Narrow Results

We study the bulk viscosity taking dust matter in the generalized teleparallel gravity. We consider different dark energy (DE) models in this scenario along with a time-dependent viscous model to construct the viscous equation of state (EoS) parameter for these DE models. We discuss the graphical representation of this parameter to investigate the viscosity effects on the accelerating expansion of the universe. It is mentioned here that the behavior of the universe depends upon the viscous coefficients showing the transition from decelerating to accelerating phase. It leads to the crossing of phantom divide line and becomes phantom dominated for specific ranges of these coefficients.

Commonly used boundary conditions in reconstructing f(T) gravity from holographic Ricci dark energy (RDE) model are found to cause some problem, we therefore propose new boundary conditions in this paper. By reconstructing f(T) gravity from the RDE with these new boundary conditions, we show that the new ones are better than the present commonly used ones since they can give the physically expected information, which is lost when the commonly used ones are taken in the reconstruction, of the resulting f(T) theory. Thus, the new boundary conditions proposed here are more suitable for the reconstruction of f(T) gravity.

This paper is devoted to study the power-law entropy corrected holographic dark energy (ECHDE) model in the framework of f(T) gravity. We assume infrared (IR) cutoff in terms of Granda–Oliveros (GO) length and discuss the constructed f(T) model in interacting as well as in non-interacting scenarios. We explore some cosmological parameters like equation of state (EoS), deceleration, statefinder parameters as well as ω_T–ω_T′ analysis. The EoS and deceleration parameters indicate phantom behavior of the accelerated expansion of the universe. It is mentioned here that statefinder trajectories represent consistent results with ΛCDM limit, while evolution trajectory of ω_T–ω_T′ phase plane does not approach to ΛCDM limit for both interacting and non-interacting cases.

In this paper, we study static spherically symmetric wormhole solutions in the framework of f(T) gravity, where T represents torsion scalar. We consider non-diagonal tetrad and anisotropic distribution of the fluid. We construct expressions for matter components such as energy density, radial pressure and transverse pressure from the field equations. Taking into account a particular equation of state (EoS) in terms of traceless fluid, we discuss the behavior of energy conditions for wormhole solutions with well-known f(T) and shape functions. We conclude that physically acceptable static wormhole solutions are obtained for both these functions.

We discuss modified teleparallel gravity with function $f(T,T_G)$ in the action, where the function depends on two arguments: torsion scalar $T$ and analogue of Gauss–Bonnet invariant $T_G$. In contradistinction to usual teleparallel gravity $f(T)$, this theory contains higher derivative terms, which may produce different instabilities. We discuss Minkowski stability problem in such kind of theories and explicitly demonstrate that for stability it must be $f_T(0,0)<0$, $f_{T_G{T}_G}>0$. We apply these restrictions for the few types of functions discussed by the early authors.

In this work, the possible existence of wormhole solutions have been investigated in the extended teleparallel $f(T)$ theory of gravity by incorporating the Lorentzian source of non-commutative geometry through the conformal motion. The physical concept of conformal symmetry becomes more arguable when it is discussed in the background of non-commutative geometry, especially with the Lorentzian source. In this context, two specific different models of the extended teleparallel theory, that is, $f_1(T)={\eta }_{1}T+\frac{{\eta }_2}{T}$, and $f_2(T)={\eta }_3{T}^n$ (where ${\eta }_1$, ${\eta }_2$, ${\eta }_3$ being real constants and $n$ a positive integer) have been studied. The corresponding energy conditions are worked out and are analyzed graphically in the presence of the conformal motion with Lorentzian source. The presence of the exotic matter has been confirmed due to the violation of null energy conditions under some particular conditions, thereby proving the existence of the wormhole geometries in both of the models under investigation. Moreover, the stability of the wormhole geometries via the Tolman-Oppenheimer-Volkov equation has been discussed. It is concluded that these wormhole solutions supported by the non-exotic matter truly exist and are well stable under the extended teleparallel gravity.

The paper deals with cosmology in modified $f(T)$ gravity theory. With some phenomenological choices for the function $f(T)$ it is possible to have cosmological solutions describing different phases of the evolution of the Universe for the homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker (FLRW) model. By proper choice of the parameters involved in the function $f(T)$ and also in the cosmological solutions it is shown that a continuous cosmic evolution starting from the emergent scenario to the present late-time acceleration is possible. Finally thermodynamical analysis of $f(T)$ gravity is presented.

Recently literature is found to be enriched with studies related to anisotropic behaviors of different compact stars in the background of $f(T)$ gravity in different energy conditions. Quintessence field, as local impacts of cosmic acceleration upon the compact stars, is also very interesting in recent studies. In this paper, the quintessential field effects on the compact stars (mainly on the neutron stars with an wide range of mass distributions), repulsive gravitational effects inside the compact stars due to dark matter distribution in them, charge distribution inside them in strong energy condition, etc. are studied. All required equations of motion using anisotropic property and concept of Massachusetts Institute of Technology bag model are acquired. Black holes surrounded by quintessential matters which satisfy the additive and linearity conditions, with the form of energy tensors were proposed and the corresponding metric was derived by Kiselev.¹ The metric, described by Krori and Barua² with Reissner–Nordström metric³ are compared to find out the different numerical values of unknown parameters. The numerical values are derived and some important parameters like anisotropic stress, adiabatic constant, surface redshift, electric intensity, compactness factor, stability etc. are analyzed deeply to get a clear idea for further study on these types of stars and to understand their nature.

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.

The existence of an extra degree of freedom (d.o.f.) in $f(T)$ gravity has been recently proved by means of the Dirac formalism for constrained Hamiltonian systems. We will show a toy model displaying the essential feature of $f(T)$ gravity, which is the pseudo-invariance of $T$ under a local symmetry, to understand the nature of the extra d.o.f.

In this work, we consider an emergent Universe in generalized gravity theories like the chameleon, f(R) and f(T) gravities. We reconstruct the potential of the chameleon field under the emergent scenario of the Universe and observe its increasing nature with the evolution of the Universe. We reveal that in the emergent Universe scenario, the equation-of-state parameter behaves like quintessence in the case of f(R) gravity and like phantom in the case of f(T) gravity.

The present work is based on the idea of an interacting framework of new holographic dark energy (HDE) with cold dark matter in the background of $f(T)$ gravity. Here, we have considered the flat modified Friedmann universe for $f(T)$ gravity which is filled with new HDE and dark matter. We have derived some cosmological parameters like deceleration parameter, equation of state (EoS) parameter, state-finder parameters, cosmographic parameters, Om parameter and graphically investigated the nature of these parameters for the above mentioned interacting scenario. The results are found to be consistent with the accelerating universe. Also, we have graphically investigated the trajectories in $\omega -{\omega }^{\prime }$ plane for different values of the interacting parameter and explored the freezing region and thawing region in $\omega -{\omega }^{\prime }$ plane. Finally, we have analyzed the stability of this model.

The self-gravitating spherically symmetric fluid models are being studied taking power-law model in extended teleparallel (or $f(T)$) gravity. We form a set of governing equations which describes the dynamics of stellar evolution in the presence of torsion scalar (dark energy candidate) by incorporating power-law model in $f(T)$ gravity along with dynamical terms like shear tensor, anisotropy, expansion scalar, dissipation, Weyl tensor and energy inhomogeneity. We explore some particular models of fluid according to various dynamical scenarios for particular values of model parameter $n$. It is found that torsion terms associated with $\left|n\right|\ll 1,n\ne 0,1$, govern stellar evolution and provide deviation from theory of general relativity (GR). For the case, $n=0,1$, tetrad field is almost negligible and the evolving models are consistent with GR model having cosmological constant. We obtain practicable rate of change of expansion and deformation of fluid models at $0<\left|n\right|\ll 1$.

We have studied LRS Bianchi type I cosmological models with barotropic perfect fluid and cosmic string in the framework of $f(T)$ theory of gravitation. We have assumed that expansion of the model is proportional to the shear scalar. Hybrid law of expansion is also used to solve the field equations. Three different functional forms of the function $f(T)$ such as $f(T)=\eta {(-T)}^m+c,$$f(T)=e^{mT}$ and $f(T)=\gamma T+\eta T^m$ are chosen for investigation. It is observed that the universe is dominated by quintessence type dark energy. The universe is accelerating, expanding and anisotropic.

In this paper, we investigate the holographic inflation in the framework of two gravitational theories like $f(T)$ gravity, where $T$ corresponds to the torsion scalar and DGP braneworld model which is based on the idea that our four-dimensional FRW universe located on five-dimensional manifold. The modified infrared cutoff is used to explore all the inflationary parameters as slow-roll parameters $({𝜖}_1,{𝜖}_2,{𝜖}_3)$, number of $e$-folds $N$, scalar spectral index $n_s$, tensor-to-scalar ratio $r$ and running of the scalar spectral index ${\alpha }_s$. Also, we investigate the behavior of inflationary parameters $n_s-N,{\alpha }_s-N,r-N,{\alpha }_s-n_s$ and $r-n_s$ through graphical presentation. The behavior of these parameters shows the consistency with Planck data $2018$.

As in the case of Lanczos–Lovelock gravity, the main advantage of $F(T)$ gravity is said to be that it leads to second-order field equations, while $F(R)$ gravity theory leads to fourth-order equations. We show that it is rather a disadvantage, since it leads to the unresolved issue of ‘Branched Hamiltonian’. The problem is bypassed in $F(R,T)$ gravity theory.

Recent research works have shown the existence of super-massive neutron stars (NSs) with mass about $2.2M_{\odot }$ or even more. The query about those super-massive NSs inspires the researchers to analyze their features and structures immensely. Here, we have inspected the behavior and properties of some of those super-massive NSs in $f(T)$ modified gravity with $T=T+\alpha T^2$, where $T$ is the torsional scalar and $\alpha$ is a regulatory parameter. In this framework of teleparallel formalism of modified gravity, we obtain the equations of motion by considering quintessence field, modified Chaplygin gas (MCG) and electromagnetic field. For our model, we use matching conditions under spherical symmetry, in order to find out the numerical values of different unknown constants of our model. This helps us to acquire various physical quantities thoroughly and to understand about the nature of those super-massive NSs deeply and quite clearly. Moreover, from our work, we can also explain the role of quintessence field and MCG in case of massive compact stars. The mass–radius relationship curve of this model can effectively describe the mass of the heaviest NS (about $2.6M_{\odot }$) ever detected via gravitational wave detection. Again, we overall investigate the anisotropic behavior, density profile, pressure profile, core repulsive force, stability, equilibrium and energy conditions of those massive compact objects. We further analyze different important parameters like anisotropic stress, surface redshift, adiabatic behavior, compactness factor, sound speed, etc. in case of super-massive NSs for better realization and future study.

The work reported in this paper explores holographic bounce. In the first phase of the study, we chose a non-singular bouncing scale factor. Then we reconstructed $f(T)$ gravity and analytically derived constraints on the bouncing parameter $\sigma$. These constraints helped us understand the scale factor’s quintessence or phantom behavior. Furthermore, we also explored the statefinder parameters for reconstructed $f(T)$ and observed the attainment of $\mathrm{\Lambda }$CDM fixed point. Next, we considered the multiplicative bouncing scale factor inspired by S. D. Odintsov and V. K. Oikonomou Phys. Rev. D 94, (2016) 064022. For this choice, we discussed the types of singularities realizable for different cases. Through the Talyor series expansion, we analytically presented cases and subcases for different ranges of $\alpha$ of the scale factor. In the last phase of the study, we demonstrated holographic bounce with the choice of the multiplicative scale factor. In this case, we considered holographic Ricci dark energy and Barrow holographic dark energy. We concluded that it is possible to generate constraints on the bouncing parameter for its feasibility for the EoS parameter. We concluded that the realization of holographic bounce is possible, and different suitable constraints can be derived for this multiplicative bouncing scale factor focusing on the realization of cosmic bounce.

In this work, we investigate the cosmological application of modified Chaplygin gas (MCG) interacting with pressureless dark matter (DM) in the $f(T)$ modified gravity framework, where $T$ is the torsion scalar in teleparallelism. The interaction term has been chosen proportional to the MCG density with positive coupling constant. In the Einstein general relativity (GR) framework, the interacting MCG has been found to have equation of state (EoS) parameter behaving like quintessence. However, the $f(T)$ gravity reconstructed via the interacting MCG has been found to have EoS crossing the phantom boundary of $-1$. Thus, one can generate a quintom-like EoS from an interacting MCG model in flat universe in the modified gravity cosmology framework. The reconstructed $f(T)$ model has been found to interpolate between dust and $\mathrm{\Lambda }$CDM. Stability of the reconstructed $f(T)$ has been investigated and it has been observed that the model is stable against gravitational perturbation. Cosmological evolution of primordial perturbations has also been investigated and the self-interacting potential has been found to increase with cosmic time and the squared speed of sound has been found to be non-negative.