Null Energy Condition Research Articles

Bulk Viscous Flat FLRW Model with Observational Constraints in f(T, B) Gravity

This paper investigates the impact of bulk viscosity within the framework of f(T,B) gravity. We consider a time-dependent viscosity model with a particular Hubble parameter expression. Here, we looked into the viability of well-motivated f(T,B) gravity model, which takes the form f=αlog(B)+βT, and has free parameters α and β. The 46 observational Hubble data (OHD) in the range 0≤z≤2.36 were used to constrain the model parameters to achieve the solution. We have used the Markov Chain Monte Carlo (MCMC) method to estimate model parameters and observe that the model appears to be in good agreement with the observations. In addition, we evaluate the effective viscous equation of state parameter for the f(T,B) model. We have examined the characteristics of different energy conditions for the stability analysis. The model is valid based on the positive behavior of null energy conditions (NEC), weak energy conditions (WEC), and dominant energy conditions (DEC); however, strong energy conditions (SEC) are in violation, suggesting that the universe is expanding faster. Our model was found in the quintom region. We also discussed how the tachyon scalar field corresponds to f(T,B) gravity.

Higher-dimensional polytropic and modified Chaplygin black holes: Thermodynamics and heat engines

In this paper, we have assumed that the thermodynamic pressure may be generated by the negative cosmological constant and the thermodynamic parameters of the n-dimensional asymptotically Anti-de Sitter (AdS)-Tangherlini black holes are identical with the polytropic gas and modified Chaplygin gas separately. The black hole mass, volume, entropy, and temperature have been written due to the assigned gases’ thermodynamic system. We found the solutions of Einstein’s field equations of n-dimensional black hole for polytropic gas and modified Chaplygin gas. The null energy condition, weak energy condition, strong energy condition, and dominant energy condition have been examined for the fluid sources in n-dimensional black hole system due to both the gases. We found that the four energy conditions completely depend on the model parameters and the dimensions of the spacetime. We have studied the heat engine phenomena for both the black holes and analyzed the work done with the efficiencies due to the Carnot and other cycles.

New wormhole solutions in a viable f (R) gravity model

Traversable wormhole solutions in General Relativity require exotic matter sources that violate the null energy condition (NEC), and such behavior may be avoided in modified gravity. In this study, we analyze the energy conditions for static, spherically symmetric traversable Morris–Thorne wormholes in a recently proposed viable [Formula: see text] gravity model. We numerically analyze solutions considering both constant and variable redshift functions, and present wormhole spacetimes respecting the NEC, supported by a phantom energy-like equation of state for the source. Moreover, we analyze the stability of the spacetimes using the generalized Tolman–Oppenheimer–Volkov equation. We demonstrate the effects of certain parameters in the [Formula: see text] model in determining energy condition violations, and establish that stable wormholes can be formulated only at the expense of violating the NEC.

Wormhole solutions in f(R,T) = R + αR2 + βlnT gravity

In this paper, the Morris–Thorne wormholes are studied in the background of [Formula: see text] gravity, where [Formula: see text] and [Formula: see text] are constants. The wormhole solutions are derived in two cases with different assumptions for the matter forms. In each case, the null energy condition (NEC) is analyzed and the range of parameters satisfying the NEC are determined. Further, the stability of solutions is examined with the help of the generalized Tolman–Oppenheimer–Volkoff (TOV) equation.

A comparative study of new generic wormhole models with stability analysis via thin-shell

This analysis is devoted to exploring the interesting aspects of wormhole geometry. The energy conditions are checked for two different new generic shape functions, which satisfy the required wormhole properties. The presence of exotic matter is confirmed due to the violation of the energy conditions in the background of f(R, T) gravity as well as in the general relativity case. The traversable wormholes respecting the null energy conditions can be realized in both considered frameworks. A thin-shell around a wormhole geometry with two different generic shape functions is obtained by using the cut and paste approach taking Schwarzschild spacetime as an exterior manifold. The stability of thin-shell is explored with linearized perturbation along the equilibrium shell radius. Stable regions and the position of the expected event horizon depend on the choice of physical parameters. It is concluded that the number of expected event horizons increases for the second shape function.

Non-local gravity wormholes

We consider Non-local Gravity in view to obtain stable and traversable wormhole solutions. In particular, the class of Non-local Integral Kernel Theories of Gravity, with the inverse d'Alembert operator in the gravitational action, is taken into account. We obtain constraints for the null energy condition and derive the field equations. Two special cases for the related Klein-Gordan equation are assumed: one where the function in the gravitational action has a linear form and another one with exponential form. In each case, we take into account two forms for scalar fields and derive the shape functions. Asymptotic flatness and flaring-out conditions are checked. Energy conditions and dynamics of the solutions are examined at the throat. The main result is that non-local gravity contributions allow stability and traversability of the wormhole without considering any exotic matter.

Tightening the Penrose inequality

The Penrose inequality estimates the lower bound of the mass of a black hole in terms of the area of its horizon. This bound is relatively loose for extremal or near extremal black holes. We propose a new Penrose-like inequality for static black holes involving the mass, area of the black hole event horizon and temperature. Our inequality includes the Penrose inequality as its corollary, and it is saturated by both the Schwarzschild and Reissner-Nordström black holes. In the spherically symmetric case, we prove this new inequality by assuming both the null and trace energy conditions.

Non-exotic wormholes in 4D Einstein–Gauss–Bonnet gravity

In the present paper, we investigate wormholes in 4D-Einstein-Gauss-Bonnet gravity without the requirement of exotic matters. We have taken the radial dependent red-shift function $\phi=\ln \left( {\frac {r_{{0}}}{r}}+1 \right)$ and shape function $b(r)={\frac{r_{0}\ln(r+1)}{\ln({r_0}+1)}}$ as well as anisotropic matter sources through equation of state (EoS) $p_r(r) = \omega \rho(r)$. We examine the energy conditions, flaring out condition, throat condition and anisotropic parameter. The volume integral quantifier is also analyzed to validate the existence and stability of the WH solution. We find, for -ve branch wormhole solutions satisfy the null energy conditions (NEC) throughout the entire space time and +ve branch reduces exactly to Morris-Thorne of GR.

Erasure Tolerant Quantum Memory and the Quantum Null Energy Condition in Holographic Systems.

Investigating principles for storage of quantum information at finite temperature with minimal need for active error correction is an active area of research. We bear upon this question in two-dimensional holographic conformal field theories via the quantum null energy condition that we have shown earlier to implement the restrictions imposed by quantum thermodynamics on such many-body systems. We study an explicit encoding of a logical qubit into two similar chirally propagating excitations of finite von Neumann entropy on a finite temperature background whose erasure can be implemented by an appropriate inhomogeneous and instantaneous energy-momentum inflow from an infinite energy memoryless bath due to which the system transits to a thermal state. Holographically, these fast erasure processes can be depicted by generalized AdS-Vaidya geometries described previously in which no assumption of specific form of bulk matter is needed. We show that the quantum null energy condition gives analytic results for the minimal finite temperature needed for the deletion which is larger than the initial background temperature in consistency with Landauer's principle. In particular, we find a simple expression for the minimum final temperature needed for the erasure of a large number of encoding qubits. We also find that if the encoding qubits are localized over an interval shorter than a specific localization length, then the fast erasure process is impossible, and furthermore this localization length is the largest for an optimal amount of encoding qubits determined by the central charge. We estimate the optimal encoding qubits for realistic protection against fast erasure. We discuss possible generalizations of our study for novel constructions of fault-tolerant quantum gates operating at finite temperature.

GW170817 Implementation on Einstein-Gauss-Bonnet Theory with Non Minimal and Non Minimal Derivative Coupling

The GW170817 event manifests that gravitational wave velocity is close to thespeed of light. As a result, several theories of gravity are no longer applicable, including EinsteinGauss-Bonnet (EGB) inflation. However, a constraint equation could be applied so that thetheory could produce a viable result. In this study, the EGB inflation is being extended byadding a non-minimal coupling (NMC) and a non-minimal derivative coupling (NMDC). Freeparameters values were evaluated to obtained viability with observational indices. We use powerlaw and exponential Gauss-Bonnet coupling functions. Each model provides observational valuesof ns and r that are compatible with the observations and has its characteristic. It specifiesthe free parameter that controls the alteration of ns and r values. The power-law model iscontrolled by the power m of the Gauss-Bonnet coupling function and the potential integrationconstant, V2. While the exponential model is controlled by the potential integration constant cand the power m of the exponential function. Some approximations do not hold true so that themodels need to be rectified. Apparently, the rectified power-law model is violating null energycondition (NEC), so we also provide the non-violating NEC power-law model.

Exploring existence of traversable wormhole solutions in the presence of some corrected Casimir energy models

In this paper, we explore the wormhole solutions (which are initially proposed by R. Garattini [Eur. Phys. J. C 79 (2019) 951] who take generalized uncertainty principle (GUP) with Casimir energy) in the framework of two well-known dark energy (DE) models, generalized Chaplygin gas (GCG) and polytropic gas. We consider the GUP corrected pressure (force per unit surface area) in the equation of state (EoS) of these DE models to construct the GCG corrected energy density and polytropic gas corrected energy density. Three models of redshift function are taken into account to construct shape function as well as wormhole geometry for both cases of corrected energy densities. We find the behavior of EoS parameters, redshift function for isotropic and anisotropic fluid, equilibrium conditions, null energy condition and exotic volume at the throat of the wormhole with radius [Formula: see text].

Perturbative unitarity and NEC violation in genesis cosmology

Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the “beyond Horndeski” theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and “beyond Horndeski” genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the “beyond Horndeski” operators.

On a Class of Harko-Kovacs-Lobo Wormholes

The Harko, Kovács, and Lobo wormhole (HKLWH) metric contains two free parameters: one is the wormhole throat r0, and the other is a dimensionless deviation parameter γ with values 0<γ<1, the latter ensuring the needed violation of the null energy condition at the throat. In this paper, we study the energetics of the HKLWH and the influence of γ on the tidal forces in the Lorentz-boosted frame. Finally, we apply a new concept, namely, the probabilistic identity of the object observed by different external observers in terms of the Fresnel coefficients derived by Tangherlini. The intriguing result is that observations can differ depending on the location of the observer, i.e., there is a nonzero probability that the HKLWH will be identified as a black hole even when γ≠0.

Non-commutative wormholes in [formula omitted] gravity satisfying the energy conditions

Traversable wormholes in General Relativity (GR) require exotic matter sources that violate the null energy condition (NEC), and such behavior may be avoided in modified gravity. Moreover, the concept of non-commutative geometry as a gravitational source can be leveraged both in GR and modified gravity to realize non-trivial space–time configurations. In this study, we use f(R) gravity in conjunction with non-commutative geometry to analyze spherically symmetric traversable Morris–Thorne wormhole solutions from the aspect of energy condition violation, considering both constant and variable redshift functions. First, we use well constrained metric and model parameters in a viable f(R) gravity model to demonstrate that wormholes respecting the NEC can be obtained with suitable choices of parameters. Additionally, we check the strong and dominant energy conditions to further validate our results. We then leverage non-commutative geometry in the framework of f(R) gravity to show that wormholes respecting the different energy conditions with a phantom-like source can be realized with suitable choices of model parameters. Our comprehensive analyses using well-constrained model parameters show that wormholes satisfying the NEC can be realized in the framework of non-commutative geometry with modified gravity.

C-functions in flows across dimensions

We explore the notion of c-functions in renormalization group flows between theories in different spacetime dimensions. We discuss functions connecting central charges of the UV and IR fixed point theories on the one hand, and functions which are monotonic along the flow on the other. First, using the geometric properties of the holographic dual RG flows across dimensions and the constraints from the null energy condition, we construct a monotonic holographic c-function and thereby establish a holographic c-theorem across dimensions. Second, we use entanglement entropies for two different types of entangling regions in a field theory along the RG flow across dimensions to construct candidate c-functions which satisfy one of the two criteria but not both. In due process we also discuss an interesting connection between corner contributions to the entanglement entropy and the topology of the compact internal space. As concrete examples for both approaches, we holographically study twisted compactifications of 4d mathcal{N} = 4 SYM and compactifications of 6d mathcal{N} = (2, 0) theories.

Validation of energy conditions in traversable wormhole for GBD modified theory

In this work, we explore the wormhole physics in a modified gravitational theory, called the generalized Brans–Dicke theory (GBD). In order to solve the equations in this theory, the parameterized forms for the shape function and the scalar-field function are taken. With the help of selecting a specific form of state equation, i.e. considering a relation between the transverse pressure (not the total pressure) and the energy density for matter that threads wormhole: [Formula: see text], we provide some special solutions of the traversable wormhole in GBD. Given that the violation of energy conditions (ECs) of matter often induces problems on the gravitational theory, it is important to inspect the ECs of matter in modified theories. In this GBD theory, we find that the weak energy condition, the null energy condition, the dominated energy condition and the strong energy condition could be satisfied for the matter near the throat of wormhole, depending on the parameterized models and the model-parameters values. Finally, using the classical reconstruction technique in the traversable wormhole physics, we derive a Lagrangian function of gravitational field for the GBD theory.

General-relativistic thin-shell Dyson megaspheres

Loosely inspired by the somewhat fanciful notion of detecting an arbitrarily advanced alien civilization, we consider a general-relativistic thin-shell Dyson mega-sphere completely enclosing a central star-like object, and perform a full general-relativistic analysis using the Israel--Lanczos--Sen junction conditions. We focus attention on the surface mass density, the surface stress, the classical energy conditions, and the forces between hemispheres. We find that in the physically acceptable region the NEC, WEC, and SEC are always satisfied, while the DEC can be violated if the Dyson mega-sphere is sufficiently close to forming a black hole. We also demonstrate that the original quasi-local version of the maximum force conjecture, F <= {1/4} F_{Stoney} = {1/4} F_{Planck}, can easily be violated if the Dyson mega-sphere is sufficiently compact, that is, sufficiently close to forming a black hole. Interestingly there is a finite region of parameter space where one can violate the original quasi-local version of the maximum force conjecture without violating the DEC. Finally, we very briefly discuss the possibility of nested thin-shell mega-spheres (Matrioshka configurations) and thick-shell Dyson mega-spheres.

A NON-SINGULAR CLOSED BOUNCING UNIVERSE WITHOUT VIOLATION OF NULL ENERGY CONDITION

A matter bouncing entropy-corrected cosmological model has been suggested. The model allows only positive curvature with negative pressure and no violation of the null energy condition. The result obtained in this paper is supported by some recent theoretical works where the combination of positive spatial curvature and vacuum energy leads to non-singular bounces with no violation of the null energy condition. An important feature of the current model is that evolutions of the cosmic pressure, energy density and equation of state parameter are independent of the values of the prefactors α and β in the corrected entropy-area relation. The validity of the classical and the new nonlinear energy conditions has been discussed. The cosmographic parameters have been analyzed.

Mode-sum prescription for the renormalized stress energy tensor on black hole spacetimes

In this paper, we describe an extremely efficient method for computing the renormalized stress-energy tensor of a quantum scalar field in spherically-symmetric black hole spacetimes. The method applies to a scalar field with arbitrary field parameters. We demonstrate the utility of the method by computing the renormalized stress-energy tensor for a scalar field in the Schwarzschild black hole spacetime, applying our results to discuss the null energy condition and the semi-classical backreaction.

Epicyclic frequencies for a generic ultracompact object

Recently, it has been shown that the radial stability of a light-ring (LR) in a spacetime generated by a stationary, axisymmetric, asymptotically flat object with a $\mathbb{Z}_2$ symmetry determines the possibility and radial stability of timelike circular orbits (TCOs) around the LR. In this paper, we generalise this result by also considering the vertical (angular) stability of the orbits through the study of the radial and vertical epicyclic frequencies. We show that the vertical stability of the LR only determines the vertical stability of the TCOs around it. A relation between the sum of the squared epicyclic frequencies and the Ricci tensor is also provided. With such relation, we show that objects with radially and vertically unstable LRs (TCOs) violate the null (strong) energy condition.