arXiv subjects

Albert Munyeshyaka

Publications and source records attributed to Albert Munyeshyaka.

15 recordsLinked to original sources

arXiv2026

Cosmic Hysteresis in Reconstructed $f(T)$ Bounce Models A Torsion-Based Thermodynamic Perspective

We investigate the emergence of cosmic hysteresis in cyclic and bouncing cosmologies within the framework of reconstructed $f(T)$ gravity. In contrast to curvature-based modifications of General Relativity, teleparallel gravity attributes gravitation to spacetime torsion encoded in the torsion scalar $T$. By reconstructing viable $f(T)$ functions corresponding to analytically prescribed nonsingular bouncing scale factors and coupling the geometry to a minimally interacting canonical scalar field, we demonstrate that asymmetric scalar field dynamics between expansion and contraction phases give rise to a non-vanishing thermodynamic work integral $\oint p_ϕ\, dV$ over complete cycles. This hysteresis manifests as closed loops in the $(w_ϕ,a)$ plane, signifying thermodynamic memory and irreversibility. We derive the modified Friedmann equations, establish exact bounce and turnaround conditions, and discuss the implications of torsion-induced hysteresis for the cosmological arrow of time. Our results confirm that cosmic hysteresis is a generic feature of cyclic universes in modified gravity, extending beyond curvature-based theories.

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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.

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arXiv2025

Constraints on multi-fluid cosmology in modified Gauss-Bonnet gravity models with different observational data sets

In the present work, we incorporate redshift-space distortion measurement to investigate the growth of large scale structure within the framework of multi-fluid cosmology in the context of modified Gauss-Bonnet gravity. Using three different modified Gauss-Bonnet gravity models, we compare the predictions of modified Gauss-Bonnet gravity expansion history-through the Friedmann equation with Hubble and BAO data sets and constrain models parameters. Within the context of multi-fluid cosmology in modified Gauss-Bonnet gravity, we obtain the structure growth equation. This equation is then combined with Sigma_8 to get f_Sigma_8 predictions-which is compared with redshift-space distortion data to constrain models parameters to obtain best-fit values including Sigma_8. This involves performing a Markov Chain Monte Carlo (MCMC) analysis for these specific forms of modified Gauss-Bonnet models.

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arXiv2025

Cosmic Hysteresis in Reconstructed $f(R)$ Bounce Models: A Thermodynamic Study

We study the emergence of cosmic hysteresis in cyclic bouncing universes within the framework of analytically reconstructed $f(R)$ gravity. Using exact bouncing scale factor solutions of exponential and power-law forms, we reconstruct the corresponding $f(R)$ models and investigate the thermodynamic behavior of a minimally coupled scalar field in these geometries. The pressure evolution during expansion and contraction phases is shown to be asymmetric, leading to a non-vanishing thermodynamic work integral over each cycle, defined by $\oint p_ϕ\, dV$. We identify closed hysteresis loops in the equation-of-state space and quantify the net energy transfer per cycle. Our results reveal that such reconstructed $f(R)$ models generically support irreversible evolution, demonstrating a natural emergence of the thermodynamic arrow of time. These findings provide new insight into the dissipative features of modified gravity and the long-term dynamics of cyclic cosmological scenarios.

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arXiv2025

Testing Gauss-Bonnet Gravity with DESI BAO Data

In the present paper, we observationally constrain f (G) gravity at the background level using Type Ia supernovae from the Pantheon Plus (PP) sample, cosmic chronometer (CC) data, and the recent Baryon Acoustic Oscillation (BAO) measurements released by DESI. For the analysis, we consider two combinations of datasets: (i) PP + CC, and (ii) PP + CC + DESI BAO. In both cases, we determine the best-fit parameters by numerically solving the modified Friedmann equations for two distinct f (G) models, namely the power-law and exponential forms. This is achieved through Markov Chain Monte Carlo (MCMC) simulations. To assess the statistical significance of the f (G) models, we employ both the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Our results show that both f (G) models are statistically favored over the standard ΛCDM model. Notably, the exponential model exhibits an additional future transition at redshift closer to -0.1, indicating a possible return to a decelerating phase. This distinctive behavior sets it apart from both the power-law model and the ΛCDM scenario, which predict continued acceleration into the future.

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arXiv2025

Matter power spectrum in a power-law $f(G)$ gravity

Cosmological models based on $f(G)$ gravity are efficient in fitting different observational datasets at both background and perturbation levels. This motivates the current study to take into account dynamical system analysis to investigate the matter power spectrum within the framework of modified Gauss-Bonnet gravity. After defining the dimensionless dynamical system variables for a power-law $f(G)$ model, We derive the full system of equations governing the energy density perturbations for both matter and Gauss-Bonnet fluids using the $1+3$ covariant formalism. After solving the energy density perturbation equations, we compute the matter power spectrum. The importance of studying first order perturbations for the defined $f(G)$ model and the relevance of different initial conditions in computing the matter power spectrum are also stressed. It is reported that matter power spectrum for $f(G)$ gravity, for a particular functional form of $f(G)$ model considered is not scale invariant as the case for General Relativity.

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arXiv2025

Observational analysis of bulk viscous modified Chaplygin gas in (2+1)-dimensional universe using MCMC

This paper investigates regarding cosmological implications of a bulk viscous modified Chaplygin gas (MCG) in (2+1)-dimensional Friedmann-Robertson-Walker spacetime, incorporating both theoretical analysis and observational constraints. We derive analytical solutions for both viscous and non-viscous cases, revealing distinct behavior in energy density evolution, Hubble parameter dynamics, and deceleration parameter transitions. A comprehensive perturbation analysis illustrates how bulk viscosity dampens the structure growth oscillations, addressing a key challenge faced by Chaplygin gas models in higher dimensions. Using Markov chain Monte Carlo (MCMC) techniques with Hubble parameter and Pantheon supernova datasets, we impose constraints on our model parameters, obtaining $H_0 = 67.90$ km s$^{-1}$ Mpc$^{-1}$, showing remarkable consistency with Planck $Λ$CDM estimations despite the dimensional reduction. Our findings suggest that lower-dimensional viscous cosmology captures essential features of cosmic evolution while providing valuable theoretical insights into the interplay between dissipative effects and exotic equations of state.

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arXiv2024

Perturbations with bulk viscosity in modified chaplygin gas cosmology

In the present work, we investigate cosmological perturbations of viscous modified chaplygin gas model. Using 1 + 3 covariant formalism, we define covariant and gauge invariant gradient variables, which after the application of scalar decomposition and harmonic decomposition techniques together with redshift transformation method, provide the energy overdensity perturbation equations in redshift space, responsible for large scale structure formation. In order to analyse the effect of the viscous modified chaplygin gas model on matter overdensity contrast, we numerically solve the perturbation equations in both long and short wavelength limits. The numerical results show that the energy overdensity contrast decays with redshift. However, the perturbations which include amplitude effects due to the viscous modified chaplygin model do differ remarkably from those in the ΛCDM. In the absence of viscous modified chaplygin model, the results reduce to those of ΛCDM.

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arXiv2023

On covariant perturbations with scalar field in modified Gauss-Bonnet gravity

We investigate cosmological perturbations of f(G) gravity in the presence of a scalar field. Using the 1 + 3 covariant formalism, we present the energy overdensity perturbation equations responsible for large scale structure formation. After applying harmonic decomposition method together with the redshift transformation technique, we obtain the fully perturbed equations in redshift space. The equations are solved to study the growth of matter overdensities contrast with redshift. For both short- and long-wavelength modes, we obtain numerical results for particular functional form f(G) models and scalar field. We find that, for this choice the energy overdensity perturbations decay with increase in redshift. However, for both short- and long- wavelength modes, the perturbations which include amplitude effects due to the f(G) models with a scalar field do differ remarkably from those in Lambda CDM. The results reduce to GR results in the limit of f(G) tends to G and in the absence of scalar field.

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arXiv2023

On Chaplygin models in f(G) gravity

The current work treats cosmological perturbation in a mixture of standard matter, Chaplygin gas as well as Gauss-bonnet fluids using a 1+3 covariant approach in the context of modified $f(G)$ gravity. We define the gradient variables to obtain linear perturbation equations. After scalar and redshift transformations, we consider both an original Chaplygin and generalized Chaplygin gas models under Gauss-bonnet gravity. For pedagogical purposes, the consideration of polynomial $f(G)$ gravity model was used to solve the perturbation equations for short- and long- wavelength modes and investigate the late time evolution. The numerical solutions were obtained. The results show that the energy overdensity perturbations decay with an increase in redshift. The treatment recovers GR results under limiting cases.

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arXiv2023

On 1 + 3 covariant perturbations of the quasi-Newtonian space-time in modified Gauss-Bonnet gravity

The consideration of a 1 + 3 covariant approach to cold dark matter universe with no shear cosmological dust model with irrotational flows is developed in the context of f (G) gravity theory in the present study. This approach reveals the existence of integrability conditions which do not appear in non-covariant treatments. We constructed the integrability conditions in modified Gauss-Bonnet f (G) gravity basing on the constraints and propagation equations. These integrability conditions reveal the linearized silent nature of quasi-Newtonian models in f (G) gravity. Finally, the linear equations for the overdensity and velocity perturbations of the quasi-Newtonian space-time were constructed in the context of modified f (G) gravity. The application of harmonic decomposition and redshift transformation techniques to explore the behaviour of the overdensity and velocity perturbations using f (G) model were made. On the other hand we applied the quasi-static approximation to study the approximated solutions on small scales which helps to get both analytical and numerical results of the perturbation equations. The analysis of the energy overdensity and velocity perturbations for both short and long wavelength modes in a dust-Gauss-Bonnet fluids were done and we see that both energy overdensity and velocity perturbations decay with redshift for both modes. In the limits to ΛCDM , it means f (G) = G the considered f (G) model results coincide with ΛCDM .

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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.

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arXiv2022

Multifluid cosmology in f (G) gravity

The treatment of 1 + 3 covariant perturbation in a multifluid cosmology with the consideration of f (G) gravity, G being the Gauss-Bonnet term, is done in the present paper. We define a set of covariant and gauge-invariant variables to describe density, velocity and entropy perturbations for both the total matter and component fluids. We then use different techniques such as scalar decomposition, harmonic decomposition, quasi-static approximation together with the redshift transformation to get simplified perturbation equations for analysis. We then discuss number of interesting applications like the case where the universe is filled with a mixture of radiation and Gauss-Bonnet fluids as well as dust with Gauss-Bonnet fluids for both short- and long-wavelength limits. Considering polynomial f (G) model, we get numerical solutions of energy density perturbations and show that they decay with increase in redshift. This feature shows that under f (G) gravity, specifically under the considered f (G) model, one expects that the formation of the structure in the late Universe is enhanced.

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arXiv2022

On inflationary parameters in scalar-tensor theories

The equivalence between $f(R)$ and scalar-tensor theories is revisited, we consequently explored different $f(R)$ models. After consideration of specific definition of the scalar field, we derived the potentials $V(ϕ)$ for each $f(R)$ model focusing on the early Universe, mostly the inflation epoch. For a given potential, we applied the slow-roll approximation approach to each $f(R)$ model and obtained the expressions for the spectral index $n_{s}$ and tensor-to-scalar ratio $r$. We determined the corresponding numerical values associated with each of the $f(R)$ models. Our results showed that for certain choice of parameter space, the values of $n_{s}$ and $r$ are consistent with the Planck survey results and others produce numerical values that are in the same range as suggested by Planck data. We further constructed the Klein-Gordon equations $(KGE)$ of each $f(R)$ model. We found numerical solutions to each KGE considering different values of free parameters and initial conditions of each $f(R)$ model. All models showed that the scalar field decreases as time increases, an indication that there is less content of the scalar field in the late Universe.

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arXiv2021

Cosmological perturbations in f(G) gravity

We explore cosmological perturbations in a modified Gauss-Bonnet f(G) gravity, using a 1+3 covariant formalism. In such a formalism, we define gradient variables to get perturbed linear evolution equations. We transform these linear evolution equations into ordinary differential equations using a spherical harmonic decomposition method. The obtained ordinary differential equations are time-dependent and then transformed into redshift dependent. After these transformations, we analyze energy-density perturbations for two-fluid systems, namely for a Gauss-Bonnet field-dust system and for a Gaus-Bonnet field-radiation system for three different pedagogical f(G) models: trigonometric, exponential, and logarithmic. For the Gauss-Bonnet field-dust system, energy-density perturbations decay with an increase in redshift for all three models. For the Gauss-Bonnet field-radiation system, the energy-density perturbations decay with an increase in redshift for all of the three f(G) models for long-wavelength modes whereas for short-wavelength modes, the energy-density perturbations decay with increasing redshift for the logarithmic and exponential f(G) models and oscillate with decreasing amplitude for the trigonometric f(G) model.

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