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- 2025| World Scientific Pub...This study investigates the cosmological implications of the 𝑓(𝑅,𝑇)=𝑅+2𝜆𝑇 gravity model. 𝑓(𝑅,𝑇) gravity is a modification of General Relativity (GR) that introduces a coupling between the Ricci scalar R and the trace of the energy–momentum tensor T. This work provides a comprehensive analysis of the model’s predictions using updated observational data, including uncorrelated Baryon Acoustic Oscillations and Cosmic Chronometers. By employing the Markov Chain Monte Carlo technique, we constrain the model parameters, demonstrating their compatibility with current observational datasets. Our findings reveal that the model naturally extends the ΛCDM model, with the parameter 𝜆 from 𝑓(𝑅,𝑇) gravity quantifying deviations from GR. Additionally, we provide a critical discussion on the challenges and limitations of the 𝑓(𝑅,𝑇) framework, addressing issues such as observational constraints, systematic uncertainties and model dependencies. This work not only refines parameter constraints for 𝑓(𝑅,𝑇) gravity, but also bridges the gap between theoretical predictions and observational tests, offering a powerful framework for exploring deviations from GR in a cosmological context.
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- The nature of dark energy remains one of the most profound questions in modern cosmology. Among the various candidates, quintessence — a dynamic scalar field with time-varying energy density — offers a promising alternative to the cosmological constant, potentially addressing the challenges posed by the accelerating expansion of the universe. This work employs a model-independent approach to analyze the evolution of quintessential dark energy, providing observational constraints without the assumptions typically imposed by specific cosmological models. Using an extensive dataset from recent observations, we derive constraints on the quintessential potential and its evolution over cosmic time, revealing insights into the dynamics of the field and the interplay between dark energy and matter. Our results support a non-static dark energy behavior that aligns with the cosmic expansion history and suggests possible deviations from the standard Lambda Cold Dark Matter (CDM) model. This study opens pathways for future observations to test quintessence models more rigorously, contributing to a deeper understanding of the role of dark energy in the past, present, and future of the universe.
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- 2025| World Scientific Pub...In this work, we investigate the cosmological implications of f(Q) gravity by introducing a nonlinear equation of state of the form p = βρ2 - ρ. This modified gravity framework, based on the non-metricity scalar Q, offers an alternative to General Relativity and provides new insights into cosmic acceleration. To test the validity of our model, we use a combined observational dataset consisting of 31 cosmic chronometer data points, 1701 Type Ia supernova measurements, and 26 baryon acoustic oscillation observations, leading to a total of 1758 data points. The statistical analysis based on this dataset allows for a viable comparison with the standard ΛCDM model. We analyze cosmographic parameters such as the deceleration parameter, jerk parameter, and statefinder parameters, to determine the impact of the model on the evolution of the universe. The results indicate that our model successfully describes cosmic expansion while presenting deviations from the standard ΛCDM scenario. Statistical comparisons based on the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) further suggest that the proposed model provides a competitive fit to observational data. Our findings show the potential of f(Q) gravity with a nonlinear EoS in the quadratic form as an alternative to the ΛCDM model. This work contributes to efforts to explore modified gravity theories as possible explanations for late cosmic acceleration and provides commentary on their implications.
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- Recently, by the end of their second observing run, advanced LIGO and Virgo have detected some binary black hole mergers. In this paper, we show that the acceleration of black holes around each other causes the emergence of two Rindler space–times. The parameters of the black hole in each region change in opposite order to the parameters of the black hole in the other region. Also, the acceleration has a direct relation to the black hole entropies and their observed redshifts.
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- In this work, we have developed an FLRW type model of a universe which displays transition from deceleration in the past to the acceleration at the present. For this, we have considered field equations of f(R,T) gravity and have taken f (R, T ) = R + 2λT , λ being an arbitrary constant. We have estimated the λ parameter in such a way that the transition red shift is found similar in the deceleration parameter, pressure and the equation of state parameter ω. The present value of Hubble parameter is estimated on the basis of the three types of observational data set: latest compilation of 46 Hubble data set, SNe Ia 580 data sets of distance modulus and 66 Pantheon data set of apparent magnitude which comprised of 40 SN Ia bined and 26 high redshift data’s in the range 0.014 ≤ z ≤ 2.26. These data are compared with theoretical results through the χ2 statistical test. Interestingly, the model satisfies all the three weak, strong and dominant energy conditions. The model fits well with observational findings. We have discussed some of the physical aspects of the model, in particular the age of the universe.
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- Cosmological models are obtained in a f(R) modified gravity with a coupled Gauss–Bonnet (GB) terms in the gravitational action. The dynamical role of the GB terms is explored with a coupled dilaton field in two different cases (I) where γ, λ and δ are arbitrary constants and (II) f(R) = R and estimate the constraints on the model parameters. In the first case we choose GB terms coupled with a free scalar field in the presence of interacting fluid and in the second case GB terms coupled with scalar field in a self interacting potential to compare the observed Universe. The evolutionary scenario of the Universe is obtained adopting a numerical technique as the field equations are highly non-linear. Defining a new density parameter ΩH, a ratio of the dark energy (DE) density to the present energy density of the non-relativistic matter, we look for a late accelerating Universe. The state finder parameters ΩH, deceleration parameter (q), jerk parameter (j) are plotted. It is noted that a non-singular Universe with oscillating cosmological parameters for a given strength of interactions is admitted in model-I. The gravitational coupling constant λ is playing an important role. The Lagrangian density of f(R) is found to dominate over the GB terms when oscillating phase of DE arises. In model-II, we do not find oscillation of the cosmological parameters as the Universe evolves. In the presence of interaction the energy from radiation sector of matter cannot flow to the other two sectors of fluid. The range of values of the strengths of interaction of the fluids are estimated for a stable Universe assuming the primordial gravitational wave speed equal to unity.
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- The accelerated expansion of the universe during recent times is well known in cosmology, whereas during early times, there was decelerated expansion. The ΛCDM model is consistent with most observations, but there are some issues with it. In addition, the transition from early deceleration to late-time acceleration cannot be explained by general relativity. Hence, it is worthwhile to examine modified gravity theories to explain this transition and to get a better understanding of dark energy. In this work, dark energy in modified f(R,T) gravity is investigated, where R is the Ricci scalar and T is the trace of the energy momentum tensor. Normally, the simplest form of f(R,T) is used, viz., f(R)=R+λT. In this work, the more complicated form f(R,T)=R+RT is investigated in Friedmann–Lemaître–Robertson–Walker spacetime. This form has not been well studied. Since the jerk parameter in general relativity is constant and j=1, in order to have as small a departure from general relativity as possible, the jerk parameter j=1 is also assumed here. This enables the complete solution for the scale factor to be found. One of these forms is used for a complete analysis and is compared with the usually studied form f(R,T)=R+RT. The solution can also be broken down into a power-law form at early times (deceleration) and an exponential form at late times (acceleration), which makes the analysis simpler. Surprisingly, each of these forms is also a solution to the differential equation j=1 (though they are not solutions to the general solution). The energy conditions are also studied, and plots are provided. It is shown that viable models can be obtained without the need for the introduction of a cosmological constant, which reduces to the ΛCDM at late times.
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- An LRS Bianchi I model is considered with constant deceleration parameter, q = α−1, where α ≥ 0 is a constant. The physical and kinematical behaviour of the models for α = 0 and α = 0 is studied in detail. The model with α = 0 describes late time acceleration, but eternal inflation demands a violation of the NEC and WEC. The acceleration is caused by phantom matter which approaches a cosmological constant at late times. The solutions with a scalar field also show that the model is compatible with a phantom field only. A comparison with the observational outcomes indicates that the universe has entered into the present accelerating phase in recent past somewhere between 0.2 z 0.5. The model obeys the “cosmic no hair conjecture”. The models with 0 <α< 1 describe late time acceleration driven by quintessence dark energy. A violation of the NEC and WEC is required to accommodate the early inflationary epoch caused by phantom matter. The models with 1 <α< 3 describe decelerating phases which are usually occur in the presence of dust or radiation. These models are also found anisotropic at early times and attain isotropy at late times. The model for α = 3 represents a stiff matter era which also has shear at early stages and becomes shear free at late times, but it evolves with an insignificant ceaseless anisotropy. The models with α > 3 violate the DEC and the corresponding scalar field models have negative potential which is physically unrealistic.
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- In the current study, we inspect the late-time acceleration of the universe in f (Q, T) gravity. In this theory, Q represents the non-metricity term and with T μ ν the energy-momentum tensor, with T its trace. To explore the solutions in the theory for the Friedmann-Lemaitre-Robertson-Walker (FLRW) model, we propose a new scale factor called the emergent scale factor, which yields the Hubble parameter, H (z), having the form H (z)= n b (− B+ 1 (A (z+ 1)) 1 b) with a new relation a (t)= 1 1+ z). By using the SNIa from Pantheon, BAO from (6dFGS, BOSS-LOWZ, SDSS DR7 MGS, BOSS-DR12, BOSS-CMASS, WiggleZ and Lya), CMB from Planck 2018, and the 36 data points from Hubble datasets using the Monte-Carlo Markov Chain (MCMC) approach, we estimate the parameters of the model. The variation in time of the deceleration parameter in the model shows that a phase of deceleration transits to a phase of acceleration. Further, we examine the behaviour of the jerk parameter and carry out a statefinder analysis. In order to complete the current study, we consider two forms for f (Q, T), specifically, f (Q, T)= a 1 Q m+ a 2 T and f (Q, T)= γ Q 2+ η Q+ λ T where a 1, a 2, m, γ, η, and λ are the model's parameters that need to satisfy observations. Finally, we present an appraisal of our current analysis.
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- Locally-rotationally-symmetric Bianchi-I space time model is studied with constant Hubble parameter in f (R,T ) = R + 2λT gravity. Although a single (primary) matter source is considered, an additional matter source appears due to the coupling between matter and f (R,T ) gravity. The constraints are obtained for a realistic cosmological scenario. The solutions are also extended to the case of a scalar field (normal or phantom) model, and it is found that the model is consistent with a phantom scalar field only. The coupled matter also acts as phantom matter. The study shows that if one expects an accelerating universe from an anisotropic model, then the solutions become physically relevant only at late times when the universe enters into an accelerated phase. Placing some observational bounds on the present equation of state of dark energy, ω0, the behavior of ω(z) is depicted, which shows that the phantom field starts dominating very recently, somewhere between 0.2 < z < 0.5. The geometrical behavior of the model remains identical to the one in general relativity
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