Wormholes and black universes without phantom fields in Einstein-Cartan theory (original) (raw)
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Magnetic black universes and wormholes with a phantom scalar
Classical and Quantum Gravity, 2012
We construct explicit examples of globally regular static, spherically symmetric solutions in general relativity with scalar and electromagnetic fields which describe traversable wormholes (with flat and AdS asymptotics) and regular black holes, in particular, black universes. A black universe is a nonsingular black hole where, beyond the horizon, instead of a singularity, there is an expanding, asymptotically isotropic universe. The scalar field in these solutions is phantom (i.e., its kinetic energy is negative), minimally coupled to gravity and has a nonzero self-interaction potential. The configurations obtained are quite diverse and contain different numbers of Killing horizons, from zero to four. This substantially widened the list of known structures of regular BH configurations. Such models can be of interest both as descriptions of local objects (black holes and wormholes) and as a basis for building nonsingular cosmological scenarios.
Magnetic wormholes and black universes with invisible ghosts
Gravitation and Cosmology, 2015
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 se-called "trapped ghosts" were previously introduced, i.e., scalars whose kinetic energy is only negative in a restricted strong-field region of space-time and positive outside it. This approach leads to certain problems, including instability (as is illustrated here by derivation of an effective potential for spherical pertubations of such systems). In this paper, we use for model construction what we call "invisible ghosts", i.e., phantom scalar fields sufficiently rapidly decaying in the weak-field region. The resulting configurations contain different numbers of Killing horizons, from zero to four.
Black bounces, wormholes, and partly phantom scalar fields
Physical Review D
Simpson and Visser recently proposed a phenomenological way to avoid some kinds of space-time singularities by replacing a parameter whose zero value corresponds to a singularity (say, r) with the manifestly nonzero expression r(u) = √ u 2 + b 2 , where u is a new coordinate, and b = const > 0. This trick, generically leading to a regular minimum of r beyond a black hole horizon (called a "black bounce"), may hopefully mimic some expected results of quantum gravity, and was previously applied to regularize the Schwarzschild, Reissner-Nordström, Kerr and some other metrics. In this paper it is applied to regularize the Fisher solution with a massless canonical scalar field in general relativity (resulting in a traversable wormhole) and a family of static, spherically symmetric dilatonic black holes (resulting in regular black holes and wormholes). These new regular metrics represent exact solutions of general relativity with a sum of stress-energy tensors of a scalar field with nonzero self-interaction potential and a magnetic field in the framework of nonlinear electrodynamics with a Lagrangian function L(F), F = Fµν F µν. A novel feature in the present study is that the scalar fields involved have "trapped ghost" properties, that is, are phantom in a strong-field region and canonical outside it, with a smooth transition between the regions. It is also shown that any static, spherically symmetric metric can be obtained as an exact solution to the Einstein equations with the stress-energy tensor of the above field combination.
Scalar Fields as Sources for Wormholes and Regular Black Holes
Particles, 2018
We review nonsingular static, spherically symmetric solutions of general relativity with minimally coupled scalar fields. Considered are wormholes and regular black holes (BHs) without a center, including black universes (BHs with expanding cosmology beyond the horizon). Such configurations require a "ghost" field with negative kinetic energy K. Ghosts can be invisible under usual conditions if K < 0 only in strong-field region ("trapped ghost"), or they rapidly decay at large radii. Before discussing particular examples, some general results are presented, such as the necessity of anisotropic matter for asymptotically flat or AdS wormholes, no-hair and global structure theorems for BHs with scalar fields. The stability properties of scalar wormholes and regular BHs under spherical perturbations are discussed. It is stressed that the effective potential V eff for perturbations has universal shapes near generic wormhole throats (a positive pole regularizable by a Darboux transformation) and near transition surfaces from canonical to ghost scalar field behavior (a negative pole at which the perturbation finiteness requirement plays a stabilizing role). Positive poles of V eff emerging at "long throats" (with the radius r ≈ r 0 + const • x 2n , n > 1, x = 0 is the throat) may be regularized by repeated Darboux transformations for some values of n.
Charged black holes and unusual wormholes in scalar-tensor gravity
Gravitation and Cosmology
We consider static, spherically symmetric, electrically or/and magnetically charged configurations of a minimally coupled scalar field with an arbitrary potential V(phi)V(\phi)V(phi) in general relativity. Using the inverse problem method, we obtain a four-parameter family of asymptotically dS, flat and AdS solutions, including those with naked singularities and both extreme and non-extreme black-hole (BH) solutions. The parameters are identified as the asymptotic cosmological constant, an arbitrary length scale, mass and charge. In all asymptotically flat BH solutions, the potential V(phi)V(\phi)V(phi) is partly negative, in accord with Bekenstein and Mayo's no-hair theorem. The well-known conformal mapping extends the BH solutions to Jordan's pictures of a general class of scalar-tensor theories (STT) of gravity under the condition that the nonminimal coupling function f(phi)f(\phi)f(phi) is everywhere positive. Relaxing the latter condition and assuming f=0f=0f=0 at some value of phi\phiphi, we obtain wormhol...
Phantom wormhole solutions in a generic cosmological constant background
Canadian Journal of Physics, 2015
There are a number of reasons to study wormholes with generic cosmological constant Λ. Recent observations indicate that the present accelerating expansion of the universe demands Λ > 0. On the other hand, some extended theories of gravitation, such as supergravity and superstring theories, possess vacuum states with Λ < 0. Even within the framework of general relativity, a negative cosmological constant permits black holes with horizons topologically different from the usual spherical ones. These solutions are convertible to wormhole solutions by adding some exotic matter. In this paper, the asymptotically flat wormhole solutions in a generic cosmological constant background are studied. By constructing a specific class of shape functions, a mass function, energy density, and pressure profile that support such a geometry are obtained. It is shown that for having such a geometry, the wormhole throat, r0, the cosmological constant, Λ, and the equation of state parameter, ω, sho...
Phys Rev D, 2008
We use a combination of numerical and analytical methods, exploiting the equations derived in a preceding paper, to classify all spherically symmetric self-similar solutions which are asymptotically Friedmann at large distances and contain a perfect fluid with equation of state p=(gamma-1)mu with 0<gamma<2/3. The expansion of the Friedmann universe is accelerated in this case. We find a one-parameter family of self-similar solutions representing a black hole embedded in a Friedmann background. This suggests that, in contrast to the positive pressure case, black holes in a universe with dark energy can grow as fast as the Hubble horizon if they are not too large. There are also self-similar solutions which contain a central naked singularity with negative mass and solutions which represent a Friedmann universe connected to either another Friedmann universe or some other cosmological model. The latter are interpreted as self-similar cosmological white hole or wormhole solutions. The throats of these wormholes are defined as two-dimensional spheres with minimal area on a spacelike hypersurface and they are all nontraversable because of the absence of a past null infinity.
Instabilities of wormholes and regular black holes supported by a phantom scalar field
Physical Review D, 2012
We test the stability of various wormholes and black holes supported by a scalar field with a negative kinetic term. The general axial perturbations and the monopole type of polar perturbations are considered in the linear approximation. Two classes of objects are considered: (i) wormholes with flat asymptotic behavior at one end and AdS on the other (M-AdS wormholes) and (ii) regular black holes with asymptotically de Sitter expansion far beyond the horizon (the so-called black universes). A difficulty in such stability studies is that the effective potential for perturbations forms an infinite wall at throats, if any. Its regularization is in general possible only by numerical methods, and such a method is suggested in a general form and used in the present paper. As a result, we have shown that all configurations under study are unstable under spherically symmetric perturbations, except for a special class of black universes where the event horizon coincides with the minimum of the area function. For this stable family, the frequencies of quasinormal modes of axial perturbations are calculated.
An anti-Schwarzshild solution: Wormholes and scalar-tensor solutions
Journal of Physics: Conference Series, 2010
We investigate a static solution with an hyperbolic nature, characterised by a pseudo-spherical foliation of space. This space-time metric can be perceived as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs to the class of static vacuum solutions termed "a degenerate static solution of class A" (see ). In the present work we review its fundamental features, discuss the existence of generalised wormholes, and derive its extension to scalar-tensor gravity theories in general.
Notes on wormhole existence in scalar-tensor and F(R) gravity
2010
Some recent papers have claimed the existence of static, spherically symmetric wormhole solutions to gravitational field equations in the absence of ghost (or phantom) degrees of freedom. We show that in some such cases the solutions in question are actually not of wormhole nature while in cases where a wormhole is obtained, the effective gravitational constant G eff is negative in some region of space, i.e., the graviton becomes a ghost. In particular, it is confirmed that there are no vacuum wormhole solutions of the Brans-Dicke theory with zero potential and the coupling constant ω > −3/2 , except for the case ω = 0 ; in the latter case, G eff < 0 in the region beyond the throat. The same is true for wormhole solutions of F (R) gravity: special wormhole solutions are only possible if F (R) contains an extremum at which G eff changes its sign.