arXiv subjects

Bita Farsi

Publications and source records attributed to Bita Farsi.

4 recordsLinked to original sources

arXiv2023

Growth of perturbations in higher dimensional Gauss-Bonnet FRW cosmology

We explore the influences of the higher order Gauss Bonnet (GB) correction terms on the growth of perturbations at the early stage of a (n + 1)-dimensional Friedmann-Robertson-Walker (FRW) universe. Considering a cosmological constant in the FRW background, we study the linear perturbations by adopting the spherically symmetric collapse (SC) formalism. In light of the modifications that appear in the field equations, we disclose the role of the GB coupling constant α, as well as the extra dimensions n > 3 on the growth of perturbations. It, in essence, is done by defining a dimensionless parameter \{beta}=(n-2)(n-3) \{alpha} H_0^2 in which H_0 is the Hubble constant. We find that the matter density contrast starts growing at the early stages of the universe and, as the universe expands, it grows faster compared to the standard cosmology. Besides, in the framework of GB gravity, the growth of matter perturbations in higher dimensions is faster than its standard counterpart (n = 3). Further, in the presence of α, the growth of perturbations increases as it increases. This is an expected result, since the higher order GB correction terms increase the strength of the gravity and thus support the growth of perturbations. For the existing cosmological model, we also investigate the behavior of quantities such as density abundance, deceleration, and the jerk parameter. Finally, we study the imprint of the GB parameter and the higher dimensions in the evolution of the mass function of the dark matter halos.

gr-qc↗

arXiv2023

Evolution of Spherical Overdensities in Energy-Momentum-Squared Gravity

Employing the spherical collapse (SC) formalism, we investigate the linear evolution of the matter over-density for energy-momentum-squared gravity (EMSG), which in practical phenomenological terms, one may imagine as an extension of the ΛCDM model of cosmology. The underlying model, while still having a cosmological constant, is a non-linear material extension of the general theory of relativity (GTR) and includes correction terms that are dominant in the high-energy regime, the early universe. Considering the Friedman{Robertson{Walker (FRW) background in the presence of a cosmological constant, we find the effects of the modifications arising from EMSG on the growth of perturbations at the early stages of the universe. Considering both possible negative and positive values of the model parameter of EMSG, we discuss its role in the evolution of the matter density contrast and growth function in the level of linear perturbations. While EMSG leaves imprints distinguishable from ΛCDM, we find that the negative range of the ESMG model parameter is not well-behaved, indicating an anomaly in the parameter space of the model. In this regard, for the evaluation of the galaxy cluster number count in the framework of EMSG, we equivalently provide an analysis of the number count of the gravitationally collapsed objects (or the dark matter halos). We show that the galaxy cluster number count decreases compared to the ΛCDM model. In agreement with the hierarchical model of structure formation, in EMSG cosmology the more massive structures are less abundant, meaning that form at later times.

astro-ph.CO↗

arXiv2022

Growth of Perturbations in Tsallis and Barrow Cosmology

We disclose the effects of the entropic corrections to the Friedmann equations on the growth of perturbations in the early stages of the universe. We consider two types of corrections to the area law of entropy, known as Tsallis and Barrow entropies. Using these corrections to entropy, we derive the modified Friedmann equations and explore the growth of perturbations in a flat universe filled with dark matter (DM) and cosmological constant. We employ the spherically symmetric collapse formalism and work in the linear regime for the perturbations. Interestingly enough, we find out that the profile of density contrast quite differs from the standard cosmology in Tsallis and barrow cosmology. We observe that the growth rate of matter perturbations crucially depend on the values of Tsallis and Barrow parameters. With increasing these correction parameters to the entropy, the total density contrast increases as well. This implies that perturbations grow up faster in a universe with modified entropy corrected Friedmann equations.

gr-qc↗

arXiv2022

Structure formation in mimetic gravity

We disclose the effects of an extra longitudinal degree of freedom on the evolution of perturbations in the framework of mimetic gravity. We consider a flat Friedmann-Robertson-Walker (FRW) background and explore the linear perturbations by adopting the spherically symmetric collapse formalism. By suitably choosing the potential of the mimetic field, we are able to solve the perturbed field equations in the linear regime and derive the matter density contrast δm in terms of the redshift parameter z. We observe that δm starts growing at the early stages and as the universe expands, it grows faster compared to the standard cosmology. This may due to the extra degree of freedom of the gravitational field which affects the growth of perturbations. We observe that in the presence of a mimetic potential, the growth rate function is smaller than ΛCDM model in small redshifts. We then consider the effects of this potential on the density abundance, the deceleration parameter and jerk parameter. We find out that mimetic potential can play the role of dark energy (DE) and affects the dynamics of matter perturbations and cosmological parameters. We also investigate the mass function and the number count for the collapsed objects in the mimetic scenario. We find that the mass function of models with potential is smaller than model without potential. With decreasing the role of DE, the mass function start to grow in smaller redshifts i.e., halo abundance is formed later. It is found that the more massive structures are less abundant and form at later times, as it should be in the hierarchical model of structure formation.

gr-qc↗