameter estimation of complicated models in a wide range of scientific fields. CoLFI provides a more efficient way for parameter estimation, especially for cases where the likelihood function is intractable or cosmological models are complex and resource-consuming. It can learn the conditional probability density $p(\boldsymbolθ|\boldsymbol{d})$ using samples generated by models, and the posterior distribution $p(\boldsymbolθ|\boldsymbol{d}_0)$ can be obtained for a given observational data $\boldsymbol{d}_0$. We tested the MNN using power spectra of the cosmic microwave background and Type Ia supernovae and obtained almost the same result as the Markov Chain Monte Carlo method. The numerical difference only exists at the level of $\mathcal{O}(10^{-2}σ)$. The method can be extended to higher-dimensional data.

astro-ph.CO↗

arXiv2022

Imprint of f(R) gravity in the cosmic magnification

f(R) gravity is one of the simplest viable modifications to General Relativity: it passes local astrophysical tests, predicts both the early-time cosmic inflation and the late-time cosmic acceleration, and also describes dark matter. In this paper, we probe cosmic magnification on large scales in f(R) gravity, using the well-known Hu-Sawicki model as an example. Our results indicate that at redshifts z < 3, values of the model exponent n > 1 lead to inconsistent behaviour in the evolution of scalar perturbations. Moreover, when relativistic effects are taken into account in the large scale analysis, our results show that as z increases, large-scale changes in the cosmic magnification angular power spectrum owing to integral values of n tend to share a similar pattern, while those of decimal values tend to share another. This feature could be searched for in the experimental data, as a potential "smoking gun" for the given class of gravity models. Furthermore, we found that at z = 1 and lower, relativistic effects lead to a suppression of the cosmic magnification on large scales in f(R) gravity, relative to the concordance model; whereas, at z > 1, relativistic effects lead to a relative boost of the cosmic magnification. In general, relativistic effects enhance the potential of the cosmic magnification as a cosmological probe.

astro-ph.CO↗

arXiv2022

Perturbations in the interacting vacuum

In this study, we present the evolution of cosmological perturbations in a universe consisting of standard matter and interacting vacuum. We use the $1 + 3$ covariant formalism in perturbation framework and consider two different models for the interacting vacuum namely a linear interacting model and interaction with creation pressure model. For both models, we derive the evolution equations governing the growth of linear perturbations for both radiation and dust dominated Universe. We find numerical solutions in appropriate limits namely long and short wavelengths. For both models, the perturbations grow with time (decay with redshift), showing that structure formation is possible in an accelerated cosmic background. The perturbation amplitudes and their relative scalings with those of standard model of cosmology depend on the values of the interaction parameters considered, and in a way that can be used to constrain the models using existing and future large scale structure data. In the vanishing limits of the coupling parameters of the interaction, we show that standard cosmology, both background and perturbed, is recovered.

gr-qc↗

arXiv2022

A cosmological model with time dependent $Λ$, $G$ and viscous fluid in General Relativity

In this paper, we investigate Bianchi type$-V$ cosmological models with bulk viscous fluid and time varying cosmological $Λ$ and Newtonian $G$ parameters. The Einstein's field equations have been transformed into a coupling non-linear, first-order differential equations, and the fourth-order Runge-Kutta method of numerical integration has been used to integrate the differential equations with appropriate initial conditions consistent with current cosmological observations. We show that the model describes a universe that starts off with a negative cosmological term, as well as a matter-dominated and decelerated early epoch that, eventually becomes $Λ$-dominated and expanding with acceleration, in concordance with current observations.

physics.gen-ph↗

arXiv2022

Large-scale structure power spectrum from scalar-tensor gravity

This work deals with the computation of the power spectrum of large-scale structure using the dynamical system approach for a multi-fluid universe in scalar-tensor theory of gravity. We use the $1+3$ covariant approach to obtain evolution equations and study the behavior of the matter power spectrum of perturbation equations. The study is based on the equivalence between $f(R)$ theory of gravity and scalar-tensor theory of gravity. We find that, for power-law $(R^{n})$ models, with $1<n<1.3$, we have the power spectrum evolving above general relativistic scale-invariant line. For $n\geq 1.3$, the power spectrum starts with constant amplitude then it experiences oscillations and eventually saturates at finite amplitude. Such behavior is consistent with other observations in the literature. The result supports the ongoing investigations of the equivalence between $f(R)$ and scalar-tensor theory at linear order.

gr-qc↗

arXiv2022

Dark-fluid constraints of shear-free universes

Integrability conditions arising from general irrotational fluid-flow considerations of a universe dominated by cosmic dark fluids will be investigated under special assumptions on the nature of the spacetime shear. Special emphasis will be placed on linearized perturbations of quasi-Newtonian and anti-Newtonian spacetimes, whereby the conditions for the existence and consistent evolution of such spacetimes in the presence of the Chaplygin gas fluid model will be derived and discussed.

gr-qc↗

arXiv2022

Activity Report of the Second African Conference on Fundamental and Applied Physics, ACP2021

The African School of Fundamental Physics and Applications, also known as the African School of Physics (ASP), was initiated in 2010, as a three-week biennial event, to offer additional training in fundamental and applied physics to African students with a minimum of three-year university education. Since its inception, ASP has grown to be much more than a school. ASP has become a series of activities and events with directed ethos towards physics as an engine for development in Africa. One such activity of ASP is the African Conference on Fundamental and Applied Physics (ACP). The first edition of ACP took place during the 2018 edition of ASP at the University of Namibia in Windhoek. In this paper, we report on the second edition of ACP, organized on March 7--11, 2022, as a virtual event.

physics.ed-ph↗

arXiv2021

Thermodynamic geometry of static and rotating regular black holes in conformal massive gravity

A version of massive gravity, namely conformal massive gravity, is employed to study the behavior of thermodynamic geometry for both the static and the rotating regular black holes. Whereas in thermodynamic geometry singularity can be interpreted as the second phase transition, seeing such phenomena as heat capacity behavior for both cases is investigated in detail. In addition, other thermodynamic quantities like the entropy and the temperature are analyzed as well. Another important property of the black holes is the so-called stability, in which utilizing the first phase transition of the heat capacity is detailed, for both cases, say the static and the spinning black holes. It is also interestingly figured out that, the behavior of temperature of the uncharged black holes in the presence of hair parameter, $λ$, can mimic the charged ones. {The effects of scalar charge, $Q$, and hair parameters with both positive and negative signs and how it affects key parameters in the study of black holes are investigated as well.} To investigate the properties of the black holes both regular thermodynamics and thermodynamic geometry approaches are considered. Then one can observe that aiming to realize the physics of the black holes, many complicated phenomena can be analyzed dramatically easier by considering the latter approach.

gr-qc↗

arXiv2021

The Rényi holographic dark energy model in Chern-Simons gravity: some cosmological implications

In this paper, we study Rényi holographic dark energy model in the context of Chern Simons (CS) modified gravity theory. Different cosmological parameters such as energy density, deceleration parameter, equation of state, square of sound speed and cosmological plane are discussed using FRW metric. Two separate solutions of CS field equations arose. The Rényi HDE model showed the transition from deceleration to acceleration phase which is fully consistent with the observational data while in the second case it represented a decelerated phase of expansion only. In fact, EoS illustrates the era of dominance of the universe by certain components. Our results predicted that the universe is under the influence of dark energy as EoS showed accelerated expansion phase. On the other hand case two advocated the influence of $Λ$ CDM. In both cases, $ω<0,ω^{\prime}<0$ indicated that the Rényi HDE model is in the freezing region and cosmic expansion is more accelerating in the context of CS modified gravity. It is also observed that the value of $ν^2_s$ is positive $ \forall z\in \Re $ a sufficient condition for the stability of the system. Hence it is concluded that the Rényi HDE model is supported by the results of general relativity in the framework of CS modified gravity.

gr-qc↗

arXiv2021

Cosmic hierarchy in $f(R)$ gravity

In this contribution, we investigate hierarchical nature of large-scale structure clustering through the oscillatory nature of the solutions of the Schrödinger-like Friedmann equation in a modified gravitational background described by the $f(R)$ gravity theory. We find the cosmological solutions to the Schrödinger equation for different ranges of $n$ of the $R^{n}$ toy model for both radiation and matter-dominated epochs of the expansion history for open, flat, and closed spacetimes. Our results show that, for certain choices of the model parameters and initial conditions, the formation and distribution of cosmic structures might indeed be hierarchical, leading to a natural explanation for the breakdown of the cosmological principle on small scales.

gr-qc↗