Motivated by recent developments in Cosmology we would like to consider an extension of the Ghost DE which we will call as varying Ghost DE. Ghost DE like other models was introduced recently as a possible way to explain accelerated expansion of the Universe. For the phenomenological origin of the varying Ghost dark energy in our Universe we can suggest an existence of some unknown dynamics between the Ghost Dark energy and a fluid which evaporated completely making sense of the proposed effect. Moreover, we assume that this was in the epochs and scales which are unreachable by nowadays experiments, like in very early Universe. In this study we will investigate the model for cosmological validity. We will apply observational and causality constraints to illuminate physically correct behavior of the model from the phenomenological one. We saw that an interaction between the varying Ghost DE and cold DM~(CDM) also provides a solution to the cosmological coincidence problem. And we found that the Ghost DE behaves as a matter like fluid in early Universe.
We investigate interacting ghost dark energy models in higher dimensional cosmology. We attempt to model dark matter within a barotropic fluid with $P_{b}=ω(t)_{b}ρ$. In this work we consider four different models based on choosing equation of state parameter and interaction term. We confirm that our models agree with observational data.
We investigate the generalized Quantum Chromodynamics (QCD) ghost model of dark energy in the framework of Einstein gravity. First, we study the non-interacting generalized ghost dark energy in a flat Friedmann-Robertson-Walker (FRW) background. We obtain the equation of state parameter, $w_D=p/ρ$, the deceleration parameter, and the evolution equation of the generalized ghost dark energy. We find that, in this case, $w_D$ cannot cross the phantom line ($w_D>-1$) and eventually the universe approaches a de-Sitter phase of expansion $(w_D\rightarrow-1)$. Then, we extend the study to the interac…
By interacting ghost dark energy (ghost DE) in the framework of Brans-Dicke theory, a spatially homogeneous and anisotropic Bianchi type I Universe has been studied. For this purpose, we use the squared sound speed $c_s^2$ whose sign determines the stability of the model. As well as we probe observational constraints on the ghost dark energy models as the unification of dark matter and dark energy by using the latest observational data. In order to do so, we focus on observational determinations of the Hubble expansion rate (namely, the expansion history) $H(z)$. After that we evaluate the evo…
In this paper, we investigate the cosmological consequences of ghost dark energy model in modified Gauss-Bonnet gravity. We construct ghost dark energy $f(G)$ model by using correspondence scenario for both interacting and non-interacting schemes. For this purpose, we consider the FRW universe with pressureless matter and power-law scale factor. We examine the behavior of the equation of state parameter and check the stability of ghost dark energy model through the squared speed of sound parameter. We also analyze the behavior of phase planes like $ω_{eff}-ω'_{eff}$ and $r-s$ graphically. It i…
A new model of dark energy namely "ghost dark energy model" has recently been suggested to interpret the positive acceleration of cosmic expansion. The energy density of ghost dark energy is proportional to the hubble parameter. In this paper we perform the statefinder diagnostic tool for this model both in flat and non-flat universe. We discuss the dependency of the evolutionary trajectories in $s-r$ and $q-r$ planes on the interaction parameter between dark matter and dark energy as well as the spatial curvature parameter of the universe. Eventually, in the light of SNe+BAO+OHD+CMB observati…
Exploring the accelerated expansion of the universe, we investigate the generalized ghost dark energy (GGDE) model from the statefinder diagnosis analysis in a flat FRW universe. First we calculate the cosmological evolution and statefinder trajectories for non-interacting case and then extend this work by considering the interaction between dark matter and dark energy components. We show that in the non-interacting case the phantom line can not be crossed and also he evolutionary trajectories of model in $s-r$ plane can not be discriminated. It has been shown that the present location of mode…
The objective of this thesis is to present a viable extension of general relativity free from cosmological singularities. A viable cosmology, in this sense, is one that is free from ghosts, tachyons or exotic matter, while staying true to the theoretical foundations of General Relativity such as general covariance, as well as observed phenomenon such as the accelerated expansion of the universe and inflationary behaviour at later times. To this end, an infinite derivative extension of relativity is introduced, with the gravitational action derived and the non-linear field equations calculated,…
We introduce a functional covariant differential as a tool for studying field space geometry in a manifestly covariant way. We then touch upon its role in gauge theories and general relativity over bounded regions, and in BRST symmetry. Due to the Gribov problem, we argue that our formalism ---allowing for a non-vanishing functional curvature---is necessary for a global treatment of gauge-invariance in field space. We conclude by suggesting that the structures we introduce satisfactorily implement the notion of a (non-asymptotic) observer in gauge theories and general relativity.
The proposal of pilgrim dark energy is based on the speculation that phantom-like dark energy possesses enough resistive force to preclude the black hole formation in the later universe. We explore this phenomenon by assuming the generalized ghost version of pilgrim dark energy. We find that most of the values of the interacting ($ξ^2$) as well as pilgrim dark energy ($u$) parameters push the equation of state parameter towards phantom region. The squared speed of sound shows that this model remains stable in most of the cases of $ξ^2$ and $u$. We also develop $ω_Λ-ω'_Λ$ plane and observe that…
A black universe is a nonsingular black hole where, beyond the horizon, there is an expanding, asymptotically isotropic universe. Such models have been previously found as solutions of general relativity with a phantom scalar field as a source of gravity and, without phantoms, in a brane world of RS2 type. Here we construct examples of static, spherically symmetric black-universe solutions in general relativity with a minimally coupled scalar field ϕwhose kinetic energy is negative in a restricted strong-field region of space-time and positive outside it. Thus in such configurations a "ghost" …
We show that a class of nonlocal gravity models, proposed to explain current cosmic acceleration without dark energy, passes two major tests: First, they can be defined so as not to alter the, observationally correct, general relativity predictions for gravitationally bound systems. Second, they are stable, ghost-free, with no additional excitations beyond those of general relativity. In this they differ from their, ghostful, localized versions. The systems' initial value constraints are the same as in general relativity, and our nonlocal modifications never convert the original gravitons into…
In this paper we extend the form of interaction term into the non-linear regime in the ghost dark energy model. A general form of non-linear interaction term is presented and cosmic dynamic equations are obtained. Next, the model is detailed for two special choice of the non-linear interaction term. According to this the universe transits at suitable time ($z\sim 0.8$) from deceleration to acceleration phase which alleviate the coincidence problem. Squared sound speed analysis revealed that for one class of non-linear interaction term $v_s^2$ can gets positive. This point is an impact of the n…
The tensor model can be regarded as theory of dynamical fuzzy spaces, and gives a way to formulate gravity on fuzzy spaces. It has recently been shown that the low-lying fluctuations around the Gaussian background solutions in the tensor model agree correctly with the metric fluctuations on the flat spaces with general dimensions in the general relativity. This suggests that the local gauge symmetry (the symmetry of local translations) is also emergent around these solutions. To systematically study this possibility, I apply the BRS gauge fixing procedure to the tensor model. The ghost kinetic…
Regarding the Veneziano ghost of QCD and its generalized form, we consider a Friedmann-Robertson-Walker (FRW) universe filled by a pressureless matter and a dark energy component interacting with each other through a mutual sign-changeable interaction of positive coupling constant. Our study shows that, at the late time, for the deceleration parameter we have $q\rightarrow-1$, while the equation of state parameter of the interacting ghost dark energy (GDE) does not cross the phantom line, namely $ω_D\geq-1$. We also extend our study to the generalized ghost dark energy (GGDE) model and show th…
We give a variational formulation of General Relativity, with coupling to a cosmological constant, to scalar fields, to vector fields and to spinor fields (all with possible mass and interaction terms). Among the matter fields, we include ghosts corresponding to diffeomorphism and Yang-Mills gauge symmetries, with kinetic terms given by gauge fixing conditions leading to hyperbolic equations of motion for all fields. The distinguishing characteristic of our Lagrangian density is its polynomiality, in all dynamical fields and Lagrange multipliers, and its validity for any number of spacetime di…
We study the stability of the ghost dark energy model versus perturbation. Since this kind of dark energy is instable in Einsteinian general relativity theory, then we study a new type of Brans-Dicke theory which has non-minimal coupling with matter which is called chameleon Brans-Dicke (CBD) model of gravity. Due to this coupling the equation of conservation energy is modified. For considering the stability of the model we use the adiabatic squared sound speed, $c_s^2$, whose sign of it determines the stability of the model in which for $c_s^2 >0 $ the model is stable and for $c_s^2 <0 $ the …
We present new, asymptotically flat, static, spherically symmetric and traversable wormhole solutions in General Relativity which are supported by a family of ghost scalar fields with quartic potential. This family consists of a particular composition of the scalar field modes, in which each mode is characterized by the same value of the angular momentum number $\ell$, yet the composition yields a spherically symmetric stress-energy-momentum and metric tensor. For $\ell = 0$ our solutions reduce to wormhole configurations which had been reported previously in the literature. We discuss the eff…
Reconstruction of f(G) Gravity from an Interacting Viscous Generalized QCD Ghost Dark Energy Model: Cosmology and Thermodynamics: Cosmology and Thermodynamics
Mukhtar Auezov South Kazakhstan University（穆赫塔尔·阿尤佐夫南哈萨克斯坦大学）；Amity University Kolkata（阿米提大学加尔各答分校）；L.N. Gumilyov Eurasian National University（L.N. 古米廖夫欧亚国立大学）
We consider general theories of a massive spin-2 particle $h_{μν}$ on a Minkowski background. A decomposition of $h_{μν}$ in terms of helicity eigenstates allows us to directly test whether any given theory possesses a consistent description as a massive spin-2 representation of the Poincaré group. We demonstrate (i) that any nonlinear theory with an Einsteinian derivative structure either contains ghosts or does not describe a weakly coupled spin-2 and (ii) that there exists a two-parameter family of non-Einsteinian cubic self-interactions which constitute a ghost-free massive spin-2 theory.
We study exact impulsive gravitational waves propagating in anti-de Sitter spacetime in the context of the ghost free infinite derivative gravity. We show that the source-free theory does not admit any AdS wave solutions other than that of Einstein's general relativity. The situation is significantly different in the presence of sources. We construct impulsive-wave solutions of the infinite derivative gravity generated by massless particles and linear sources in four and three dimensions. The singularities corresponding to distributional curvature at the locations of the sources get smeared by…
We investigate the slow-motion and weak-field approximation of the general ghost-free parity-violating (PV) theory of gravity in the parametrized post-Newtonian (PPN) framework and derive the perturbative field equations, which are modified by the PV terms of this theory. The complete PPN parameters are obtained by solving the perturbative field equations. We find that all the PPN parameters are exactly the same as those in general relativity, except for an extra parameter $κ$, which is caused by the new curl-type term in the gravitomagnetic sector of the metric in this theory. We calculate th…
This work is based on pilgrim dark energy conjecture which states that phantom-like dark energy possesses the enough resistive force to preclude the formation of black hole. The non-flat geometry is considered which contains the interacting generalized ghost pilgrim dark energy with cold dark matter. Some well-known cosmological parameters (evolution parameter ($ω_Λ$) and squared speed of sound) and planes ($ω_Λ$-$ω_Λ'$ and statefinder) are constructed in this scenario. The discussion of these parameters is totally done through pilgrim dark energy parameter ($u$) and interacting parameter ($d^…
The bimetric variational principle is a subtle reinterpretation of general relativity that assumes the spacetime connection to be generated by an independent metric. Unlike the so called Palatini formalism that promotes the connection into a fundamental field, the new variational principle results in a physically distinct theory since the potential for the connection carries new degrees of freedom. The connection-generating metric naturally allows also an antisymmetric component. This sets torsion propagating! It is also shown here that while in the most straightforward generalization of the E…
We show that a massless canonical scalar field minimally coupled to general relativity can become a tachyonic ghost at low energies around a background in which the scalar's gradient is spacelike. By performing a canonical transformation we demonstrate that this low energy ghost can be recast, at the level of the action, in a form of a fluid that undergoes a Jeans-like instability affecting only modes with large wavelength. This illustrates that low-energy tachyonic ghosts do not lead to a catastrophic quantum vacuum instability, unlike the usual high-energy ghost degrees of freedom.
In our previous paper, we have presented a covariant BRST quantization of unimodular gravity which may account for the smallness of the cosmological constant, and have shown that the physical degrees of freedom in the theory are the same as general relativity. The formulation has been given by using rank-2 antisymmetric tensor fields for both ghosts and antighosts. Here we give an alternative formulation using a vector field for the antighost but keeping the same structure for the ghosts. This gives a significantly simpler covariant quantization with less ghosts and no tripole modes in the gho…
We construct explicit examples of globally regular static, spherically symmetric solutions in general relativity with scalar and electromagnetic fields, describing traversable wormholes with flat and AdS asymptotics and regular black holes, in particular, black universes. (A black universe is a regular black hole with an expanding, asymptotically isotropic space-time beyond the horizon.) Such objects exist in the presence of scalar fields with negative kinetic energy ("phantoms", or "ghosts"), which are not observed under usual physical conditions. To account for that, we consider what we call…
Barbour, Foster and Ó Murchadha have recently developed a new framework, called here {\it{the 3-space approach}}, for the formulation of classical bosonic dynamics. Neither time nor a locally Minkowskian structure of spacetime are presupposed. Both arise as emergent features of the world from geodesic-type dynamics on a space of 3-dimensional metric--matter configurations. In fact gravity, the universal light cone and Abelian gauge theory minimally coupled to gravity all arise naturally through a single common mechanism. It yields relativity -- and more -- without presupposing relativity. This…
We construct explicit examples of globally regular static, spherically symmetric solutions in general relativity with scalar and electromagnetic fields describing traversable wormholes with flat and AdS asymptotics and regular black holes, in particular, black universes. (A black universe is a regular black hole with an expanding, asymptotically isotropic space-time beyond the horizon.) The existence of such objects requires invoking scalars with negative kinetic energy ("phantoms", or "ghosts"), which are not observed under usual physical conditions. To account for that, the so-called "trappe…
General Relativity enjoys the freedom of different geometrical interpretations in terms of curvature, torsion or non-metricity. Within this geometrical trinity, a simpler geometrical formulation of General Relativity manifests itself in the latter, where gravity is entirely attributed to non-metricity. In this Letter, we consider non-linear extensions of Coincident General Relativity $f(\mathring{\mathbb{Q}})$ for phenomenological applications on both cosmological as well as galactic scales. The theory not only delivers dark energy on large scales but also recovers MOND on galactic scales, tog…
Three static models with two interacting phantom and ghost scalar fields were considered: a model of a traversable wormhole, a brane-like model and a spherically symmetric problem. It was shown numerically that regular solutions exist for all three cases.
In this paper, we point out thermodynamical description of ghost dark energy and its generalization to the early universe. Thereinafter, we find expressions for the entropy changes of these dark energy candidates. In addition, considering thermal fluctuations, thermodynamics of the dark energy component interacting with a dark matter sector is addressed. {We will also find the effects of considering the coincidence problem on the mutual interaction between the dark sectors, and thus the equation of state parameter of dark energy.} Finally, we derive a relation between the mutual interaction of…
We construct consistent theories of multiple interacting spin-2 fields in arbitrary spacetime dimensions using a vielbein formulation. We show that these theories have the additional primary constraints needed to eliminate potential ghosts, to all orders in the fields, and to all orders beyond any decoupling limit. We postulate that the number of spin-2 fields interacting at a single vertex is limited by the number of spacetime dimensions. We then show that, for the case of two spin-2 fields, the vielbein theory is equivalent to the recently proposed theories of ghost-free massive gravity and …
Massive vector fields feature in several areas of particle physics, e.g., as carriers of weak interactions, dark matter candidates, or as an effective description of photons in a plasma. Here we investigate vector fields with self-interactions by replacing the mass term in the Proca equation with a general potential. We show that this seemingly benign modification inevitably introduces ghost instabilities of the same kind as those recently identified for vector-tensor theories of modified gravity (but in this simpler, minimally coupled theory). It has been suggested that nonperturbative dynami…
We investigate the instability of the ghost dark energy model against perturbations in different cases. To this goal we use the squared sound speed $v_s^2$ whose sign determines the stability of the model. When $v_s^2<0$ the model is unstable against perturbation. At first we discuss the noninteracting ghost dark energy model in a flat FRW universe and find out that such a model is unstable due to the negativity of the $v_s^2$ in all epoches. The interacting ghost dark energy model in both flat and non-flat universe are studied in the next parts and in both cases we find that the squared sound…
In this paper we consider several phenomenological models of variable $Λ$. Model of a flat Universe with variable $Λ$ and $G$ is accepted. It is well known, that varying $G$ and $Λ$ gives rise to modified field equations and modified conservation laws, which gives rise to many different manipulations and assumptions in literature. We will consider two component fluid, which parameters will enter to $Λ$. Interaction between fluids with energy densities $ρ_{1}$ and $ρ_{2}$ assumed as $Q=3Hb(ρ_{1}+ρ_{2})$. We have numerical analyze of important cosmological parameters like EoS parameter of the co…
We consider the classical linear stability of a static universe filled with a non-interacting mixture of isotropic radiation and a ghost scalar field. Unlike the conventional Einstein static model, this cosmology is stable against homogeneous and isotropic perturbations. This is shown by means of exact oscillatory solutions about the original static state. We also examine the linear response of the static ghost universe to all types of inhomogeneous fluctuations, namely density, vorticity and gravitational-wave perturbations. The results show that the static background constrains the linear ev…
We present a survey of the known cosmological and black hole solutions in ghost-free bigravity and massive gravity theories. These can be divided into three classes. First, there are solutions with proportional metrics, which are the same as in General Relativity with a cosmological term, which can be positive, negative or zero. Secondly, for spherically symmetric systems, there are solutions with non-bidiagonal metrics. The g-metric fulfills Einstein equations with a positive cosmological term and a matter source, while the f-metric is anti-de Sitter. The third class contains solutions with b…
Special gravity refers to interacting theories of massless gravitons in Minkowski space-time which are invariant under the abelian gauge invariance $h_{ab}\rightarrow h_{ab}+\partial_{(a}χ_{b)}$ only. In this article we determine the most general form of special gravity free of Ostrogradski ghosts, meaning its equation of motion is of at most second order. Together with the recent works, this result could be helpful in formulating proofs of General Relativity as the unique physical theory of self-interacting massless gravitons. We also study how to construct gauge invariant couplings to matter…
A new dark energy model called "ghost dark energy" was recently suggested to explain the observed accelerating expansion of the universe. This model originates from the Veneziano ghost of QCD. The dark energy density is proportional to Hubble parameter, $ρ_D=αH$, where $α$ is a constant of order $Λ_{\rm QCD}^3$ and $Λ_{\rm QCD}\sim 100 MeV$ is QCD mass scale. In this paper, we extend the ghost dark energy model to the universe with spatial curvature in the presence of interaction between dark matter and dark energy. We study cosmological implications of this model in detail. In the absence of …
We investigate the correspondence between the ghost and Chaplygin scalar field dark energy models in the framework of Einstein gravity. We consider a spatially non-flat FRW universe containing the interacting dark energy with dark matter. We reconstruct the potential and the dynamics for the Chaplygin scalar field model according to the evolutionary behavior of the ghost dark energy which can describe the phantomic accelerated expansion of the universe.
Recently it has been argued that a possible source for the dark energy may arise due to the contribution to the vacuum energy of the QCD ghost in a time-dependent background. In this paper we establish a connection between interacting ghost dark energy and tachyon field. It is demonstrated that the evolution of the ghost dark energy dominated universe can be described completely by a single tachyon scalar field. The potential and the dynamics of the tachyon field are reconstructed according to the evolutionary behavior of ghost energy density.
Is Phantom Divide Crossing in General Relativity Completely Impossible? Shortcomings in canonical and minimally coupled scalar field and Possible Solutions in $k$-essence Models
High Energy Accelerator Research Organization (KEK)（高能加速器研究机构）；Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University（名古屋大学小林-益川粒子宇宙起源研究所）；ICREA（加泰罗尼亚高级研究中心）；Institute of Space Sciences (IEEC-CSIC)（空间科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）；Aristotle University of Thessaloniki（塞萨洛尼基亚里士多德大学）；Khazar University（哈扎尔大学）
The self-accelerating universe realizes the accelerated expansion of the universe at late times by large-distance modification of general relativity without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld model provides an explicit example of the self-accelerating universe. Recently, the DGP model becomes very popular to study the observational consequences of the modified gravity models as an alternative to dark energy models in GR. However, it has been shown that the self-accelerating universe in the DGP model contains a ghost at the linearized level. The ghost carries …
In this paper we investigate ghost dark energy model in the presence of non-linear interaction between dark energy and dark matter. We also extend the analysis to the so called generalized ghost dark energy (GGDE) which $ρ_D=αH+βH^2$. The model contains three free parameters as $Ω_D, ζ(=\frac{8πG β}{3})$ and $b^2$ (the coupling coefficient of interactions). We propose three kinds of non-linear interaction terms and discuss the behavior of equation of state, deceleration and dark energy density parameters of the model. We also find the squared sound speed and search for signs of stability of th…
We discuss the cosmological consequences of a model based on a non-local infrared modification of Einstein equations. We find that the model generates a dynamical dark energy that can account for the presently observed value of $Ω_{\rm DE}$, without introducing a cosmological constant. Tuning a free mass parameter $m$ to a value $m\simeq 0.67 H_0$ we reproduce the observed value $Ω_{\rm DE}\simeq 0.68$. This leaves us with no free parameter and we then get a pure prediction for the EOS parameter of dark energy. Writing $w_{\rm DE}(a)=w_0+(1-a) w_a$, we find $w_0\simeq-1.04$ and $w_a\simeq -0.0…
We apply the dynamical systems tools to study the linear dynamics of a self-interacting scalar field trapped in the braneworld, for a wide variety of self-interaction potentials. We focus on Randall-Sundrum (RS) and on Dvali-Gabadadze-Porrati (DGP) braneworld models exclusively. These models are complementary to each other: while the RS brane produces ultra-violet (UV) corrections to general relativity, the DGP braneworld modifies Einstein's theory at large scales, i. e., produces infra-red (IR) modifications of general relativity. This study of the asymptotic properties of both braneworld mod…
