Narrow Results

The main aim of this paper is to explore the Morris–Thorne (MT) static traversable wormhole structures supported by dark energy within the framework of the quadratic $f(R,T)$ formalism, where the gravitational Lagrangian is defined by an arbitrary function of the Ricci scalar R as well as the trace of the stress–energy tensor T. To achieve this aim, we consider the function $f(R,T)=R+\alpha R^2+\lambda T$, where $\alpha$ and $\lambda$ denote coupling parameters, together with two different forms of the shape function introduced in the selected metric as well as assume a constant redshift function. We obtained the exact solution to each choice of shape function and investigated wormholes that can emerge depending on the presence of exotic or non-exotic matter. For the selected theoretical setups, we initially examine the general behavior of the corresponding solutions and then discuss, respectively, the throat, flaring out, asymptotically flatness, hydrostatic equilibrium, and energy conditions. We observed that the quadratic component and linear trace term of $f(R,T)$ gravity and the hybrid forms of shape functions play an important role in validating the energy conditions for viable wormhole solutions.

In [1], we pointed out that in the Dark Dimension scenario [2] theoretical issues arise when the prediction for the vacuum energy $\rho$, that is obtained from swampland conjectures in string theory, is confronted with the corresponding result for $\rho$ in the effective field theory (EFT) limit. One of the problems concerns the widely spread belief that in higher dimensional EFTs with compact dimensions the vacuum energy is automatically finite. On the contrary, our analysis shows that $\rho$ contains (previously missed) UV-sensitive terms. Our work was challenged in [3]. Here we show why in our opinion the claims in [3] are flawed, and provide further support to our findings. We conclude presenting ideas on the physical mechanism that should dispose of the large UV contributions to $\rho$.

According to the “dark dimension” (DD) scenario, we might live in a universe with a single compact extra dimension, whose mesoscopic size is dictated by the measured value of the cosmological constant. This scenario is based on swampland conjectures that lead to the relation ${\rho }_{\mathrm{\text{swamp}}}\sim m_{{}_{\mathrm{\text{KK}}}}^4$ between the vacuum energy ${\rho }_{\mathrm{\text{swamp}}}$ and the size of the extra dimension $m_{{}_{\mathrm{\text{KK}}}}^{-1}$ ($m_{{}_{\mathrm{\text{KK}}}}$ is the mass scale of a Kaluza–Klein tower), and on the corresponding result ${\rho }_{{}_{\mathrm{\text{EFT}}}}$ from the effective field theory (EFT) limit. We show that ${\rho }_{{}_{\mathrm{\text{EFT}}}}$ contains previously missed UV-sensitive terms, whose presence invalidates the widely spread belief (based on existing literature) that the calculation gives automatically the finite result ${\rho }_{{}_{\mathrm{\text{EFT}}}}\sim m_{{}_{\mathrm{\text{KK}}}}^4$ (with no need for fine-tuning). This renders the matching between ${\rho }_{\mathrm{\text{swamp}}}$ and ${\rho }_{{}_{\mathrm{\text{EFT}}}}$ a nontrivial issue. We then comment on the necessity to find a mechanism that implements the suppression of the aforementioned UV-sensitive terms. This should finally allow to frame the DD scenario in a self-consistent framework, also in view of its several phenomenological applications based on EFT calculations.

This paper focuses on the examination of various thermodynamic properties of RN-AdS black hole surrounded by quintessence, which is considered as one of the most widely accepted models of dark energy. The investigation involves the determination of temperature, entropy, heat capacity, pressure, equation of state, and Gibbs free energy for this particular black hole. By employing the laws of thermodynamics pertaining to black holes and considering the influence of quintessence, the impact on these quantities is thoroughly analyzed. Additionally, graphical representations of these quantities are depicted, and based on the critical points obtained, the stability and phase transition of the system are assessed. The study further delves into the examination of stability, instability, and the Swallowtail behavior of the system. Notably, due to the discontinuity in the heat capacity, a phase transition occurs within the system, resembling Van der Waals-like phase transitions that transpire between small, intermediate, and large black holes.

This paper is the continuation of the groundwork laid in [A. Iram, A. A. Siddiqui and T. Feroze, Int. J. Mod. Phys. D31(11) (2022) 2240006], where the Segre classification scheme was applied to categorize spherically symmetric static spacetime metrics into four possible Segre types $[(1,111)],$$[1,(111)],$$[(1,1)(11)],$ or $[1,1(11)]$. The solution for type $[(1,111)]$ leads to the Schwarzschild de-Sitter/anti de-Sitter metrics. The eigenvalue degeneracy in Segre types identifies the kind of matter distribution in space and aid in the consideration of novel solutions for the corresponding energy momentum tensor. We deal with an anisotropic distribution of matter correlated with electric field intensity for types $[(1,1)(11)]$ and $[1,1(11)]$. The type $[1,(111)]$ refers to ideal fluid characterized by uniformly distributed pressure. A comprehensive examination is conducted in scenarios where all physical and stability criteria are satisfied. Nevertheless, for type $[(1,1)(11)]$, the strong energy and causality prerequisites are breached due to negative pressure, suggesting the existence of dark energy where the attributes of standard matter cannot be met. Furthermore, the numerical test for models is conducted for the compact objects $4\text{U}1538-52$, $\text{PSR}\text{J}1614-2230$, $4\text{U}1608-52$, and $\text{EXO}1785-248$. Each star’s density, pressure, and compactness factor are observed, showing the regularity at the origin. These observations impart that the discover models are plausible since they accurately represent the observable system.

This paper explores a comprehensive approach to modeling compact stars that incorporates both normal matter and dark energy. We employ the Durgapal–Fuloria ansatz within the context of Rastall gravity to derive a relativistic analytical solution. The model is thoroughly analyzed both analytically and graphically, to assess its physical properties and facilitates a comparison with the results of classical general relativity. Our findings demonstrate that the model we have put forward is in substantial agreement with observational data for three different compact star representatives like Her X-1, PSR J0348+0432, and RX J1856.3-37.2. We evaluate the model’s viability by examining its energy conditions, stability, and adherence to the Buchdahl limit, all of which are found to be satisfactory. The analysis confirms the stable solution for Rastall parameter spanning −0.005 to 0.11, and it converges to standard general relativity when the coupling parameter approaches to zero.

In this work, we investigate cosmological perturbations of viscous modified chaplygin gas model. Using $1+3$ covariant formalism, we define covariant and gauge invariant gradient variables, which after the application of scalar decomposition and harmonic decomposition techniques together with redshift transformation method, provide the energy overdensity perturbation equations in redshift space, responsible for large-scale structure formation. In order to analyze the effect of the viscous modified chaplygin gas model on matter overdensity contrast, we numerically solve the perturbation equations in both long and short wavelength limits. The numerical results show that the energy overdensity contrast decays with redshift. However, the perturbations which include amplitude effects due to the viscous modified chaplygin model do differ remarkably from those in the $\mathrm{\Lambda }$CDM. In the absence of viscous modified chaplygin model, the results reduce to those of $\mathrm{\Lambda }$CDM.

In this work, the cosmic solutions, particularly the well-known $\mathrm{\Lambda }$CDM model, are investigated in the framework of the Gauss–Bonnet (GB) gravity, where the gravitational action incorporates the GB invariant function. We utilize a specialized formulation of the deceleration parameter in terms of the Hubble parameter $H$, given by $q=-1-\frac{Ḣ}{H^2}$, to solve the field equations. To identify the appropriate model parameters, we align them to the most recent observational datasets, which include 31 data points from the Cosmic Chronometers, Pantheon+, and BAO datasets. The physical characteristics of the cosmographic parameters, such as pressure and energy density, that correlate to the limited values of the model parameters, are examined. The evolution of the deceleration parameter suggests a transition from a decelerated to an accelerated phase of the universe. Additionally, we examine the stability of the assumed model and provide an explanation for late-time acceleration using the energy conditions. The behavior of the equation of state parameter has been analyzed through dynamical variables by constraining various parameters in light of the recent observational data. This study has resulted in a quintessence-like evolution.

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.

This study investigates the $f(R,T)$ gravitational framework by introducing a novel functional form, $f(R,T)=R+\alpha T^{\beta }+\lambda ln(T)$, which incorporates power-law and logarithmic dependencies on the trace of the energy-momentum tensor $T$. These terms enrich the phenomenological interaction between matter and geometry, enabling a comprehensive exploration of their role in cosmic evolution. We derive the modified field equations and analyze their implications for late-time cosmic acceleration, matter clustering, and the formation of large-scale structures. The model reveals notable deviations from the standard $\mathrm{\Lambda }$CDM cosmology, including distinctive signatures in the matter power spectrum, the Integrated Sachs–Wolfe (ISW) effect, and the anisotropies of the cosmic microwave background (CMB). The parameters $\alpha$, $\beta$, and $\lambda$ introduce scale-dependent modifications to the gravitational constant $G_{\mathrm{\text{eff}}}$, which influence the growth of structures and the expansion history. These features allow the model to address fundamental cosmological challenges such as the cosmological constant problem and the coincidence problem by dynamically regulating the evolution of dark matter and dark energy densities. We emphasize the potential of this framework to unify dark energy and dark matter dynamics while offering testable predictions for future large-scale cosmological surveys such as Euclid and LSST. The results underscore the versatility of the $f(R,T)$ model in explaining the observed universe’s accelerated expansion and the growth of cosmic structures without requiring exotic fields or modifications to General Relativity.

We consider a model of interacting cosmological constant/quintessence, where dark matter and dark energy behave as, respectively, two coexisting phases of a fluid, a thermally excited Bose component and a condensate, respectively. In a simple phenomenological model for the dark components interaction we find that their energy density evolution is strongly coupled during the universe evolution. This feature provides a possible way out for the coincidence problem affecting many quintessence models.

We present a whole frame for the cosmic strings, inflation and dark energy with the complex scalar field which can be regarded as the order parameter of our universe. One can find that the comic strings emerge in the zeros of the complex scalar field in the early universe. And with the evolution of complex scalar field, inflation and dark energy can be understood in this frame.

There are two physical actions that have a natural setting in terms of the coadjoint representation of the algebra of diffeomorphisms and of affine Lie algebras. One is the usual geometric action that comes from coadjoint orbits. The other action lives on the phase space that is transverse to the orbits and are called transverse actions, where Yang-Mills theory in two dimensions is an example. Here we show that the transverse action associated with the Virasoro algebra might contain clues for a theory for dark energy. These actions might also suggests a mechanism for symmetry changing.

The early cosmic inflation, when taken along with the recent observations that the universe is currently dominated by a low density vacuum energy, leads to at least two potential problems which modern cosmology must address. First, there is the old cosmological constant problem, with a new twist: the coincidence problem. Secondly, cosmology still lacks a model to predict the observed current cosmic acceleration and to determine whether or not there is a future exit out of this state (as previously in the inflationary case). This constitutes (what is called here) a dynamical problem. Here a framework is proposed to address these two problems, based on treating the cosmic background vacuum (dark) energy as both dynamical and interacting. The universe behaves as a vacuum-driven cosmic engine which, in search of equilibrium, always back-reacts to vacuum-induced accelerations by increasing its inertia (internal energy) through vacuum energy dissipation. The process couples cosmic vacuum (dark) energy to matter to produce future-directed increasingly comparable amplitudes in these fields by setting up oscillations in the decaying vacuum energy density and corresponding sympathetic ones in the matter fields. By putting bounds on the relative magnitudes of these coupled oscillations the model offers a natural and conceptually simple channel to discuss the coincidence problem, while also suggesting a way to deal with the dynamical problem. A result with important observational implications is an equation of state w(t) which specifically predicts a variable, quasi-periodic, acceleration for the current universe. This result can be directly tested by future observational techniques such as SNAP.

One possibility to explain the current accelerated expansion of the universe may be related with the presence of cosmologically evolving scalar whose mass depends on the local matter density (chameleon cosmology). We point out that matter quantum effects in such scalar–tensor theory produce the chameleon scalar field dependent conformal anomaly. Such conformal anomaly adds higher derivative terms to chameleon field equation of motion. As a result, the principal possibility for instabilities appears. These instabilities seem to be irrelevant at small curvature but may become dangerous in the regions where gravitational field is strong.

Diverse cosmological observations indicate the existence of dark energy, comprising ~ 70% of the total cosmic energy density and driving the accelerated cosmic expansion. Possible explanations for the dark energy include a cosmological constant and quintessence — a time-varying, inhomogeneous field with negative pressure. In this article we summarize how the dark energy imprints features on the cosmic microwave background. Observation of these features could be useful in discriminating amongst various theories, and could reveal clues as to the nature of the dark energy.

We study the concentration parameters, their mass dependence and redshift evolution, of dark-matter halos in different dark-energy cosmologies with constant and time-variable equation of state, and compare them with "standard" ΛCDM and OCDM models. The dependence of averaged halo concentrations on mass and redshift permits a simple fit of the form (1+z)c=c₀(M/M₀)^α, with α≈-0.1 throughout. We find that the cluster concentration depends on the dark energy equation of state at the cluster formation redshift z_coll through the linear growth factor D₊(z_coll). As a simple correction accounting for dark-energy cosmologies, we propose scaling c₀ from ΛCDM with the ratio of linear growth factors, c₀→c₀D₊(z_coll)/D₊,ΛCDM(z_coll).

This is a short overview of spatially flat (or open) four-dimensional accelerating cosmologies for some simple exponential potentials obtained by string or M theory compactification on some non-trivial curved spaces, which may lead to some striking results, e.g., the observed cosmic acceleration and the scale of the dark energy from first principles.

We discuss the implications of a new scalar field coupled to neutrinos.

The recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. This may be the evidence of the breakdown of the standard Friedmann equation. Instead of a linear function of the matter density, we consider a general function of the matter density to modify the Freidmann equation. We propose a new model which explains the recent acceleration and the past deceleration. Furthermore, the new model also gives a decelerated universe in the future. The new model gives Ω_m0=0.46 and z_T=0.44.