Papers by albert munyeshyaka
Perturbations with Bulk Viscosity in Modified Chaplygin Gas Cosmology
The European Physical Journal C, Jan 17, 2024
We investigate cosmological perturbations of f (G) gravity in the presence of a scalar field. Usi... more 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 longwavelength 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 longwavelength modes, the perturbations which include amplitude effects due to the f (G) models with a scalar field do differ remarkably from those in CDM. The results reduce to GR results in the limit of f (G) → G and in the absence of scalar field.
On 1 + 3 covariant perturbations of the quasi-Newtonian space-time in modified Gauss-Bonnet gravity
arXiv (Cornell University), May 2, 2023
International Journal of Geometric Methods in Modern Physics, Nov 7, 2022
The treatment of 1 + 3 covariant perturbation in a multifluid cosmology with the consideration of... more 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 aproximation 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 shortand 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.
International Journal of Geometric Methods in Modern Physics, Jun 16, 2022
The equivalence between f (R) and scalar-tensor theories is revisited, we consequently explored d... more 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.
International Journal of Modern Physics D
This work treats cosmological perturbation in a mixture of standard matter, Chaplygin gas as well... more This 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 [Formula: see text] 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 [Formula: see text] 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.
International Journal of Modern Physics D
The consideration of a [Formula: see text] covariant approach to cold dark matter universe with n... more The consideration of a [Formula: see text] covariant approach to cold dark matter universe with no shear cosmological dust model with irrotational flows is developed in the context of [Formula: see text] gravity theory in this study. This approach reveals the existence of integrability conditions which do not appear in noncovariant treatments. We constructed the integrability conditions in modified Gauss–Bonnet [Formula: see text] gravity basing on the constraints and propagation equations. These integrability conditions reveal the linearized silent nature of quasi-Newtonian models in [Formula: see text] gravity. Finally, the linear equations for the overdensity and velocity perturbations of the quasi-Newtonian spacetime were constructed in the context of modified [Formula: see text] gravity. The application of harmonic decomposition and redshift transformation techniques to explore the behavior of the overdensity and velocity perturbations using [Formula: see text] model was made. ...
In this study, we present the evolution of cosmological perturbations in a universe consisting of... more 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.
arXiv (Cornell University), Sep 12, 2022
The treatment of 1 + 3 covariant perturbation in a multifluid cosmology with the consideration of... more 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 aproximation 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 shortand 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.
International Journal of Geometric Methods in Modern Physics
The treatment of (1+3)-covariant perturbation in a multifluid cosmology with the consideration of... more The treatment of (1+3)-covariant perturbation in a multifluid cosmology with the consideration of [Formula: see text] gravity, [Formula: see text] being the Gauss–Bonnet term, is done in this 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 a 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 [Formula: see text] model, we get numerical solutions of energy density perturbations and show that they decay with increase in redshift. This feature shows that under ...
International Journal of Geometric Methods in Modern Physics
The equivalence between [Formula: see text] and scalar–tensor theories is revisited, we consequen... more The equivalence between [Formula: see text] and scalar–tensor theories is revisited, we consequently explored different [Formula: see text] models. After consideration of specific definition of the scalar field, we derived the potentials [Formula: see text] for each [Formula: see text] model focusing on the early Universe, mostly the inflation epoch. For a given potential, we applied the slow-roll approximation approach to each [Formula: see text] model and obtained the expressions for the spectral index [Formula: see text] and tensor-to-scalar ratio [Formula: see text]. We determined the corresponding numerical values associated with each of the [Formula: see text] models. Our results showed that for certain choice of parameter space, the values of [Formula: see text] and [Formula: see text] 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 (KGEs...
International Journal of Geometric Methods in Modern Physics
In this study, we present the evolution of cosmological perturbations in a universe consisting of... more 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 [Formula: see text]CDM — 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....
Cosmological perturbations in f(G) gravity
International Journal of Modern Physics D
We explore cosmological perturbations in a modified Gauss–Bonnet [Formula: see text] gravity, usi... more We explore cosmological perturbations in a modified Gauss–Bonnet [Formula: see text] gravity, using a [Formula: see text] 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 Gauss–Bonnet field-radiation system for three different pedagogical [Formula: see text] models: trigonometric, exponential and logarithmic. For the Gauss–Bonnet field-dust system, energy-density perturbations decay with increase in redshift for all the three models. For the Gauss–Bonnet field-radiation system, the energy-density perturbations decay with increase in re...
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Papers by albert munyeshyaka