Narrow Results

The new formal analogy between superfluid systems and cosmology, which emerges by taking into account the back-reaction of the vacuum to the quanta of sound waves,¹ enables us to put forward some common features between these two different areas of physics. We find the condition that allows us to justify a General Relativity (GR) derivation of the hydrodynamical equation for the superfluid in a four-dimensional space whose metric is the Unruh one.² Furthermore, we show how, in the particular case taken into account, our hydrodynamical equation can be deduced within a four-dimensional space from the wave equation of a massless scalar field.

We study the semiclassical Einstein equation and inflaton in the oscillatory region of the FRW Universe. We study the relative deviation of particle production in coherent and squeezed vacuum states for a minimally coupled scalar field in the oscillatory region of flat FRW Universe. We also study the behavior of inflaton field in non-oscillatory region. We examine whether the solution obtained in slow-roll approximation satisfies Klein–Gordon equation and commutation relation as well.

We explore the validity of the generalized second law (GSL) of thermodynamics in flat FRW universe with apparent horizon and event horizon as the boundary. We found that in a universe with holographic dark energy and dark matter, interacting with each other, the GSL is satisfied at the apparent horizon and partially satisfied at the event horizon under thermal equilibrium conditions. We also analyzed the GSL under non-equilibrium conditions and shows that the fulfillment of GSL at the apparent horizon implies that the temperature of the dark energy is greater than that of the horizon. Thus, there occurs a flow of dark energy towards the apparent horizon. As a result, the entropy of the dark energy decreases and that of the horizon increases. This is verified by finding the evolution of the dark energy entropy and horizon entropy in a dark energy dominated universe under non-equilibrium conditions.

A cosmological model with an inhomogeneous viscous dark fluid coupled with dark matter in a flat Friedmann–Robertson–Walker (FRW) universe is investigated. The influence of dark matter on the behavior of an inhomogeneous viscous fluid of this kind, responsible for cosmic acceleration and for the appearance of different types of singularities, is analyzed in detail. In particular, the critical points corresponding to the solutions of the background equations in a useful approximation are obtained explicitly.

In this paper, we have researched tachyon field, k-essence and quintessence dark energy (DE) models for Friedmann–Robertson–Walker (FRW) universe with varying G and $\mathrm{\Lambda }$ in f(R, T) gravitation theory. The theory of f(R, T) is proposed by Harko et al. [Phys. Rev. D84, 024020, 2011]. In this theory, R is the Ricci scalar and T is the trace of energy–momentum tensor. For the solutions of field equations, we have used linearly varying deceleration parameter (LVDP), the equation of state (EoS) and the ratio between $\mathrm{\Lambda }$ and Hubble parameter. Also, we have discussed some physical behavior of the models with various graphics.

In this paper, generalized Friedmann–Robertson–Walker universe for Strange Quark Matter (SQM) and Normal Matter (NM) coupled with Domain Wall (DW) in form perfect fluid have been investigated in $f(R,T)$ modified theory. $f(R,T)$ function has been assumed as $f(R,T)=R+\mu T$ and Modified Field Equations (MEFEs) have been attained. The MEFEs of the constructed model do not allow solutions of open and closed FRW universe. Obtained modified field equations of higher dimensional flat FRW universe have been solved by using Linearly Varying Deceleration Parameter (LVDP) suggested by Akarsu and Dereli [Int. J. Theor. Phys. 51, 612 (2012)]. Pressure and energy density of the matter distributions for the Linearly Expansion Model (LEM) and Constant Expansion Model (CEM) have been attained as decreasing values by the cosmic time t and constant $\mu$ in $f(R,T)$ gravity. But for the Exponential Expansion Model (XEM), these quantities have been obtained as time-independent. It is found out that the domain wall behaves like invisible matter when $s_1=0$ due to negative domain wall tension (${\sigma }_w^{\mathrm{\text{LSQM}}}=-B_c$) for SQM with DW of LEM. Strange quark matters for LDW and CDW models behave like cold dark matter when $s_1=0$ and $s_3=0$, respectively. One can say that SQM with DW for LEM and CEM may cause the expansion of the universe in these special situations. Also, solutions of XEM have shown that domain wall matter behaves like stiff matter when $\mu =20\pi$. All solutions of the constructed model in $f(R,T)$ theory have been reduced to General Relativity (GR) theory solutions by assuming $\mu =0$. Finally, all solutions obtained have been discussed in detail.

According to a number of astronomical studies, the universe is currently undergoing an accelerated expansion phase. We employed the recently proposed gravity, which holds that the current accelerated expansion phase of the universe is caused by a torsion scalar. The investigation is carried out using a parameterization of $q(z)=-1+\frac{3}{2}(\frac{{(z+1)}^{q_2}}{q_1+{(z+1)}^{q_2}})$ (where $q_1$ and $q_2$ are the model parameters) and the $H(z)$ using $H(z)=H_{0}exp[{\int }_0^z(1+q(z))d(ln(1+z))]$ is obtained. The model shows good agreement with recent observations, such as the LCDM model at late times. Finally, we examined how the cosmological parameters behaved in for constrained values.

In this paper, we investigate a Cartesian coordinate metric viscous fluid cosmological model with interactions, using a novel parametrization of the Hubble parameter and constraining free parameters with observational data. Our analysis reveals a comprehensive picture of the universe’s expansion dynamics, including a transition from deceleration to acceleration. The model exhibits quintessence-like behavior, approaching the $\mathrm{\Lambda }$CDM scenario at present, and phantom-like behavior in the distant future. Our findings align with observational evidence and theoretical expectations, reinforcing the validity of our model. We also analyze the energy density, isotropic pressure and equation of state parameter, providing insights into the dynamic nature of dark energy. Our model satisfies the WEC, NEC and DEC but violates the SEC, indicating unconventional behavior consistent with observations of the universe’s accelerating expansion.

We show that the Einstein field equations for a Friedmann-Robertson-Walker(FRW) universe are completely equivalent to a generalized Ermakov-Milne-Pinney system. This extends recent work of R. Hawkins and J. Lidsey, and provides an alternate method for deriving exact solutions of the field equations.

In this paper, we study strange quark matter for Friedman–Robertson–Walker (FRW) universes in the context of general relativity. For this purpose we consider quark–gluon plasma as a perfect fluid in the FRW universes. After solving Einstein's field equations, we have analyzed this matter for the different types of universe. Also, we discuss the features of collapsing strange quark matter.

A new analogy between superfluid systems and cosmology is here presented, which relies strongly on the following ingredient: the back-reaction of the vacuum to the quanta of sound waves. We show how the presence of thermal phonons, the excitations above the quantum vacuum for T > 0, enable us to deduce an hydrodynamical equation formally similar to the one obtained for a perfect fluid in a Universe obeying the Friedmann–Robertson–Walker metric.

Based on the chiral effective action method, we derive the thermal radiation from the apparent horizon in a Friedmann–Robertson–Walker (FRW) universe. The dimension reduction technique plays a crucial role in the derivation. The result further confirms the thermal properties of the apparent horizon in a FRW universe.

We consider two different entropic corrections to Bekenstein entropy, namely Renyi entropy and logarithmic-corrected entropy, and develop the entropic force, heat flow across the horizon and pressure. We also derive the expressions for Newton’s law of gravitation and verify with Bekenstein entropy by taking $\lambda =1$ in the case of Renyi entropy and $\beta =1$ for logarithmic entropy. The modified Friedmann equations are also being developed by using Newton’s first law of thermodynamics in both cases. In the presence of these equations, we also analyze the validity of generalized second law of thermodynamics for both entropy corrections on the apparent horizon. It is found that this law remains valid throughout the region under certain assumptions for nonflat FRW universe.

A new theory, named G(General)-theory in 14 dimensions, has been proposed that is reduced to $F(R)$-gravity and produces the metric of FRW universe in four dimensions. In this theory, the Universe is born in six stages. First, there is nothing in the world. Then, two strings, one with positive energy and one with negative energy in 14th, dimension are created such that the sum over their energies is zero. These strings are excited and for flowing their energies, other dimensions are produced. Second, these strings decay to G0-branes. Third, these branes join each other and construct Gp-branes which tensor fields live on. The rank of these fields can change from zero to p for $p\le 6$ and from zero to six for $p>6$. Four, by compacting Gp-branes on three circles, supersymmetry is born which contains the equal number of degrees of freedom for fermions and bosons. Six, by reducing G-theory to four dimensions, FRW universe is emerges and initial tensor fields produce the predicted shape of $F(R)$-gravity.

This paper is an attempt to revisit the Friedmann–Robertson–Walker (FRW) cosmological models under the new scenario of observational cosmology, which has established that the current universe is expanding with an increasing rate, in contrast to the earlier belief that the rate of expansion is constant or slowing down. This paper represents a model which encompasses both, earlier decelerating and the current accelerating universe, passing through a transition phase. The universe is assumed to be filled with two fluids, barotropic and dark energy. We have considered two cases; first, when these fluids are assumed to be non-interacting and second, when they interact with each other. Some physical, kinematic and geometric properties of the model are also discussed along with the acceptability and stability of the solution. The results found are very compatible with the established results as well as recent observations.

This work is dedicated to the cosmological scenario of the Barrow holographic dark energy (BHDE) in the light of time varying deceleration parameter. We investigate the evolution of a spatially flat Friedmann–Robertson–Walker (FRW) universe filled with pressure-less dark matter and BHDE density. We analyze the evolutionary history of essential cosmological parameters, including the deceleration parameter, the equation of state parameter, and the density parameter, using the Hubble horizon as the IR cut-off, and find appropriate characteristics in the BHDE model. Also, we examine the various structures of the model by reconstructing the scalar field potential to describe the universe accelerated expansion phase. For more accuracy of the dark energy model characteristics, we include the statefinder pair (r,s) which mimics the $\mathrm{\Lambda }$CDM model.

In this study, unimodular $f({𝒢},T)$ gravity is proposed as an alternative theory of gravity. Field equations of unimodular $f({𝒢},T)$ gravity are suggested. The field equation and their solutions are obtained for exponential expansion and power-law expansion models. Energy conditions of unimodular $f({𝒢},T)$ gravity are defined by using Raychaudhuri equations. Energy conditions for exponential expansion and power-law expansion models are investigated. Hubble parameter and deceleration parameter are calculated by considering power-law expansion model and obtained solutions. It has been shown that both deceleration parameter and Hubble parameter are compatible with the current observational data.

In this paper, we investigate the scalar field dark energy (DE) models within the context of $F(T,T_G)$ gravity. Scalar field models are known for their dynamic nature. Parameters in the dynamical equation of state are responsible for the acceleration of the cosmos in recent epochs. We determined that the power-law cosmology fits the OHD and Pantheon data nicely. Using the Bayesian analysis and likelihood function in conjunction with the Markov Chain Monte Carlo (MCMC) method, we examine the three sets of the model parameters $H_0=63.26_{-0.97}^{+0.77}$$q=-0.039_{0.0029}^{+0.0029}$ obtained by 57 points of the $H(z)$ data, $H_0=70.93_{-0.0037}^{+0.0024}$$q=-0.3384_{0.0026}^{+0.0027}$ 1048 points of the Pantheon data and $H_0=70.76732_{-0.00088}^{+0.0030}$$q=-0.6478_{0.0075}^{+0.0093}$ by using joint data ($H(z)+\mathrm{\text{BAO}}+\mathrm{\text{Pantheon}}$), respectively. We solved the modified Einstein’s field equations by taking the bulk-viscosity component $\xi ={\xi }_0{\rho }^{\alpha }$. We also establish a correspondence between the $F(T,T_G)$ gravity model and scalar field DE models such as quintessence, $k$-essence, and DBI-essence. The nature of scalar fields and their corresponding potentials are being graphically analyzed. To check the validity of $F(T,T_G)$ gravity model, we also analyze the behavior of energy conditions.