P THAKUR
Articles written in Pramana – Journal of Physics
Volume 88 Issue 3 March 2017 Article ID 0051 Research Article
Recent observational constraints on generalized Chaplygin gas in UDME scenario
Recent observational predictions suggest that our Universe is passing through an accelerating phase in the recent past. This acceleration may be realized with the negatively pressured dark energy. Generalized Chaplygin gas may be suitable to describe the evolution of the Universe as a candidate of unified dark matterenergy (UDME) model. Its EoS parameters are constrained using (i) dimensionless age parameter ($H_{0}t_{0}$) and (ii) the observed Hubble (H(z) − z) data (OHD) + baryon acoustic oscillation (BAO) data + cosmic microwavebackground (CMB) shift data + supernovae (Union2.1) data. Dimensionless age parameter puts loose bounds on the EoS parameters. Best-fit values of the EoS parameters $H_{0}, A_{s}$ and $\alpha$ ($A_{s}$ and $\alpha$ are defined in the energy density for generalized Chaplygin gas (GCG) and in EoS) are then determined from OHD+BAO+CMB+Union2.1 data and contours are drawn to obtain their allowed range of values. The present age of the Universe ($t_0$) and the present Hubble parameter ($H_0$) have been estimated with 1σ confidence level. Best-fit values of deceleration parameter (q), squared sound speed ($c^{2}_{s}$ ) and EoS parameter ($\omega$) of this model are then determined. It is seen that GCG satisfactorily accommodates an accelerating phase and structure formation phase.
Volume 89 Issue 2 August 2017 Article ID 0027 Research Article
Recent observational constraints on EoS parameters of a class of emergent Universe
Emergent Universe (EU) model is investigated here using the recent observational data of thebackground tests. The background test data comprise observed Hubble data, baryon acoustic oscillation data, cosmic microwave background shift data and Union compilation(2.1) data. The flat EU model obtained by Mukherjee $\it{et al}$ is permitted with a non-linear equation of state (in short, EoS) $(p = Aρ − Bρ^{1/2})$, where $A$ and $B$ are constants. The best-fit values and permitted range of values of the EoS parameters are determined in general EU model and in specific EU model $(A = 0)$ by using chi-square minimization technique. The best-fit values of the EoS parameters are used to study the evolution of the squared adiabatic sound speed $c^{2}_{s}$ , state parameter $\omega$ anddeceleration parameter $q$ for different red-shifts $z$. The present age of the Universe $t_0$ has been determined in general EU model as well as for EU model with $A = 0$. The late accelerating phase of the Universe in the EU model is accommodated satisfactorily.
Volume 89 Issue 2 August 2017 Article ID 0029 Research Article
Observational constraints on extended Chaplygin gas cosmologies
We investigate cosmological models with extended Chaplygin gas (ECG) as a candidate for dark energy and determine the equation of state parameters using observed data namely, observed Hubble data, baryon acousticoscillation data and cosmic microwave background shift data. Cosmological models are investigated considering cosmic fluid which is an extension of Chaplygin gas, however, it reduces to modified Chaplygin gas (MCG) andalso to generalized Chaplygin gas (GCG) in special cases. It is found that in the case of MCG and GCG, the best-fit values of all the parameters are positive. The distance modulus agrees quite well with the experimental Union2data. The speed of sound obtained in the model is small, necessary for structure formation. We also determine the observational constraints on the constants of the ECG equation.
Volume 95 All articles Published: 24 May 2021 Article ID 0091 Research Article
Thermodynamics in the emergent Universe model
The emergent Universe scenario is obtained in flat Universe with equation of state (EoS) ($P =B\rho-A\rho^{{1}/{2}}$) (where $A$ and $B$ are constants) as per Mukherjee et al. The EoS of this emergent Universe (EU) model can describe the current accelerated expansion and the initial singularity of the Universe. Following the standard thermodynamical criteria, stability of the EU models has been discussed. It is noted from thermal stability and positivity of adiabatic sound speed that $B$ satisfies the values $B=\frac{1}{3}, 1$ in the EU model. So, the emergent models with $B=0, -\frac{1}{3}$ are not supported with the stability issue. Further, the third law of thermodynamics is obeyed in this case for $B <-1$ (or with $A=0$, but it is outside the EU), i.e., any of the four discrete value of $B$ $ (= 0, -\frac{1}{3}, \frac{1}{3}, 1)$ does not support this third law. The specific heat at constant volume, $c_{v}$ obeys the relation $c_{v}\geq 0$ for $T\geq 0$ in the EU models. Two characteristic volume scales, critical volume $V_{c}$ and flip volume $V_{f}$ are obtained from zero pressure and zero deceleration condition in this model. Physically, these should follow the relation $V_{f}> V_{c}$, which are actually followed in the EU model for $B=\frac{1}{3}$.
Volume 97, 2023
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