Search Results (47)

In this article, we investigate the influence of viscosity on the evolution of the cosmos within the framework of the newly proposed $f(Q,L_m)$ gravity. We have considered a linear functional form $f(Q,L_m)=\alpha Q+\beta L_m$ with a bulk viscous coefficient $\zeta ={\zeta }_0+{\zeta }_{1}H$ for our analysis and obtained exact solutions to the field equations associated with a flat FLRW metric. In addition, we utilized Cosmic Chronometers (CC), CC + BAO, CC + BAO + GRB, and GRB data samples to determine the constrained values of independent parameters in the derived exact solution. The likelihood function and the Markov Chain Monte Carlo (MCMC) sampling technique are combined to yield the posterior probability using Bayesian statistical methods. Furthermore, by comparing our results with the standard cosmological model, we found that our considered model supports the acceleration of the universe in late time. Full article

Wormholes are non-trivial topological structures that arise as exact solutions to Einstein’s field equations, theoretically connecting distinct regions of spacetime via a throat-like geometry. While static traversable wormholes necessarily require exotic matter that violates the classical energy conditions, subsequent studies have sought to minimize such violations by introducing time-dependent geometries embedded within cosmological backgrounds. This review provides a comprehensive survey of evolving wormhole solutions, emphasizing their formulation within both general relativity and alternative theories of gravity. We explore key developments in the construction of non-static wormhole spacetimes, including those conformally related to static solutions, as well as dynamically evolving geometries influenced by scalar fields. Particular attention is given to the wormholes embedded into Friedmann–Lemaître–Robertson–Walker (FLRW) universes and de Sitter backgrounds, where the interplay between the cosmic expansion and wormhole dynamics is analyzed. We also examine the role of modified gravity theories, especially in hybrid metric–Palatini gravity, which enable the realization of traversable wormholes supported by effective stress–energy tensors that do not violate the null or weak energy conditions. By systematically analyzing a wide range of time-dependent wormhole solutions, this review identifies the specific geometric and physical conditions under which wormholes can evolve consistently with null and weak energy conditions. These findings clarify how such configurations can be naturally integrated into cosmological models governed by general relativity or modified gravity, thereby contributing to a deeper theoretical understanding of localized spacetime structures in an expanding universe. Full article

An approach is presented to address singularities in general relativity using a complex Riemannian spacetime extension. We demonstrate how this method can be applied to both black hole and cosmological singularities, specifically focusing on the Schwarzschild and Kerr black holes and the Friedmann–Lemaître–Robertson–Walker (FLRW) Big Bang cosmology. By extending the relevant coordinates into the complex plane and carefully choosing integration contours, we show that it is possible to regularize these singularities, resulting in physically meaningful, singularity-free solutions when projected back onto real spacetime. The removal of the singularity at the Big Bang allows for a bounce cosmology. The approach offers a potential bridge between classical general relativity and quantum gravity effects, suggesting a way to resolve longstanding issues in gravitational physics without requiring a full theory of quantum gravity. Full article

The cosmic time dilation observed in Type Ia supernova light curves suggests that the passage of cosmic time varies throughout the evolution of the Universe. This observation implies that the rate of proper time is not constant, as assumed in the standard FLRW metric, but instead is time-dependent. Consequently, the commonly used FLRW metric should be replaced by a more general framework, known as the Conformal Cosmology (CC) metric, to properly account for cosmic time dilation. The CC metric incorporates both spatial expansion and time dilation during cosmic evolution. As a result, it is necessary to distinguish between comoving and proper (physical) time, similar to the distinction made between comoving and proper distances. In addition to successfully explaining cosmic time dilation, the CC metric offers several further advantages: (1) it preserves Lorentz invariance, (2) it maintains the form of Maxwell’s equations as in Minkowski spacetime, (3) it eliminates the need for dark matter and dark energy in the Friedmann equations, and (4) it successfully predicts the expansion and morphology of spiral galaxies in agreement with observations. Full article

Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate $\Lambda =\Lambda \left(H\right)$. This running is inherited from the cosmic evolution of the vacuum energy density (VED), ${\rho }_{\mathrm{vac}}$, which is sensitive to quantum effects in curved spacetime that ultimately trigger that running. The VED is a function of the Hubble rate and its time derivatives ${\rho }_{\mathrm{vac}}={\rho }_{\mathrm{vac}}(H,\dot{H},\ddot{H},\dots )$. Two nearby points of cosmic evolution during the FLRW epoch are smoothly related as $\delta {\rho }_{\mathrm{vac}}\sim \mathcal{O}\left(H^2\right)$. In the very early universe, in contrast, the higher powers of the Hubble rate take over and bring about a period of fast inflation. They originate from quantum effects on the effective action of a vacuum, which we compute. Herein, we focus on the lowest possible power for inflation to occur: $H^4$. During the inflationary phase, H remains approximately constant and very large. Subsequently, the universe enters the usual FLRW radiation epoch. This new mechanism (‘RVM inflation’) is not based on any supplementary ‘inflaton’ field; it is fueled by pure QFT effects on the dynamical background and is different from Starobinsky’s inflation, in which H is never constant. Full article

In the initial part of this paper, we survey (in arbitrary spacetime dimension) the general FLRW cosmologies with non-interacting perfect fluids and with a canonical or phantom scalar field, minimally coupled to gravity and possibly self-interacting; after integrating the evolution equations for the fluids, any model of this kind can be described as a Lagrangian system with two degrees of freedom, where the Lagrange equations determine the evolution of the scale factor and the scalar field as functions of the cosmic time. We analyze specific solvable models, paying special attention to cases with a phantom scalar; the latter favors the emergence of nonsingular cosmologies in which the Big Bang is replaced, e.g., with a Big Bounce or a periodic behavior. As a first example, we consider the case with dust (i.e., pressureless matter), radiation, and a scalar field with a constant self-interaction potential (this is equivalent to a model with dust, radiation, a free scalar field and a cosmological constant in the Einstein equations). In the phantom subcase (say, with nonpositive spatial curvature), this yields a Big Bounce cosmology, which is a non-absurd alternative to the standard ($\Lambda$CDM) Big Bang cosmology; this Big Bounce model is analyzed in detail, even from a quantitative viewpoint. We subsequently consider a class of cosmological models with dust and a phantom scalar, whose self-potential has a special trigonometric form. The Lagrange equations for these models are decoupled passing to suitable coordinates $(x,y)$, which can be interpreted geometrically as Cartesian coordinates in a Euclidean plane: in this description, the scale factor is a power of the radius $r=\sqrt{x^2+y^2}$. Each one of the coordinates $x,y$ evolves like a harmonic repulsor, a harmonic oscillator, or a free particle (depending on the signs of certain constants in the self-interaction potential of the phantom scalar). In particular, in the case of two harmonic oscillators, the curves in the plane described by the point $(x,y)$ as a function of time are the Lissajous curves, well known in other settings but not so popular in cosmology. A general comparison is performed between the contents of the present work and the previous literature on FLRW cosmological models with scalar fields, to the best of our knowledge. Full article

We investigate the cosmological evolution of the universe for a spatially flat FLRW background space within the context of $f(T,B)$ gravity, which is a recently formulated teleparallel theory that connects both $f\left(T\right)$ and $f\left(R\right)$ gravity under suitable limits. The analysis focuses on four different $f(T,B)$ cosmological models corresponding to various choices of scale factor, namely, emergent, logamediate, and intermediate. In addition to this, we assume a power law-like function of $f(T,B)$ gravity. The reconstruction of $f(T,B)$ gravity considers the Holographic Ricci Dark Energy (HRDE) as the background fluid. We analyze the equation of state parameters and the squared speed of sound for the reconstructed models. Finally, we conduct a thermodynamical analysis for each reconstructed model. The generalized second law of thermodynamics (GSLT) is valid for the four different $f(T,B)$ cosmological models. Full article

We apply a combined study in order to investigate the dynamics of cosmological models incorporating nonlinear electrodynamics (NLED). The study is based on the simultaneous investigation of such fundamental aspects as stability and causality, complemented with a dynamical systems investigation of the involved models, as well as Bayesian inference for parameter estimation. We explore two specific NLED models: the power-law and the rational Lagrangian. We present the theoretical framework of NLED coupled with general relativity, followed by an analysis of the stability and causality of the various NLED Lagrangians. We then perform a detailed dynamical analysis to identify the ranges where these models are stable and causal. Our results show that the power-law Lagrangian model transitions through various cosmological phases, evolving from a Maxwell radiation-dominated state to a matter-dominated state. For the rational Lagrangian model, including the Maxwell term, stable and causal behavior is observed within specific parameter ranges, with critical points indicating the evolutionary pathways of the universe. To validate our theoretical findings, we perform Bayesian parameter estimation using a comprehensive set of observational data, including cosmic chronometers, baryon acoustic oscillation (BAO) measurements, and supernovae type Ia (SNeIa). The estimated parameters for both models align with the expected values for the current universe, particularly the matter density ${\Omega }_m$ and the Hubble parameter h. However, the parameters of the models are not tightly constrained within the prior ranges. Our combined studies approach rules out the mentioned models as an appropriate description of the cosmos. Our results highlight the need for further refinement and exploration of NLED-based cosmological models to fully integrate them into the standard cosmological framework. Full article

A Friedmann–Lemaitre–Robertson–Walker space–time model with all curvatures $k=0, \pm 1$ is explored in $f(R,T)$ gravity, where R is the Ricci scalar, and T is the trace of the energy–momentum tensor. The solutions are obtained via the parametrization of the scale factor that leads to a model transiting from a decelerated universe to an accelerating one. The physical features of the model are discussed and analyzed in detail. The study shows that $f(R,T)$ gravity can be a good alternative to the hypothetical candidates of dark energy to describe the present accelerating expansion of the universe. Full article

Considering the Friedmann–Lemaître–Robertson–Walker (FLRW) metric and the Einstein scalar field system as an underlying gravitational model to construct fractional cosmological models has interesting implications in both classical and quantum regimes. Regarding the former, we just review the most fundamental approach to establishing an extended cosmological model. We demonstrate that employing new methodologies allows us to obtain exact solutions. Despite the corresponding standard models, we cannot use any arbitrary scalar potentials; instead, it is determined from solving three independent fractional field equations. This article concludes with an overview of a fractional quantum/semi-classical model that provides an inflationary scenario. Full article

We construct and examine a holonomy-corrected chiral fields model of cosmological relevance. Specifically, we holonomize the Hamiltonian corresponding to a quintom field scenario with additional kinetic interaction (governed by the constant chiral metric, $m_{ab}$) on a flat FLRW background and contrast the resulting model with the corresponding purely classical system. In particular, it is shown that the single LQC bouncing stage is ensured to be realized, provided the full chiral kinetic energy function does not change sign during evolution. (As preparation, a particularly simple k-essence field is examined within the effective LQC scheme; some exact solutions are obtained in the process.) Additionally, under the said assumption, it is established that the landmark bouncing mechanism of standard (effective) LQC is still guaranteed to be featured even when taking any finite number of fields ${\varphi }^1,\dots {\varphi }^m$ and $m_{ab}$ to be dependent on such fields (the particular zero-potential case corresponding to a family of simple purely kinetic k-essence multi-field cosmology models). Full article

A viable radiation-dominated era in the early universe is best described by the standard (FLRW) model of cosmology. In this short review, we demonstrate reconstruction of the forms of $F\left(R\right)$ in the modified theory of gravity and the metric compatible $F\left(\mathrm{T}\right)$ together with the symmetric $F\left(Q\right)$ in alternative teleparallel theories of gravity, from different perspectives, primarily rendering emphasis on a viable FLRW radiation era. Inflation has also been studied for a particular choice of the scalar potential. The inflationary parameters are found to agree appreciably with the recently released observational data. Full article

The present analysis deals with the study of the $f(Q,T)$ theory of gravity, which was recently considered by many cosmologists. In this theory of gravity, the action is taken as an arbitrary function $f(Q,T)$, where Q is non-metricity and T is the trace of the energy–momentum tensor for matter fluid. In this study, we took two different forms of the function $f(Q,T)$ as $f(Q,T)=a_{1}Q+a_{2}T$ and $f(Q,T)=a_3{Q}^2+a_{4}T$, and discussed the physical properties of the models. Also, we obtained the various cosmological parameters for the Friedmann–Lemaître–Robertson–Walker (FLRW) universe by defining the transit form of a scale factor that yielded the Hubble parameter in redshift form, as $H\left(z\right)=\frac{H_0}{(\lambda +1)}\left(\lambda +{(1+z)}^{\delta }\right)$. We obtained the best-fit values of model parameters using the least squares method for observational constraints on available datasets, like Hubble $H\left(z\right)$, Supernova SNe-Ia, etc., by applying the root mean squared error formula (RMSE). For the obtained approximate best-fit values of model parameters, we observed that the deceleration parameter $q\left(z\right)$ shows a signature-flipping (transition) point within the range of $0.623\le z_0\le 1.668$. Thus, it shows the decelerated expansion transiting into the accelerated universe expansion with $\omega \to -1$ as $z\to -1$ in the extreme future. Full article

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the ‘running vacuum model’ (RVM) form, in which the VED evolves mildly as $\delta {\rho }_{\mathrm{vac}}\left(H\right)\sim {\nu }_{\mathrm{eff}}{m}_{\mathrm{Pl}}^2\mathcal{O}\left(H^2\right)$, where H is the Hubble rate and ${\nu }_{\mathrm{eff}}$ is a (small) free parameter. This dynamical scenario is grounded on recent studies of quantum field theory (QFT) in curved spacetime and also on string theory. It turns out that what we call the ‘cosmological constant’, $\mathsf{\Lambda }$, is no longer a rigid parameter but the nearly sustained value of $8\pi G\left(H\right){\rho }_{\mathrm{vac}}\left(H\right)$ around any given epoch $H\left(t\right)$, where $G\left(H\right)$ is the gravitational coupling, which can also be very mildly running (logarithmically). Of particular interest is the possibility suggested in past works that such a running may help to cure the cosmological tensions afflicting the $\mathsf{\Lambda }$CDM. In the current study, we reanalyze the RVM in full and we find it becomes further buttressed. Using modern cosmological data, namely a compilation of the latest SNIa+BAO+$H\left(z\right)$+LSS+CMB observations, we probe to what extent the RVM provides a quality fit better than the concordance $\mathsf{\Lambda }$CDM model, with particular emphasis on its impact on the ${\sigma }_8$ and $H_0$ tensions. We utilize the Einstein–Boltzmann system solver CLASS and the Monte Carlo sampler MontePython for the statistical analysis, as well as the statistical DIC criterion to compare the running vacuum against the rigid vacuum (${\nu }_{\mathrm{eff}}=0$). On fundamental grounds, ${\nu }_{\mathrm{eff}}$ receives contributions from all the quantized matter fields in FLRW spacetime. We show that with a tiny amount of vacuum dynamics (${\nu }_{\mathrm{eff}}\ll 1$) the global fit can improve significantly with respect to the $\mathsf{\Lambda }$CDM and the mentioned tensions may subside to inconspicuous levels. Full article

Using an open thermodynamic systems theory, the effect of particle creation on the evolution and dynamics of the standard cosmological FLRW model in a higher-dimensional spacetime with functionally dependent cosmological and gravitational constants $\mathsf{\Lambda }$ and G is investigated. The gravitational field equations have been transformed into a dimensionless system of non-linear, first-order, coupled differential equations (DEs) as functions of the universe’s density parameters ${\Omega }_i$ and rate of particle creation $\mathsf{\Psi }$ in redshift space, which can be numerically casted. Two cosmological models are obtained, depending on the choice of particle creation rate—$\mathsf{\Psi }\sim H^2$ and $\mathsf{\Psi }\sim n^2$ for dust-, radiation- and dark-energy-dominated universes, respectively. The dynamic behaviour of each model is discussed. Full article