Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the first part of the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been chosen such that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. An effective interaction term, between matter and dark energy, has been proposed and calculated. Its negative value at the present time indicates conversion of matter into dark energy. Using this term, the time dependence of the rates of change of matter and dark energy has been determined. It is found that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. The present study provides us with a simple way to determine the time evolution of dark energy for a homogeneous and isotropic universe of zero spatial curvature, without involving any self-interaction potential or cosmological constant in the formulation.Comment: 22 pages, 16 figures, 2 tables. In the present version we have two models of density parameter calculation. Calculations of time derivatives of densities have been added to the previous version. Four new graphs have been added to the older version. Changes have been made to different sections of this articl

Galaxy interactions in filaments and sheets: insights from EAGLE simulations

We study the colour and star formation rates of paired galaxies in filaments and sheets using the EAGLE simulations. We find that the major pairs with pair separation $<50$ kpc are bluer and more star forming in filamentary environments compared to those hosted in sheet-like environments. This trend reverses beyond a pair separation of $\sim 50$ kpc. The interacting pairs with larger separations ($>50$ kpc) in filaments are on average redder and low-star forming when compared to those embedded in sheets. The galaxies in filaments and sheets may have different stellar mass and cold gas mass distributions. Using a Kolmogorov-Smirnov test, we find that for paired galaxies with pair separation $<50$ kpc, there are no significant differences in these properties in sheets and filaments. The filaments transport gas towards the cluster of galaxies. Some earlier studies find preferential alignment of galaxy pairs with filament axis. Such alignment of galaxy pairs may lead to different gas accretion efficiency in galaxies residing in filaments and sheets. We propose that the enhancement of star formation rate at smaller pair separation in filaments is caused by the alignment of galaxy pairs. A recent study with the SDSS data (Das et al., 2023) reports the same findings. The confirmation of these results by the EAGLE simulations suggests that the hydrodynamical simulations are powerful theoretical tools for studying the galaxy formation and evolution in the cosmic web.Comment: 9 pages, 2 figures, 2 table

Galaxy interactions in filaments and sheets: effects of the large-scale structures versus the local density

The major interactions are known to trigger star formation in galaxies and alter their colour. We study the major interactions in filaments and sheets using the SDSS data to understand the influence of large-scale environments on the galaxy interactions. We identify the galaxies in filaments and sheets using the local dimension and also find the major pairs residing in these environments. The star formation rate and colour of the interacting galaxies as a function of pair separation are separately analyzed in filaments and sheets. The analysis is repeated for three volume limited samples covering different magnitude ranges. The major pairs residing in the filaments show a significantly higher star formation rate (SFR) and bluer colour than those residing in the sheets up to the projected pair separation of $\sim 50$ kpc. We observe a complete reversal of this behaviour for both the SFR and colour of the galaxy pairs having a projected separation larger than 50 kpc. Some earlier studies report that the galaxy pairs align with the filament axis. Such alignment inside filaments indicates anisotropic accretion that may cause these differences. We do not observe these trends in the brighter galaxy samples. The pairs in filaments and sheets from the brighter galaxy samples trace relatively denser regions in these environments. The absence of these trends in the brighter samples may be explained by the dominant effect of the local density over the effects of the large-scale environment.Comment: 11 pages, 3 figures, 1 table, Accepted for publication in Research in Astronomy and Astrophysics (RAA

Do minor interactions trigger star formation in galaxy pairs?

We analyze the galaxy pairs in a set of volume limited samples from the SDSS to study the effects of minor interactions on the star formation rate (SFR) and colour of galaxies. We carefully design control samples of the isolated galaxies by matching the stellar mass and redshift of the minor pairs. The SFR distributions and colour distributions in the minor pairs differ from their controls at $>99\%$ significance level. We also simultaneously match the control galaxies in stellar mass, redshift and local density to assess the role of the environment. The null hypothesis can be rejected at $>99\%$ confidence level even after matching the environment. Our analysis shows a quenching in the minor pairs where the degree of quenching decreases with the increasing pair separation and plateaus beyond 50 kpc. We also prepare a sample of minor pairs with $H_{\alpha}$ line information. We calculate the SFR of these galaxies using the $H_{\alpha}$ line and repeat our analysis. We observe a quenching in the $H_{\alpha}$ sample too. We find that the majority of the minor pairs are quiescent systems that could be quenched due to minor interactions. Combining data from the Galaxy Zoo and Galaxy Zoo2, we find that only $\sim 1\%$ galaxies have a dominant bulge, $4\%-7\%$ galaxies host a bar, and $5\%-10\%$ galaxies show the AGN activity in minor pairs. This indicates that the presence of bulge, bar and AGN activity plays an insignificant role in quenching the galaxies in minor pairs. The more massive companion satisfies the criteria for mass quenching in most of the minor pairs. We propose that the stripping and starvation likely caused the quenching in the less massive companion at a later stage of evolution.Comment: 24 pages, 13 figures, 6 tables, significantly revised, main conclusions unchanged, Accepted for publication in Research in Astronomy and Astrophysics. this article supersedes arXiv:2108.0587

Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been so chosen for the present model that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. A term, representing interaction between matter and dark energy, has been calculated. Its negative value at the present time indicates conversion of matter into dark energy. It is evident from the present study that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. This model has an inherent simplicity in the sense that it allows one to determine the time evolution of dark energy for a homogeneous and isotropic universe, without involving any self-interaction potential or cosmological constant in the formulation

Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the first part of the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been chosen such that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. An effective interaction term, between matter and dark energy, has been proposed and calculated. Its negative value at the present time indicates conversion of matter into dark energy. Using this term, the time dependence of the rates of change of matter and dark energy has been determined. It is found that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. The present study provides us with a simple way to determine the time evolution of dark energy for a homogeneous and isotropic universe of zero spatial curvature, without involving any self-interaction potential or cosmological constant in the formulation

Time Evolution of Energy Density, EoS Parameter and the Density Parameters for Matter and Dark Energy in Brans-Dicke Gravity

The present study has been conducted in the framework of Brans-Dicke theory, using FRW metric with flat space, to determine the time dependence of the Brans-Dicke parameter (ω), energy density and the equation of state parameter of the cosmic fluid in an universe expanding with acceleration, following a phase of decelerated expansion. For this purpose, a scale factor has been so chosen that the deceleration parameter, obtained from it, changes its sign from positive to negative as time goes on. Considering the total pressure to be due to the entity named dark energy, the time dependence of energy parameters for matter and dark energy have been determined. Time dependence of the equation of state (EoS) parameter for dark energy has also been studied. Time variations of all these parameters have been shown graphically. It is quite evident from the present study that the dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities mentioned above. This model has an inherent simplicity in the sense that it allows one to determine the time evolution of dark energy without involving any self interaction potential or cosmological constant in the formulation

Time Evolution of Energy Density, EoS Parameter and the Density Parameters for Matter and Dark Energy in Brans-Dicke Gravity

The present study has been conducted in the framework of Brans-Dicke theory, using FRW metric with flat space, to determine the time dependence of the Brans-Dicke parameter (ω), energy density and the equation of state parameter of the cosmic fluid in an universe expanding with acceleration, following a phase of decelerated expansion. For this purpose, a scale factor has been so chosen that the deceleration parameter, obtained from it, changes its sign from positive to negative as time goes on. Considering the total pressure to be due to the entity named dark energy, the time dependence of energy parameters for matter and dark energy have been determined. Time dependence of the equation of state (EoS) parameter for dark energy has also been studied. Time variations of all these parameters have been shown graphically. It is quite evident from the present study that the dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities mentioned above. This model has an inherent simplicity in the sense that it allows one to determine the time evolution of dark energy without involving any self interaction potential or cosmological constant in the formulation

Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been so chosen for the present model that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. A term, representing interaction between matter and dark energy, has been calculated. Its negative value at the present time indicates conversion of matter into dark energy. It is evident from the present study that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. This model has an inherent simplicity in the sense that it allows one to determine the time evolution of dark energy for a homogeneous and isotropic universe, without involving any self-interaction potential or cosmological constant in the formulation

Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity

In the framework of Brans-Dicke (BD) theory, the first part of the present study determines the time dependence of BD parameter, energy density and equation of state (EoS) parameter of the cosmic fluid in a universe expanding with acceleration, preceded by a phase of deceleration. For this purpose, a scale factor has been chosen such that the deceleration parameter, obtained from it, shows a signature flip with time. Considering the dark energy to be responsible for the entire pressure, the time evolution of energy parameters for matter and dark energy and the EoS parameter for dark energy have been determined. An effective interaction term, between matter and dark energy, has been proposed and calculated. Its negative value at the present time indicates conversion of matter into dark energy. Using this term, the time dependence of the rates of change of matter and dark energy has been determined. It is found that the nature of dependence of the scalar field upon the scale factor plays a very important role in governing the time evolution of the cosmological quantities studied here. The present study provides us with a simple way to determine the time evolution of dark energy for a homogeneous and isotropic universe of zero spatial curvature, without involving any self-interaction potential or cosmological constant in the formulation