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Register a new userBulk viscous matter and the cosmic acceleration of the universe in $f(Q,T)$ gravity
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Added by
Pardyumn Kumar Sahoo
Publication date
2021
fields
Physics
and research's language is
English
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We studied bulk viscosity in the modified $f(Q,T)$ gravity theory formalism, where $Q$ represents the non-metricity and $T$ denotes the trace of energy-momentum tensor within a flat Friedmann-Lema^{i}tre-Robertson-Walker metric (FLRW). We consider the effective equation of state, which includes a bulk viscosity term explicitly. We find the exact solutions relating to bulk viscosity by assuming a specific form of $f(Q,T)=alpha Q+beta T$, where $alpha$ and $beta$ are constants. Furthermore, we constrained our model with revised Hubble datasets consisting of 57 data points and newly published Pantheon samples with 1048 points to obtain the best fitting values of the model parameters. Our model is found to be in good agreement with observations. Furthermore, we analysed the cosmological behavior of the density parameter, the equation of state (EoS) parameter ($omega$), and the deceleration parameter ($q$). The universe appears to be evolving from a decelerated to an accelerated phase. The EoS parameter is further found to be in the quintessence phase, indicating that the universe is accelerating. We can deduce that the accumulation of bulk viscosity as effective dark energy is responsible for the current accelerated expansion of the universe.
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The standard formulation of general relativity fails to describe some recent interests in the universe. It impels us to go beyond the standard formulation of gravity. The $f(Q)$ gravity theory is an interesting modified theory of gravity, where the gravitational interaction is driven by the nonmetricity $Q$. This study aims to examine the cosmological models with the presence of bulk viscosity effect in the cosmological fluid within the framework of $f(Q)$ gravity. We construct three bulk viscous fluid models, i.e. (i) for the first model, we assuming the Lagrangian $f(Q)$ as linear dependence on $Q$, (ii) for the second model the Lagrangian $f(Q)$ as a polynomial functional form, and (iii) the Lagrangian $f(Q)$ as a logarithmic dependence on $Q$. Furthermore, we use 57 points of Hubble data and 1048 Pantheon dataset to constraint the model parameters. Then, we discuss all the energy conditions for each model, which helps us to test the self-consistency of our models. Finally, we present the profiles of the equation of state parameters to test the models present status.
The recently proposed $f(Q, T)$ gravity (Xu et al. Eur. Phys. J. C textbf{79} (2019) 708) is an extension of the symmetric teleparallel gravity. The gravitational action $L$ is given by an arbitrary function $f$ of the non-metricity $Q$ and the trace of the matter-energy momentum tensor $T$. In this paper, we examined the essence of some well prompted forms of $f(Q,T)$ gravity models i.e. $f(Q,T)= mQ+bT$ and $f(Q,T)= m Q^{n+1}+b T$ where $m$, $b$, and $n$ are model parameters. We have used the proposed deceleration parameter, which predicts both decelerated and accelerated phases of the Universe, with the transition redshift by recent observations and obtains energy density ($rho$) and pressure ($p$) to study the various energy conditions for cosmological models. The equation of state parameter ($omegasimeq -1$) in the present model also supports the accelerating behavior of the Universe. In both, the models, the null, weak, and dominant energy conditions are obeyed with violating strong energy conditions as per the present accelerated expansion.
The article communicates an alternative route to suffice the late-time acceleration considering a bulk viscous fluid with viscosity coefficient $zeta =zeta _{0}+ zeta _{1} H + zeta _{2} H^{2}$, where $zeta _{0}, zeta _{1}, zeta _{2}$ are constants in the framework of $f(R,T)$ modified gravity. We presume the $f(R,T)$ functional form to be $f=R+2alpha T$ where $alpha$ is a constant. We then solve the field equations for the Hubble Parameter and study the cosmological dynamics of kinematic variables such as deceleration, jerk, snap and lerk parameters as a function of cosmic time. We observe the deceleration parameter to be highly sensitive to $alpha$ and undergoes a signature flipping at around $tsim 10$ Gyrs for $alpha=-0.179$ which is favored by observations. The EoS parameter for our model assumes values close to $-1$ at $t_{0}=13.7$Gyrs which is in remarkable agreement with the latest Planck measurements. Next, we study the evolution of energy conditions and find that our model violate the Strong Energy Condition in order to explain the late-time cosmic acceleration. To understand the nature of dark energy mimicked by the bulk viscous baryonic fluid, we perform some geometrical diagnostics like the ${r,s}$ and ${r,q}$ plane. We found the model to mimic the nature of a Chaplygin gas type dark energy model at early times while a Quintessence type in distant future. Finally, we study the violation of continuity equation for our model and show that in order to explain the cosmic acceleration at the present epoch, energy-momentum must violate.
New high-precision observations are now possible to constrain different gravity theories. To examine the accelerated expansion of the Universe, we used the newly proposed $f(Q,T)$ gravity, where $Q$ is the non-metricity, and $T$ is the trace of the energy-momentum tensor. The investigation is carried out using a parameterized effective equation of state with two parameters, $m$ and $n$. We have also considered the linear form of $f(Q,T)= Q+bT$, where $b$ is constant. By constraining the model with the recently published 1048 Pantheon sample, we were able to find the best fitting values for the parameters $b$, $m$, and $n$. The model appears to be in good agreement with the observations. Finally, we analyzed the behavior of the deceleration parameter and equation of state parameter. The results support the feasibility of $f(Q,T)$ as a promising theory of gravity, illuminating a new direction towards explaining the Universes dark sector.
The paper presents late time cosmology in $f(Q,T)$ gravity where the dark energy is purely geometric in nature. We start by employing a well motivated $f(Q,T)$ gravity model, $f(Q,T)=mQ^{n}+bT$ where $m,n$ and $b$ are model parameters. Additionally we also assume the universe to be dominated by pressure-less matter which yields a power law type scale factor of the form $% a(t)=c_{2}(At+c_{1})^{frac{1}{A}}$, where $A=dfrac{3(8pi +b)}{n(16pi +3b)% }$ and $c_{1}$ & $c_{2}$ are just integration constants. To investigate the cosmological viability of the model, constraints on the model parameters were imposed from the updated 57 points of Hubble data sets and 580 points of union 2.1 compilation supernovae data sets. We have thoroughly investigated the nature of geometrical dark energy mimicked by the parametrization of $f(Q,T)=mQ^{n}+bT$ with the assistance of statefinder diagnostic in ${s,r}$ and ${q,r}$ planes and also performed the $Om$ -diagnostic analysis. The present analysis makes it clear-cut that $f(Q,T)$ gravity can be promising in addressing the current cosmic acceleration and therefore a suitable alternative to the dark energy problem. Further studies in other cosmological areas are therefore encouraging to further investigate the viability of $f(Q,T)$ gravity.
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