Littérature scientifique sur le sujet « Bronnikov »

Study of accretion onto wormholes is rather rare compared to that onto black holes. In this paper, we consider accretion flow of cosmological dark energy modeled by barotropic fluid onto the celebrated Ellis–Bronnikov wormhole (EBWH) built by Einstein minimally coupled scalar field ϕ, violating the null energy condition. The accreting fluid is assumed to be phantom, quintessence, dust and stiff matter. We begin by first pointing out a mathematical novelty showing how the EBWH can lead to the Schwarzschild black hole under a complex Wick rotation. Then, we analyze the profiles of fluid radial velocity, density and the rate of mass variation of the EBWH due to accretion and compare the profiles with those of the Schwarzschild black hole. We also analyze accretion to the massless EBWH that has zero ADM mass but has what we call nonzero Wheelerian mass (“mass without mass”), composed of the non-trivial scalar field, that shows gravitational effects. Our conclusion is that the mass of SBH due to phantom accretion decreases consistently with known results, while, in contrast, the mass of EBWH increases. Exactly an opposite behavior emerges for non-phantom accretion to these two objects. Accretion to massless EBWH (i.e., to nonzero Wheelerian mass) shares the same patterns as those of the massive EBWH; hence there is no way to distinguish massive and massless cases by means of accretion flow. The contrasting mass variations due to phantom accretion could be a reflection of the distinct topology of the central objects.

In this paper, we investigate the simplest wormhole solution—the Ellis–Bronnikov one—in the context of the asymptotically safe gravity (ASG) at the Planck scale. We work with three models, which employ the Ricci scalar, Kretschmann scalar, and squared Ricci tensor to improve the field equations by turning the Newton constant into a running coupling constant. For all the cases, we check the radial energy conditions of the wormhole solution and compare them with those that are valid in general relativity (GR). We verified that asymptotic safety guarantees that the Ellis–Bronnikov wormhole can satisfy the radial energy conditions at the throat radius, r0, within an interval of values of the latter, which is quite different from the result found in GR. Following this, we evaluate the effective radial state parameter, ω(r), at r0, showing that the quantum gravitational effects modify Einstein’s field equations in such a way that it is necessary to have a very exotic source of matter to generate the wormhole spacetime–phantom or quintessence-like matter. This occurs within some ranges of the throat radii, even though the energy conditions are or are not violated there. Finally, we find that, although at r0 we have a quintessence-like matter, upon growing r, we inevitably came across phantom-like regions. We speculate whether such a phantom fluid must always be present in wormholes in the ASG context or even in more general quantum gravity scenarios.

Tsukamoto [N. Tsukamoto, Phys. Rev. D 95, 064035 (2017)] developed a method, which is an improvement over that of Bozza [V. Bozza, Phys. Rev. D 66, 103001 (2002)], for calculating light deflection angle in the strong gravity field of a spherically symmetric static spacetime. The method is directly applicable to the massless Ellis–Bronnikov wormhole (EBWH), while Bozza’s method is not applicable. We wish to show that it is still possible to obtain the same deflection angle by applying Bozza’s method but only in an indirect way, that is, first calculate the deflection by the parent massive EBWH and then take its massless limit.

We check the consistency of the entropy of Bardeen and Ayón Beato-García-Bronnikov black holes with the entropy of particular conformal field theory via Cardy-Verlinde formula. We also compute the first-order semiclassical corrections of this formula due to self-gravitational effects by modifying pure extensive and Casimir energy in the context of Keski-Vakkuri, Kraus and Wilczek analysis. It is concluded that the correction term remains positive for both black holes, which leads to the violation of the holographic bound.

The Bronnikov model of nonlinear electrodynamics is investigated in general relativity. The magnetic black hole is considered and we obtain a solution giving corrections to the Reissner-Nordström solution. In this model spacetime at r → ∞ becomes Minkowski’s spacetime. We calculate the magnetic mass of the black hole and the metric function. At some parameters of the model there can be one, two or no horizons. The Hawking temperature and the heat capacity of black holes are calculated. We show that a second-order phase transition takes place and black holes are thermodynamically stable at some range of parameters.

In the Ellis–Bronnikov wormhole (WH) metrics, the motion of a particle along the curved rotating channels is studied. By taking into account a prescribed shape of a trajectory, we derive the reduced 1 + 1 metrics, obtain the corresponding Langrangian of a free particle and analytically and numerically solve the corresponding equations of motion. We have shown that if the channels are twisted and lagged behind rotation, under certain conditions, beads might asymptotically reach infinity, leaving the WH, which is not possible for straight corotating trajectories. The analytical and numerical study is performed for two- and three-dimensional cases and initial conditions of particles are analyzed in the context of possibility of passing through the WH.

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).