Search Results (4)

In this work, the Friedmann–Robertson–Walker (FRW) Universe is considered a thermodynamic system, where the cosmological constant generates the thermodynamic pressure. Using a unified first law, we have determined the amount of energy $dE$ crossing the apparent horizon. Since heat is one of the forms of thermal energy, so the heat flows $\delta Q$ through the apparent horizon = amount of energy crossing the apparent horizon. Using the first law of thermodynamics, on the apparent horizon, we found $TdS=A(\rho +p)H{\tilde{r}}_{h}dt+A\rho d{\tilde{r}}_h$ where $T,S,A,H,{\tilde{r}}_h,\rho ,p$ are respectively the temperature, entropy, area, Hubble parameter, horizon radius, fluid density and pressure. Since the apparent horizon is dynamical, so we have assumed that $d{\tilde{r}}_h$ cannot be zero in general, i.e., the second term $A\rho d{\tilde{r}}_h$ is non-zero on the apparent horizon. Using Friedmann equations with the unified first law, we have obtained the modified entropy-area relation on the apparent horizon. In addition, from the modified entropy-area relation, we have obtained modified Friedmann equations. From the original Friedmann equations and also from modified Friedmann equations, we have obtained the same entropy. We have derived the equations for the main thermodynamical quantise, such as temperature, volume, mass, specific heat capacity, thermal expansion, isothermal compressibility, critical temperature, critical volume, critical pressure and critical entropy. To determine the cooling/heating nature of the FRW Universe, we have obtained the coefficient of Joule–Thomson expansion. Next, we have discussed the heat engine phenomena of the thermodynamical FRW Universe. We have considered the Carnot cycle and obtained its completed work. In addition, we studied the work completed and the thermal efficiency of the new heat engine. Finally, we have obtained the thermal efficiency of the Rankine cycle. Full article

We assume the anisotropic model of the Universe in the framework of a varying speed of light c and a varying gravitational constant G theories and study different types of singularities. We write the scale factors for the singularity models in terms of cosmic time and find some conditions for possible singularities. For future singularities, we assume the forms of a varying speed of light and varying gravitational constant. For regularizing the Big Bang singularity, we assume two forms of scale factors: the sine model and the tangent model. For both models, we examine the validity of null and strong energy conditions. Starting from the first law of thermodynamics, we study the thermodynamic behaviors of a number n of universes (i.e., multiverse) for (i) varying c, (ii) varying G and (iii) varying both c and G models. We find the total entropies for all the cases in the anisotropic multiverse model. We also find the nature of the multiverse if the total entropy is constant. Full article

In this work, we study the existence of strange stars in the background of $f(\mathbb{T},\mathcal{T})$ gravity in the Einstein spacetime geometry, where $\mathbb{T}$ is the torsion tensor and $\mathcal{T}$ is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified Tolman–Oppenheimer–Volkoff (TOV) equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in $f(\mathbb{T},\mathcal{T})$ gravity as well as their observational signatures. Full article

We demonstrate two periodic or quasi-periodic generalizations of the Chaplygin gas (CG) type models to explain the origins of dark energy as well as dark matter by using the Weierstrass ξ(t), σ(t) and ζ (t) functions with two periods being infinite. If the universe can evolve periodically, a non-singular universe can be realized. Furthermore, we examine the cosmological evolution and nature of the equation of state (EoS) of dark energy in the Friedmann–Lemaître–Robertson–Walker cosmology. It is explicitly illustrated that there exist three type models in which the universe always stays in the non-phantom (quintessence) phase, whereas it always evolves in the phantom phase, or the crossing of the phantom divide can be realized. The scalar fields and the corresponding potentials are also analyzed for different types of models. Full article