Narrow Results

This study is dedicated to the dynamic exploration of Barrow holographic dark energy (BHDE) inflation within the Granda–Oliveros (GO) cut-off framework. The investigation employs the Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model and takes place within the context of the Chern–Simons (CS) model of modified gravity (MG) theory. The aim of this analysis is to apply Hubble parameter and observational data from the Pantheon sample of Supernovae (SNIa) in conjunction with the most recent cosmic chronometer dataset to derive constraints on the BHDE scenario. We employ Markov chain Monte Carlo (MCMC) analysis to utilize the most recent cosmological data to constrain its parameters. Thus, we use the dark energy density parameter, deceleration parameter (DP) and equation of state (EoS) parameter to study cosmic evolution. The transition from decelerated to accelerated expanding phase for the BHDE universe is explained through dynamical behavior of DP. The evolutionary trajectories of ${\omega }_D-{\omega }_D^{\prime }$ plain are examined. Additionally, we use the Bayesian information criterion (BIC) and the Akaike information criterion (AIC) for model selection, and we compare our model to the $\mathrm{\Lambda }$CDM model, which serves as our reference.

In this paper, we investigate the constrained transit cosmological models in the most recently proposed modified gravity theory, $f(R,L_m,T)$-gravity. We obtain the modified field equations for a flat homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime metric. We constrain the equation of continuity by imposing the equation of state for the perfect fluid source $p=-\frac{1}{3}\rho +p_0$ so that we get energy conservation equation as $\dot{\rho }+3H(\rho +p)=0$ (because generally, energy conservation law is not satisfied in $f(R,L_m,T)$-gravity). Using this constraint, we establish a relation between the energy density parameters ${\mathrm{\Omega }}_{m0}$, ${\mathrm{\Omega }}_{r0}$, and ${\mathrm{\Omega }}_{f0}$ and the Hubble function. After that, we made observational constraints on $H(z)$ to obtain the best-fit present values of ${\mathrm{\Omega }}_{m0}$, ${\mathrm{\Omega }}_{r0}$, and $H_0$. Then, we use these best-fit values of energy parameters to investigate cosmological parameters such as the deceleration parameter, the effective equation of state ${\omega }_{\mathrm{\text{eff}}}$, and the energy density parameters ${\mathrm{\Omega }}_m$, ${\mathrm{\Omega }}_r$, and ${\mathrm{\Omega }}_f$ to learn more about the components and history of the expanding universe. We found an effective EoS parameter in the range $-1\le {\omega }_{\mathrm{\text{eff}}}\le \frac{1}{3}$ with a deceleration–acceleration transition redshift value of $z_t=0.6377,0.6424$ along two datasets cosmic chronometer (CC) and Pantheon SNIa, respectively.

In this paper, we investigate dust-fluid flat cosmological models in the recently proposed modified $f(R,T,L_m)$-gravity theory. We derive the field equations for the flat FLRW spacetime metric for the arbitrary function $f(R,T,L_m)=\frac{R}{2}+\alpha T+\beta L_m^n-\gamma$, where $R$ is the Ricci curvature scalar, $L_m$ is the matter Lagrangian, $T$ is the trace of the energy–momentum tensor $T_{ij}$, and $\alpha$, $\beta$, $\gamma$, and $n$ are the model parameters. We solve these equations to obtain the Hubble function in terms of matter energy density parameters ${\mathrm{\Omega }}_{\alpha 0}$, ${\mathrm{\Omega }}_{\beta 0}$, ${\mathrm{\Omega }}_{\gamma 0}$, and Hubble constant $H_0$. Then, we use the cosmic chronometer (CC) Hubble points dataset and the Pantheon dataset to do MCMC analysis of the Hubble function and find the best fit model parameters for the lowest ${\chi }^2$ values. Subsequently, we investigate the effective equation of state parameter ${\omega }_{\mathrm{\text{eff}}}$ and deceleration parameter $q(z)$ for the present past epoch of the universe. We also perform analysis for energy conditions and statefinder parameters to discuss the different stages of the dark energy models.

In this paper, we explore the cosmic acceleration and the physical aspects of the dark energy nature in Hoyle–Narlikar’s creation-field theory, taking into account observational constraints. We find analytical solutions to the modified field equations for a specific choice of creation field $C(t)=t+\int \frac{k}{a^n}+c_1$ with certain background fluid sources as a barotropic fluid and a radiation fluid in a flat Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime universe. From there, we use observational constraints with data from the cosmic chronometer (CC) Hubble data and the apparent magnitude from the Pantheon SNe sample to get the model parameters’ constrained values with $\sigma 1$ and $\sigma 2$ confidence levels. We look at the physical properties of the models we’ve made using these estimated values of model parameters. To do this, we look at the deceleration parameter $q(z)$, the effective equation of state parameter ${\omega }_{\mathrm{\text{eff}}}$, the jerk parameter $j(z)$, the snap parameter $s(z)$, and the energy conditions. We also examine the cosmic behavior of square sound speed $c_s^2$ for the test of causality and viability of the models. We measure the present age of the universe and transition redshift $z_t$ for the signature-flipping point. We obtain two transit-phase accelerating universe models with $H_0=67.0\pm 3.2,69.6\pm 3.2$ Km/s/Mpc, $q_0=-0.4586,-0.5917$, $z_t=0.6383,0.6751$, and ${\omega }_{\mathrm{\text{eff}}0}=-0.6391,-0.7278$, respectively, with barotropic and radiation fluid sources.

In this paper, we investigate the freezing quintessence scenario in late-time cosmic expansion using a nonlinear $f(R,L_m)$ gravity model, $f(R,L_m)=\frac{R}{2}+L_m^{\alpha }$, where $\alpha$ is a free parameter. We consider a solution for this model using an appropriate parametrization of the scale factor, and then the model is constrained by observational datasets, including CC, Pantheon$+$ (SN), and CC$+$SN$+$BAO. Our analysis yields results aligning closely with observational data. The Hubble parameter, deceleration parameter, matter-energy density, and EoS parameter of our model exhibit expected trends over cosmic time, supporting its physical validity. Furthermore, the model demonstrates consistency with the $\mathrm{\Lambda }$CDM model in late times, displaying freezing behavior in the $\omega -{\omega }^{\prime }$ plane and stability against density perturbations. Our findings suggest that the modified $f(R,L_m)$ gravity model is a credible approach to describing the universe’s accelerating phase.

In a recent work [S. Kumar, R. C. Nunes and S. K. Yadav, Phys. Rev. D 98, 043521 (2018)], we have investigated a dark matter (DM)-photon coupling model in which the DM decays into photons in the presence of dark energy (DE) with constant equation of state (EoS) parameter. Here, we study an extension of the DM-photon coupling model by considering a time-varying EoS of DE via Chevalier–Polarski–Linder (CPL) parametrization. We derive observational constraints on the model parameters by using the data from cosmic microwave background (CMB), baryonic acoustic oscillations (BAO), the local value of Hubble constant from Hubble Space Telescope (HST), and large-scale structure (LSS) information from the abundance of galaxy clusters, in four different combinations. We find that in this DM-photon coupling scenario the mean values of $w_{\mathrm{\text{de}}0}$ are in quintessence region $(w_{\mathrm{\text{de}}0}>-1)$ whereas they were in the phantom region $(w_{\mathrm{\text{de}}0}<-1)$ in our previous study with all data combinations. The constraints on the DM-photon coupling parameter do not reflect any significant deviation from the previous results. Due to the decay of DM into photons, we obtain higher values of $H_0$, consistent with the local measurements, similar to our previous study. But, the time-varying DE leads to lower values of ${\sigma }_8$ in the DM-photon coupling model with all data combinations, in comparison to the results in our previous study. Thus, allowing time-varying DE in the DM-photon coupling scenario is useful to alleviate the $H_0$ and ${\sigma }_8$ tensions.

This study examines a dark energy cosmological model in the classical domain, where a generic scalar field is considered a dark energy source. Einstein’s field equations are solved in a model independent way, i.e. using a scheme of cosmological parametrization. A simple parametrization of the density parameter as a function of the cosmic scale factor is considered and investigated extensively. The results obtained are noteworthy as it shows a smooth transition from a decelerating to an accelerating phase in the recent past. Certain external datasets are used here to limit the model parameters involved in the functional form of the considered parametrization of energy density of the scalar filed. The updated Hubble datasets containing 57 data points, 1048 points of recently compiled Pantheon datasets, and also the Baryon Acoustic Oscillation (BAO) datasets are used here to determine the best-fitting model parameter values. The expressions of several significant cosmological parameters are represented as a function of redshift “$z$” and illustrated visually for the best fit values of the model parameters to better comprehend cosmic evolution. The obtained model is also compared with the $\mathrm{\Lambda }$CDM model. Our model has a distinct behavior in future and shown a big crunch type collapse. The best fit values of the model parameters are also used to compute the current values of several physical and geometrical parameters, as well as phase transition redshift. To examine the nature of dark energy, certain cosmological tests and diagnostic analyses are done on the derived model.

We investigate an isotropic and homogeneous flat dark energy model in $f(Q,C)$ gravity theory that is linear in non-metricity Q and quadratic in boundary term C as $f(Q,C)=Q+\alpha C^2$, where $\alpha$ is a model parameter. We have solved the field equations in flat Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetime geometry and considered a relation in the form of Hubble function in total energy density parameters ${\mathrm{\Omega }}_{m0}$, ${\mathrm{\Omega }}_{0(Q,C)}$, and Hubble constant $H_0$. We have compared our results with two observational datasets $H(z)$ and Pantheon SNe Ia datasets by using MCMC analysis and have obtained the best fit present values of parameters. We have used these best fit values throughout in result analysis and discussion. We have found the equation of state (EoS) parameter as $-1\le \omega \le 0.2$ over $-1\le z\le 3$. We have also investigated the Om diagnostic function and present age of the universe for these two datasets.

In the field of theoretical physics, the fundamental characteristics and evolutionary mechanism of dark energy remain a subject of ongoing enquiry. While the simplest explanation for dark energy within General Relativity Theory is the cosmological constant, this concept faces a significant challenge known as the fine-tuning problem. In this study, we adopt an alternative approach, where we attribute the evolution of dark energy to the gravitational sector rather than the matter source. We explore the recently proposed theory of gravity known as modified symmetric teleparallel gravity, $f(Q)$, in which gravitational interactions are governed by the non-metricity term Q. Within this manuscript, we consider $f(Q)=\alpha Q^{\beta }$ that incorporates nonlinear forms of the non-metricity scalar, where $\alpha$ and $\beta$ are free model parameters. Subsequently, we determine the values of these model parameters that align with the observed values of cosmographic parameters. Our analysis delves into the behavior of various cosmological parameters, including the deceleration parameter, density, and equation of state parameters, as well as the cosmological distances for our cosmological model.

In the context of symmetric teleparallel modified gravity, known as $f(Q)$ theory, we examine an accelerating cosmological model using a log-square-root form of the non-metricity function. Solving the field equations with a parametrized deceleration parameter in terms of redshift, we derive the model parameters. Our analysis utilizes data from 57 Hubble observations, 1048 Pantheon supernovae, and BAO datasets, with a Markov Chain Monte Carlo (MCMC) technique for statistical analysis. The results show that the deceleration parameter transitions from positive (early deceleration) to negative (current acceleration), aligning with observational data. Statefinder parameters indicate that the model evolves from quintessence-like behavior to convergence with the $\mathrm{\Lambda }$CDM model. The equation of state parameter suggests a phantom dark energy scenario, implying faster-than-expected acceleration. The matter density and cosmic pressure predictions are consistent with observational trends. We can conclude that the $f(Q)$ gravity model offers a robust framework for explaining the accelerated expansion of the universe, providing a viable alternative to the $\mathrm{\Lambda }$CDM model, and highlighting the potential of non-metric gravity theories to advance our understanding of dark energy and cosmic dynamics.

In this paper we investigate an observable universe in Bianchi type V space–time by taking into account the cosmological constant as the source of energy. We have performed an ${\chi }^2$ test to obtain the best fit values of the model parameters of the universe in the derived model. We have used two types of data sets, viz: (i) 31 values of the Hubble parameter and (ii) the 1048 Pantheon data set of various supernovae distance moduli and apparent magnitudes. From both the data sets, we have estimated the current values of the Hubble constant, density parameters ${({\mathrm{\Omega }}_m)}_0$ and ${({\mathrm{\Omega }}_{\mathrm{\Lambda }})}_0$. The present value of deceleration parameter of the universe in derived model is obtained as $q_0=-0.59_{-0.03}^{+0.04}$ and $-0.59_{-0.03}^{+0.02}$ in accordance with $H(z)$ and Pantheon data, respectively. Also we observe that there is a signature flipping in the sign of deceleration parameter from positive to negative and a transition redshift exists. Thus, the universe in the derived model represents a transitioning universe which is in an accelerated phase of expansion at present epoch. We have estimated the current age of the universe $(t_0)$ and present value of the jerk parameter $(j_0)$. Our obtained values of $t_0$ and $j_0$ are in good agreement with their values estimated by Planck collaborations and WMAP observations.

In this paper, we have proposed a model of accelerating universe with binary mixture of bulk viscous fluid and dark energy (DE) and probed the model parameters: present values of Hubble’s constant $H_0$, equation of state paper of DE ${\omega }_{\mathrm{\text{de}}}$ and density parameter of DE ${({\mathrm{\Omega }}_{\mathrm{\text{de}}})}_0$ with recent observational $H(z)$ data (OHD) as well as joint Pantheon compilation of SN Ia data and OHD. Using cosmic chronometric technique, we obtain $H_0=69.80\pm 1.64{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$ and $70.0258\pm 1.72{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$ by restricting our derived model with recent OHD and joint Pantheon compilation SN Ia data and OHD, respectively. The present age of the universe in derived model is estimated as $t_0=13.82\pm 0.33\mathrm{\text{Gyr}}$. Also, we observe that derived model represents a model of transitioning universe with transition redshift $z_t=0.7286$. We have constrained the present value of jerk parameter as $j_0=0.969\pm 0.0075$ with joint OHD and Pantheon data. From this analysis, we observed that the model of the universe, presented in this paper, shows a marginal departure from $\mathrm{\Lambda }$CDM model.

In this work, we investigate gravitational baryogenesis in the framework of $f(P)$ gravity to understand the applicability of this class of modified gravity in addressing the baryon asymmetry of the universe. For the analysis, we set $f(P)=\alpha P$, where $\alpha$ is the model parameter. We found that in $f(P)$ gravity, the CP-violating interaction acquires a modification through the addition of the nontopological cubic term $P$ in addition to the Ricci scalar $R$ and the mathematical expression of the baryon-to-entropy ratio depends not only on the time derivative of $R$ but also the time derivative of $P$. Additionally, we also investigate the consequences of a more complete and generalized CP-violating interaction proportional to $f(P)$ instead of $P$ in addressing the baryon asymmetry of the universe. For this type of interaction, we report that the baryon-to-entropy ratio is proportional to $Ṙ$, $Ṗ$ and $f^{\prime }(P)$. We report that for both of these cases, rational values of $\alpha$ generate acceptable baryon-to-entropy ratios compatible with observations.

$f(Q,T)$ gravity is a novel extension of the symmetric teleparallel gravity where the Lagrangian $L$ is represented through an arbitrary function of the nonmetricity $Q$ and the trace of the energy–momentum tensor $T$.²³ In this work, we have constrained a widely used $f(Q,T)$ gravity model of the form $f(Q,T)=Q^{a+1}+bT$ from the primordial abundances of the light elements to understand its viability in cosmology. We report that the $f(Q,T)$ gravity model can elegantly explain the observed abundances of Helium and Deuterium while the Lithium problem persists. From the constraint on the expansion factor in the range $0.9425\lesssim Z\lesssim 1.1525$, we report strict constraints on the parameters $a$ and $b$ in the range $0.01\lesssim a\lesssim -0.01$ and $-5.86\lesssim b\lesssim 12.52$, respectively.

In this work, we study the interaction between the dark matter (DM) component and the dark energy (DE) component using the Tsallis holographic dark energy (THDE) density expression for a Bianchi type-II space–time within the framework of general relativity (GR). To obtain the exact solutions of Einstein’s field equations, we use two constraints: (i) the expansion scalar $(𝜃)$ of the Universe is proportional to the component ${\sigma }_1^1$ of the shear tensor $\left({\sigma }_i^i\right)$, i.e. $𝜃\propto {\sigma }_1^1$ and (ii) we assume that the scale factor follows the Hybrid Expansion Law (HEL). We have discussed some geometrical and physical parameters of our model such as the deceleration parameter (DP) and the equation of state (EoS) parameter. In these parameters, we plot their behavior in terms of redshift $z$. We observe that the DP evolves from the early decelerating phase to the current accelerating phase with a current value consistent with the observation data. The EoS parameter evolves from a state of a stiff-matter fluid-dominated era $({\omega }_T=1)$ for high redshift $z$ to a $\mathrm{\Lambda }$CDM era $({\omega }_T=-1)$ in the later times. Also, we have established a correspondence between the THDE model and the tachyon scalar field dark energy model. We have reconstructed the potential and the tachyon scalar field, which describes the current accelerated expansion of the Universe. Finally, using 51 values of observed Hubble measurement and the technique $R^2$-test, we found the best fit value for the model parameters $\alpha$, $\beta$ and $H_0$ (current value of the Hubble parameter). These results are consistent with the new measurements of $H_0$.³⁷

In recent times, astounding observations of both over- and under-luminous type Ia supernovae have emerged. These peculiar observations hint not only at surpassing the Chandrasekhar limit but may also suggest potential modifications in the physical attributes of their progenitors such as their cooling rate. This, in turn, can influence their temporal assessments and provide a compelling explanation for these intriguing observations. In this spirit, we investigate here the cooling process of white dwarfs in $f(R,T)$ gravity with the simplest model $f(R,T)=R+\lambda T$, where $\lambda$ is the model parameter. Our modeling suggests that the cooling timescale of white dwarfs exhibits an inverse relationship with the model parameter $\lambda$. This unveils that in the realm of $f(R,T)$ gravity, the energy release rate for white dwarfs increases as $\lambda$ increases. Furthermore, we also report that the luminosity of the white dwarfs also depends on $\lambda$ and an upswing in $\lambda$ leads to an amplification in the luminosity. As a result, utilizing white dwarf luminosity could possibly define bounds on $f(R,T)$ gravity models.

This work explores the dark energy nature of logarithmic $f(R,L_m)$-gravity models with observational constraints, where $L_m$ represents the matter Lagrangian for perfect fluid and R is the Ricci-scalar curvature. With a matter Lagrangian $L_m=\rho$, a flat FLRW space–time metric and an arbitrary function $f(R,L_m)=\frac{R}{2}+L_m-\mu ln{L}_m$, where $\mu$ is a positive model parameter, we have derived the field equations of this model. By using the Hubble function, we have been able to solve the energy conservation equation and create a relationship between ${\mathrm{\Omega }}_{m0}$, ${\mathrm{\Omega }}_{q0}$ and ${\mathrm{\Omega }}_{\mu 0}$. Using the most recent two observational datasets as 32 $H(z)$ and 1048 Pantheon SNe Ia datasets, we conducted MCMC analysis. We have obtained best fit values of model parameters with $1-\sigma ,2-\sigma ,3-\sigma$ errors and using these values, we have explored the cosmological properties of the model. We have performed om diagnostic analysis for the model and estimated the present age of the universe. Thus our derived model presents a transit phase decelerating to accelerating model without dark energy term $\mathrm{\Lambda }$. We found that the effective equation of state parameter is in the range $-1\le \omega \le 0$ over $-1\le z<\infty$ with present value $\omega \approx -0.6971,-0.7120$.

In this paper, we have explored a transitioning cosmic model, depicting late-time accelerated expansion in the $f(R,T^{\varphi })$ theory of gravity for an isotropic and homogeneous universe, where the trace of the energy–momentum tensor $T^{\varphi }$ is the function of the self-interacting scalar field $\varphi$. We have proposed an explicit solution to the derived model by utilizing a scale factor of the hybrid form $a(t)=t^{\alpha }{e}^{\beta t}$, where $\alpha$ and $\beta$ are constants. To estimate the best-fit values of model parameters, statistical analysis based on the Markov Chain Monte Carlo (MCMC) method has been employed on 57 $H(z)$ points in the range $0.07\le z\le 2.36$ and Pantheon Sample consisting of a total of 1048 SNe Ia in the range of $0.01<z<2.3$. We have described the dynamical features of the model, like energy density, cosmic pressure, and the equation of state parameter, in the context of the scalar field $\varphi$. We have also described the potential and behavior of the scalar field for quintessence and phantom scenarios. For the joint dataset of OHD and Pantheon, the deceleration parameter depicts a transitioning universe with signature flipping at $z_t=0.78$ with the present value of the deceleration parameter $q_0=-0.565$. The violation of SEC for the derived model indicates cosmic expansion at a faster rate. We have used state-finders to diagnose the model. The findings for our theoretical model indicate that the derived model agrees with observed findings within a particular range of limitations.

In this paper we investigate a simple parametrization scheme of the quintessence model given by Wetterich [Phys. Lett. B594, 17 (2004)]. The crucial parameter of this model is the bending parameter b, which is related to the amount of dark energy in the early universe. Using the linear perturbation and the spherical infall approximations, we investigate the evolution of matter density perturbations in the variable dark energy model, and obtain an analytical expression for the growth index f. We show that increasing b leads to less growth of the density contrast δ, and also decreases the growth index. Giving a fitting formula for the growth index at the present time, we verify that the approximation relation also holds in this model. To compare predictions of the model with observations, we use the Supernovae type Ia (SNIa) Gold Sample and the parameters of the large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS). The best fit values for the model parameters by marginalizing on the remained ones, are , and at 1σ confidence level. As a final test we calculate the age of universe for different choices of the free parameters in this model and compare it with the age of old stars and some high redshift objects. Then we show that the predictions of this variable dark energy model are consistent with the age observation of old star and can solve the "age crisis" problem.

This paper is a broadband review of the current status of nonbaryonic dark matter (DM) research, starting from a historical overview of the evidences of existence of DM, then discussing how DM is distributed from small scale to large scale, continuing with a discussion on DM nature, DM candidates and their detection. I finally discuss some of the limits of the ΛCDM model, with particular emphasis on the small scale problems of the paradigm.