Littérature scientifique sur le sujet « Phantom scalar field »

We establish a correspondence between a conformally invariant complex scalar field action (with a conformal self-interaction potential) and the action of a phantom scalar field minimally coupled to gravity (with a cosmological constant). In this correspondence, the module of the complex scalar field is used to relate conformally the metrics of both systems while its phase is identified with the phantom scalar field. At the level of the equations, the correspondence allows to map solution of the conformally nonlinear Klein–Gordon equation with vanishing energy–momentum tensor to solution of a phantom scalar field minimally coupled to gravity with cosmological constant satisfying a massless Klein–Gordon equation. The converse is also valid with the advantage that it offers more possibilities owing to the freedom of rewriting a metric as the conformal transformation of another metric. In three dimensions, the coupling of this matter action to conformal gravity is put in equivalence with topologically massive gravity with a cosmological constant and with a phantom source. Finally, we provide some examples of this correspondence.

A model of a thick brane in 5D bulk supported by two phantom scalar fields is considered. The comparison with a thick brane supported by two usual scalar fields is carried out. The distinctions between a thick brane supported by one usual scalar field and our model have been pointed out.

We present an AdS black hole solution with Ricci flat horizon in Einstein-phantom scalar theory. The phantom scalar fields just depend on the transverse coordinates x and y, which are parameterized by the parameter α. We study the thermodynamics of the AdS phantom black hole. Although its horizon is a Ricci flat Euclidean space, we find that the thermodynamical properties of the black hole solution are qualitatively the same as those of AdS Schwarzschild black hole. Namely, there exists a minimal temperature and the large black hole is thermodynamically stable, while the smaller one is unstable, so there is a so-called Hawking-Page phase transition between the large black hole and the thermal gas solution in the AdS space-time in Poincare coordinates. We also calculate the entanglement entropy for a strip geometry dual to the AdS phantom black holes and find that the behavior of the entanglement entropy is qualitatively the same as that of the black hole thermodynamical entropy.

A thick de Sitter brane constructed by a canonical or phantom scalar field and localization mass spectra of the bulk scalar and vector fields on the de Sitter brane is investigated. It is found that the scalar and vector zero modes are always localized on the de Sitter brane. For the de Sitter brane generated by a canonical scalar field, the spectrum of the scalar (vector) KK modes consists of one or two bound modes (one bound mode) and a set of continuous modes. However, for the de Sitter brane generated by a phantom scalar field, the spectrum of the scalar (vector) KK modes consists of two (one) or more bound KK modes as well as a set of continuous modes. The continuous spectrum on both branes starts with a same value for the scalar (vector) KK modes.

We develop a generalization of semiclassical field theory for the case of non-Hermitian Hamiltonians with CPT symmetry and construct a classical cosmological, scalar-field based model describing a smooth transition from ordinary dark energy to the phantom one. Our model arises from a Lagrangian with a complex potential leading to a non-trivial vacuum with real vacuum energy. Equivalence with models involving two scalar fields one of which is phantom-like is discussed.

In this work we have investigated the emergent scenario of the Universe described by loop quantum cosmology model, DGP brane model and Kaluza–Klein cosmology. Scalar field along with barotropic fluid as normal matter is considered as the matter content of the Universe. In loop quantum cosmology it is found that the emergent scenario is realized with the imposition of some conditions on the value of the density of normal matter in case of normal and phantom scalar field. This is a surprising result indeed considering the fact that scalar field is the dominating matter component! In case of tachyonic field, emergent scenario is realized with some constraints on the value of ρ1 for both normal and phantom tachyon. In case of DGP brane-world realization of an emergent scenario is possible almost unconditionally for normal and phantom fields. Plots and table have been generated to testify this fact. In case of tachyonic field emergent scenario is realized with some constraints on [Formula: see text]. In Kaluza–Klein cosmology emergent scenario is possible only for a closed Universe in case of normal and phantom scalar field. For a tachyonic field, realization of emergent Universe is possible for all models (closed, open and flat).

In this paper, we study the possibility of building two-field models of dark energy with equation of state across -1. Specifically we will consider two classes of models: one consists of two scalar fields (quintessence + phantom) and another includes one scalar (phantom) and one spinor field (neutrino). Our studies indicate to some extent that two-field models give rise to a simple realization of the dynamical dark energy model with the equation of state across w=-1.

We construct a dark energy model where a scalar field non-minimally coupled to gravity plays the role of the dark component. We compare cosmological consequences of this non-minimal coupling of the scalar field and gravity in the spirit of the dark energy paradigm in Jordan and Einstein frames. Some important issues such as phantom divide line crossing, existence of the bouncing solutions and the stability of the solutions are compared in these two frames. We show that while a non-minimally coupled scalar field in the Jordan frame is a suitable dark energy component with capability to realize phantom divide line crossing, its conformal transformation in the Einstein frame does not have this capability. The conformal transformation from Jordan frame to Einstein frame transforms the equation of state parameter of the dark energy component to its minimal form with a redefined scalar field and in this case it is impossible to realize a phantom phase with possible crossing of the phantom divide line.

We construct a model of phantom energy using the graded Lie algebra SU(2/1). The negative kinetic energy of the phantom field emerges naturally from the graded Lie algebra, resulting in an equation of state with w < -1. The model also contains ordinary scalar fields and anticommuting (Grassmann) vector fields which can be taken as two-component dark matter. A potential term is generated for both the phantom fields and the ordinary scalar fields via a postulated condensate of the Grassmann vector fields. Since the phantom energy and dark matter arise from the same Lagrangian, the phantom energy and dark matter of this model are coupled via the Grassmann vector fields. In the model presented here phantom energy and dark matter come from a gauge principle rather than being introduced in an ad hoc manner.

AbstractIn this article, we investigated a generalised scalar field cosmology characterised by a time-dependent coupling function, a time-dependent cosmological constant, a chameleonic field, and a time-dependent equation of state parameter. Based on a particular choice of the Hubble parameter as function of the scalar field and its time derivative, we have investigated the dynamics of the Friedmann–Robertson–Walker flat universe. We have observed that for a particular choice of the free parameters in the theory, the universe accelerates with time and asymptotically tends toward a static universe. For specific values of the free parameters, the asymptotically static universe may be dominated by phantom energy or a cosmological constant without the need to implement in the theory multiple scalar fields or additional interactions. It is also pointed out that an asymptotically static universe is as well one special class of solutions for Horndeski and generalised galileons cosmologies. Special properties and features are discussed accordingly.

Questa tesi si occupa della questione della stabilità lineare di wormholes (tunnel spaziotemporali) statici e a simmetria sferica, supportati da campi scalari di tipo fantasma autointeragenti, nel contesto della Relatività Generale per spazitempi di dimensione arbitraria. In letteratura, attraverso un'analisi gauge-invariante delle configurazioni di tipo wormhole, spesso si riesce a disaccoppiare le equazioni di campo linearizzate, ottenendo un'equazione delle onde (master equation) che, tuttavia, tipicamente è singolare dove il coefficiente radiale della metrica ha un punto critico, cioè nella gola del tunnel. Per risolvere questo problema, nei lavori passati è stato proposto un metodo di regolarizzazione che trasforma l'equazione delle onde singolare in una regolare; questo metodo è solitamente denominato "S-deformazione" (e spesso richiede parzialmente un'implementazione numerica, specialmente nel caso di campi scalari con un'autointerazione non banale). Il primo risultato del mio lavoro è la riduzione delle equazioni di campo linearizzate ad un sistema delle onde vincolato e completamente regolare, per due funzioni gauge-invarianti delle perturbazioni dei coefficienti della metrica e del campo scalare, opportunamente definite; il secondo risultato è una strategia per disaccoppiare questo sistema, ottenendo una sola master equation delle onde per un'altra quantità gauge-invariante. Nessun passaggio di questa costruzione determina l'apparizione di singolarità nella gola del tunnel o in altri punti (sempre che il campo scalare imperturbato non abbia punti critici, cosa che accade in moti esempi); quindi non è necessario regolarizzare a posteriori la master equation utilizzando il metodo di S-deformazione. Questo formalismo gauge-invariante e libero da singolarità, che generalizza a dimensione arbitraria l'approccio del mio articolo [1], è applicato ad alcune soluzioni di tipo wormhole statiche note (la maggior parte, ma non tutte, considerate in [1]). La più importante applicazione è ad un wormhole Anti-de Sitter (AdS), la cui stabilità lineare non pare sia mai stata analizzata da altri autori finora; utilizzando il presente metodo è possibile derivare una master equation completamente regolare che descrive le perturbazioni del wormhole AdS e quindi dimostrare che quest'ultimo è linearmente instabile, dopo aver dettagliatamente analizzato le proprietà spettrali di un operatore di tipo Schrödinger che compare nella master equation. Sulla stessa linea, è ottenuto un risultato parziale per l'analogo wormhole di tipo de Sitter (dS), caso tecnicamente più sottile a causa della presenza di orizzonti. Come ulteriore applicazione, ho riottenuto in maniera libera da singolarità le master equations per le perturbazioni di dei wormholes di Ellis-Bronnikov e di Torii-Shinkai. Ad integrazione, l'instabilità lineare dei wormholes AdS e di Torii-Shinkai sono riottenute utilizzando un metodo alternativo, privo di singolarità ma gauge-dipendente: in questo caso, si ottiene una master equation per la perturbazione della coordinata radiale, e l'indipendenza dal gauge del risultato di instabilità è testata a posteriori. Questo approccio alternativo e gauge-dipendente generalizza quello introdotto in [2] per il wormhole di Ellis-Bronnikov a simmetria riflessiva. Vorrei citare infine [3], dal quale ho riportato alcuni fatti sui wormholes appena menzionati in assenza di perturbazione. BIBLIOGRAFIA: [1] F. Cremona, L. Pizzocchero, and O. Sarbach. Gauge-invariant spherical linear perturbations of wormholes in einstein gravity minimally coupled to a self-interacting phantom scalar field. Physical Review D, 101, 05 2020. [2] F. Cremona, F. Pirotta, and L. Pizzocchero. On the linear instability of the Ellis-Bronnikov-Morris-Thorne wormhole. Gen. Relativ. Gravitat., 51:19, 2019. [3] F. Cremona. Geodesic structure and linear instability of some wormholes. Proceeding for the conference: Domoschool 2019 (submitted).
In this thesis I deal with the linear stability analysis of static, spherically symmetric wormholes supported by phantom self-interacting scalar fields, in the framework of General Relativity with arbitrary spacetime dimension. In the previous literature, a gauge-invariant stability analysis of wormhole configurations often succeeds in decoupling the linearized field equations, yielding a wave-type master equation which, however, is typically singular where the radial coefficient of the metric has a critical point, that is, at the wormhole throat. In order to overcome this problem a regularization method has been proposed in previous works, which transforms the singular wave equation to a regular one; this method is usually referred to as “S-deformation” (and sometimes requires a partly numerical implementation, especially, in the case of scalar fields with nontrivial self-interaction). The first result of my work is the reduction of the linearized field equations to a completely regular, constrained wave system for two suitably defined gauge-invariant functions of the perturbations in the metric coefficients and in the scalar field; the second result is a strategy for decoupling this system, obtaining a single wave-type master equation for another gauge-invariant quantity. No step of this construction causes the appearing of singularities at the wormhole throat or elsewhere (provided that the unperturbed scalar field has no critical points, which occurs in many examples); therefore, it is not necessary to regularize a posteriori the master equation via the S-deformation method. This gauge-invariant and singularity-free formalism, which generalizes to arbitrary spacetime dimensions the approach of my paper [1], is then applied to some known static wormhole solutions (most, but not all of them considered in [1]). The most relevant application is a certain Anti-de Sitter (AdS) wormhole, whose linear stability analysis does not seem to have been performed previously by other authors; by using the present method, it is possible to derive a completely regular master equation describing the perturbations of the AdS wormhole and prove that the latter is actually linearly unstable, after providing a detailed analysis of the spectral properties of the Schrödinger type operator appearing in the master equation. A partial instability result is derived along the same lines for the analogous de Sitter (dS) wormhole, a technically more subtle case due to the presence of horizons. As a further application, I rederive in a singularity-free fashion the master equations for the perturbed Ellis-Bronnikov and Torii-Shinkai wormholes. As a supplement, the linear instability results for the AdS and for the Torii-Shinkai wormholes are also recovered using an alternative, singularity free but gauge-dependent method: in this case a regular master equation is derived for the perturbed radial coordinate, and the gauge-independence of the instability result is tested a posteriori. This alternative, gauge-dependent approach generalizes that introduced in my paper [2] for the reflection symmetric Ellis-Bronnikov wormhole. Let me also cite [3], from which I report some facts about the previously mentioned wormholes in absence of perturbations. BIBLIOGRAPHY: [1] F. Cremona, L. Pizzocchero, and O. Sarbach. Gauge-invariant spherical linear perturbations of wormholes in einstein gravity minimally coupled to a self-interacting phantom scalar field. Physical Review D, 101, 05 2020. [2] F. Cremona, F. Pirotta, and L. Pizzocchero. On the linear instability of the Ellis-Bronnikov-Morris-Thorne wormhole. Gen. Relativ. Gravitat., 51:19, 2019. [3] F. Cremona. Geodesic structure and linear instability of some wormholes. Proceeding for the conference: Domoschool 2019 (submitted).

Abstract An experimental and numerical investigation of phantom cooling effects on cooled and uncooled rotating high pressure turbine blades in a full scale 1+1/2 stage turbine test is carried out. Objectives set to capture, separate, and quantify the effects of upstream vane film-cooling and leakage flows on the downstream rotor blade surface heat flux. Multiple series of 1+1/2 stage rotating high pressure turbine tests were carried out in the Air Force Research Laboratory, Turbine Research Facility, at Wright-Patterson Air Force Base, Ohio. A non-proprietary research turbine test article is uniquely instrumented with high frequency double-sided thin film heat flux gauges custom made at AFRL. High bandwidth, time resolved surface heat flux is measured on multiple film-cooled and non-film-cooled HPT rotor blades downstream of both film-cooled and non-film-cooled vane sectors. Upstream wake passing and heat flux is characterized on both rotor pressure and suction side surfaces, along with quantifying rotor phantom cooling effects from nonuniform 1st stage vane film cooling and leakage flows. Fast response heat flux measurements quantify how rotor phantom cooling impacts the blade pressure side greatest; increasing along the pressure side towards the trailing edge. It is discovered upstream vane film-cooling alone can account for 50% of the rotor blade cooling effect, and even outweigh the rotor blade film cooling effect far from the blade showerhead holes. Added unsteady aero numerical simulation demonstrate how variations in inlet total temperature and incidence angle can also contribute to circumferentially non-uniform rotor heat flux. Better understanding from this investigation aids modelling and design efforts in optimizing film cooling performance in real high pressure turbine flow fields. Understanding the behavior of such non-uniform circumferential rotor phantom cooling effects can be critical to optimize the efficiency, fuel consumption, range, and durability of advanced turbomachines.

In recent times, Ultrasound for therapeutic applications is becoming increasingly popular due to its high practicality and efficiency. However, determination of adequate dosages presents a great challenge due to the difficulty of measuring tissue temperatures during the process. Further, accurate calculation of temperature field induced by ultrasound within the tissue is difficult to develop because of the time-scale differences between pressure and temperature analyses. In order to overcome this issue, practical and accurate methods to couple both analyses are needed. In the present study, Westervelt’s nonlinear wave equation is used to simulate ultrasonic propagation driven by an unfocused piston source in an axisymmetric biological tissue phantom. Using the Finite Difference Time Domain (FDTD) method, a pressure field was calculated for different sinusoidal bursts, frequencies, and source pressures. Average heat generation fields were calculated from the pressure field within an adequate time range for practical purposes. The Pennes bioheat transfer equation with the calculated heat generation fields were used to acquire transient temperature distributions. Effect of source pressure, frequency, source radius, and trial duration on the temperature profiles was examined. It can be observed from the simulations that continuous wave signals increase temperature at a focus in shorter times, while discrete pulses with adequate duty factors can be useful in maintaining required temperatures constant while diffusing heat along the tissue. The methodology presented here can be of use in many applications such as increasing necrotic volume for tissue ablation purposes.

Torque behavior in Taylor-Couette flow has been discussed for decades of years and a series of torque behavior models have been proposed to deepen the understanding of dynamic behavior. In industry fields, the empirical relations based on torque measurement of scaled models in laboratory have been widely used to predict the torque behavior of rotating machinery. However, they fail sometimes, especially in ultimate flow regime. Therefore, a uniform theory based on physical mechanism is needed to model the torque behavior. Under the efforts of many scholars, fortunately, Eckhardt-Grossmann-Lohse theory throws light upon this problem from momentum transfer behavior based on N-S equations. It argues that angular velocity current seems to be constant in the gap, meanwhile, bulk flow theory shows that turbulent bulk and boundary layer play different roles in momentum transfer behavior. Compared with the thickness of boundary layer, surface texture is the same level in dimension. What is the interaction mechanism between the boundary layer and surface texture and how much does surface texture affect torque behavior? In this paper, we mainly focus on how surface texture affects the momentum transfer behavior. To investigate the surface effect on momentum transfer behavior, global torque behavior was measured through rotating multicomponent dynamometer of Kistler and surface texture effect on dynamic behavior of boundary layer was observed through high speed camera of Phantom. To compare the effect of different surface texture, the surface morphology was mapped by stereoscopic microscope of Zeiss. The results indicate that irregular surface texture strengthens the momentum transfer behavior through boundary layer transportation.