arXiv subjects

Shambel Sahlu

Publications and source records attributed to Shambel Sahlu.

26 records · Page 2Linked to original sources

arXiv2022

Linear Cosmological perturbations in $f(Q)$ Gravity

In this manuscript, we studied the accelerated expansion history of the universe and the formations of large-scale structures using $f(Q)$ gravity model. The expansion rate of the universe within distance modulus and redshift has been performed in $f(Q)$ gravity. This work is also devoted to investigating the linear cosmological perturbations in the $f(Q)$ gravity model using the $1 + 3$ covariant formalism. The scalar and harmonic decomposition techniques are applied to find the evolution equation. The growth of matter density contrasts is analyzed with redshift $z$. Using the nonmetricity modified gravity model which is $f (Q) = Q+αQ^n$, the accelerating expanding universe and the formations of the large-scale structures have been explained. The quasi-static approximation technique is applied to analyze the growth of matter contrasts. The growth of density contrast is studied within the dust and radiation-dominated universe in the presence of nonmetricity gravity.

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arXiv2021

Covariant density and velocity perturbations of the quasi-Newtonian cosmological model in $f(T)$ gravity

We investigate classes of shear-free cosmological dust models with irrotational fluid flows within the framework of $f(T)$ gravity. In particular, we use the $1 + 3$ covariant formalism and present the covariant linearised evolution and constraint equations describing such models. We then derive the integrability conditions describing a consistent evolution of the linearised field equations of these quasi-Newtonian universes in the $f(T)$ gravitational theory. Finally, we derive the evolution equations for the density and velocity perturbations of the quasi-Newtonian universe. We explore the behaviour of the matter density contrast for two models - $f(T)= μT_{0}(T/T_{0})^{n}$ and the more generalised case, where $f(T)= T+ μT_{0} (T/T_{0})^{n}$, with and without the application of the quasi-static approximation. Our numerical solutions show that these $f(T)$ theories can be suitable alternatives to study the background dynamics, whereas the growth of energy density fluctuations change dramatically from the expected $Λ$CDM behaviour even for small deviations away from the general relativistic limits of the underlying $f(T)$ theory. Moreover, applying the so-called quasi-static approximation yields exact-solution results that are orders of magnitude different from the numerically integrated solutions of the full system, suggesting that these approximations are not applicable here.

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arXiv2020

On multifluid perturbations in scalar-tensor cosmology

In this paper the scalar-tensor theory is applied to the study of perturbations in a multi-fluid universe, using the 1+3 covariant approach. Both scalar and harmonic decompositions are instituted on the perturbation equations. In particular, as an application, we study perturbations on a background FRW cosmology consisting of both radiation and dust in the presence of a scalar field. We consider both radiation-dominated and dust-dominated epochs, respectively, and study the results. During the analysis, quasi-static approximation is instituted. It is observed that the fluctuations of the energy density decrease with increasing redshift, for different values of $n$ of a power law $R^{n}$ model

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arXiv2020

Perturbations in a Chaplygin gas Cosmology

In this paper we study the perturbations of a cosmic multi-fluid medium consisting of radiation, dust and a Chaplygin gas. To do so, we follow the 1 + 3 covariant formalism and derive the evolution equations of the fluctuations in the energy density for each species of fluid in the multi-fluid system. The solutions to these coupled systems of equations are then computed in both short-wavelength and long-wavelength modes. Our preliminary results suggest that unlike most dark energy models that discourage large-scale structure formation due to the rapid cosmological expansion (which gives little time for fluctuations to coalesce), the Chaplygin-gas model supports the formation of cosmic structures. This is manifested in the solutions of the perturbation equations which show the growth of density fluctuations over time.

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arXiv2020

Scalar perturbations in $f(T)$ gravity using the $1+3$ covariant approach

The cosmological scalar perturbations of standard matter are investigated in the context of extended teleparallel $f(T)$ gravity theories using the $1+3$ covariant formalism. After a review of the background, gravitational field equations of $f(T)$ gravity and the introduction of the covariant perturbation variables, the usual scalar and harmonic decomposition have been performed, and the analysis of the growth of the density contrasts in the quasi-static approximation for two non-interacting fluids scenarios, namely torsion-dust and torsion-radiation mixtures is presented for the generic $f(T)$ gravity theory. Special applications to two classes of $f(T)$ gravity toy models, namely $f(T) = μT_0 (T/T_0)^n$ and $f(T) = T + μT_0 (-T/T_0)^n$, have then been made within the observationally viable regions of their respective parameter spaces, and the growth of the matter density contrast for both torsion-dust and torsion-radiation epochs of the Universe has been examined. The exact solutions of the dust perturbations, with growing amplitudes in cosmic time, are obtained for some limiting cases of n. Similarly, the long- and short-wavelength modes in the torsion-radiation case are treated, with the amplitudes either oscillating or monotonically growing with time. Overall, it is noted that $f(T)$ models contain a richer set of observationally viable structure growth scenarios that can be tested against up- and-coming observational data and can accommodate currently known features of the large-scale structure power spectrum in the general relativistic and $Λ$CDM limits.

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arXiv2020

Accelerating universe in modified teleparallel gravity theory

This paper studies the cosmology of accelerating expansion of the universe in modified teleparallel gravity theory. We discuss the cosmology of $f(T,B)$ gravity theory and its implication to the new general form of the equation of state parameter $w_{TB}$ for explaining the late-time accelerating expansion of the universe without the need for the cosmological constant scenario. We examine the numerical value of $w_{TB}$ in different paradigmatic $f(T,B)$ gravity models. In those models, the numerical result of $w_{TB}$ is favored with observations in the presence of the torsion scalar T associated with a boundary term B and shows the accelerating expansion of the universe.

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arXiv2019

The Chaplygin gas as a model for modified teleparallel gravity?

This paper explores the possibility of treating the exotic Chaplygin-gas (CG) fluid model as some manifestation of an $f(T)$ gravitation. To this end, we use the different cosmological CG equations of state, compare them with the equation of state for the modified teleparallel gravity and reconstruct the corresponding Lagrangian densities. We then explicitly derive the equation of state parameter of the torsion fluid $w_T$ and study its evolution for vacuum-torsion, radiation-torsion, dust-torsion, stiff fluid torsion, and radiation-dust-torsion multi-fluid systems. The obtained Lagrangians have, in general, matter dependence due to the matter-torsion coupling appearing in the energy density and pressure terms of the modified teleparallel gravity theory. For the simplest CG models, however, it is possible to reconstruct $f(T)$ Lagrangians that depend explicitly on the torsion scalar $T$ only. The preliminary results show that, in addition to providing Chaplygin-gas-like solutions to the modified teleparallel gravitation, which naturally behave like dark matter and dark energy at early and late times respectively, the technique can be used to overcome some of the challenges attributed to the CG cosmological alternative.

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arXiv2018

Inflation constraints for classes of f(R) models

In this paper, we explore the equivalence between two theories, namely f(R) and scalar-tensor theories of gravity. We use this equivalence to explore several f(R) toy models focusing on the inflation epoch of the early universe. The study is done based on the definition of the scalar field in terms of the first derivative of f(R) model. We have applied the slow-roll approximations during inflationary parameters consideration. The comparison of the numerically computed inflationary parameters with the observations is done. We have inspected that some of the f(R) models produce numerical values of $n_{s}$ that are in the same range as the suggested values from observations. But for the case of the tensor-to-scalar ratio $r$, we realized that some of the considered f(R) models suffer to produce a value which is in agreement with the observed values for different considered space parameter.

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