Epicyclic Oscillations around Simpson–Visser Regular Black Holes and Wormholes

We study epicyclic oscillatory motion along circular geodesics of the Simpson–Visser meta-geometry describing in a unique way regular black-bounce black holes and reflection-symmetric wormholes by using a length parameter l. We give the frequencies of the orbital and epicyclic motion in a Keplerian disc with inner edge at the innermost circular geodesic located above the black hole outer horizon or on the our side of the wormhole. We use these frequencies in the epicyclic resonance version of the so-called geodesic models of high-frequency quasi-periodic oscillations (HF QPOs) observed in microquasars and around supermassive black holes in active galactic nuclei to test the ability of this meta-geometry to improve the fitting of HF QPOs observational data from the surrounding of supermassive black holes. We demonstrate that this is really possible for wormholes with sufficiently high length parameter l.

Marginally bound circular orbits in the composed black-hole-ring system

The physical and mathematical properties of the non-linearly coupled black-hole-orbiting-ring system are studied analytically to second order in the dimensionless angular velocity $$M_{\text {ir}}\omega _{\text {H}}$$ M ir ω H of the black-hole horizon (here $$M_{\text {ir}}$$ M ir is the irreducible mass of the slowly rotating central black hole). In particular, we determine analytically, to first order in the dimensionless ring-to-black-hole mass ratio $$m/M_{\text {ir}}$$ m / M ir , the shift $$\Delta \Omega _{\text {mb}}/\Omega _{\text {mb}}$$ Δ Ω mb / Ω mb in the orbital frequency of the marginally bound circular geodesic that characterizes the composed curved spacetime. Interestingly, our analytical results for the frequency shift $$\Delta \Omega _{\text {mb}}$$ Δ Ω mb in the composed black-hole-orbiting-ring toy model agree qualitatively with the recently published numerical results for the corresponding frequency shift in the physically related (and mathematically much more complex) black-hole-orbiting-particle system. In particular, the present analysis provides evidence that, at order $$O(m/M_{\text {ir}})$$ O ( m / M ir ) , the recently observed positive shift in the angular frequency of the marginally bound circular orbit is directly related to the physically intriguing phenomenon of dragging of inertial frames by orbiting masses in general relativity.

Scalar radiation emitted by a source around a spherically symmetric black hole

We analyze the scalar radiation emitted by a source in a circular geodesic orbit around a spherically symmetric black hole. The black hole (BH) spacetime considered is quite general, in the sense that it encompasses the solutions of Schwarzschild and Reissner–Nordström, and also the Bardeen solution of a regular BH. We use the framework of quantum field theory in curved spaces to compute the one-particle emission amplitude of scalar particles and related quantities.

Casimir effect around an Ellis wormhole

We discuss the Casimir effect in a small cavity, moving in a circular orbit around an Ellis wormhole. We show that the interplay between the spacetime geometry and the cavity orbital motion gives rise to a distortion in the Casimir energy density, causing a reduction of its absolute value. Quite interestingly, such effect can be observed also when the cavity moves on a circular geodesic (albeit unstable) orbit at the wormhole throat, where a comoving observer becomes locally inertial (namely, the observer’s reference frame reduces to a geodesic, Fermi-Walker transported one). In that respect, the discussed effect appears as a nonlocal quantum effect by means of which some properties of the underlying spacetime geometry, hidden to local classical measurements, can be captured and unveiled.

The influence of cosmological constant in temperature oscillations of a gas moving close to circular geodesic

The aim of this work is to analyze and to verify the effects of the charge and cosmological constant on the temperature oscillations that occur in a gas in a circular motion close to geodesic under the action of a Reissner–Nordström–de Sitter metric. The temperature oscillations are determined from Tolman’s law written in Fermi normal coordinates for a comoving observer. The temperature oscillations are calculated for a theoretical model obtained in the literature. Comparing the different configurations analyzed, it is possible to verify that the cosmological constant term causes a small displacement in the oscillation peaks. We also calculated the ratio between frequencies for some particular cases of the Reissner–Nordström–de Sitter metric and verified that the cases with null cosmological constant are closer of the 3/2 value found in QPOs. In another hand, the addition of the cosmological constant causes a direct increase of the ratio between frequencies.

Circular geodesic radiation in Schwarzschild spacetime: A semiclassical approach

Extreme curvature settings and nontrivial causal structure of curved spacetimes may have interesting theoretical and practical implications for quantum field theories. Radiation emission in black hole spacetimes is one such scenario in which the semiclassical approach, i.e. quantum fields propagating in a nondynamical background spacetime, adds a very simple conceptual point of view and allows us to compute the emitted power in a straightforward way. Within this context, we reexamine sources in circular orbit around a Schwarzschild black hole, investigating the emission of scalar, electromagnetic and gravitational radiations. The analysis of the differences and similarities between these cases provide an excellent overview of the powerful conceptual and computational tool that is quantum field theory in curved spacetime.

Circular geodesic of Bardeen and Ayon–Beato–Garcia regular black-hole and no-horizon spacetimes

In this paper, we study circular geodesic motion of test particles and photons in the Bardeen and Ayon–Beato–Garcia (ABG) geometry describing spherically symmetric regular black-hole or no-horizon spacetimes. While the Bardeen geometry is not exact solution of Einstein's equations, the ABG spacetime is related to self-gravitating charged sources governed by Einstein's gravity and nonlinear electrodynamics. They both are characterized by the mass parameter m and the charge parameter g. We demonstrate that in similarity to the Reissner–Nordstrom (RN) naked singularity spacetimes an antigravity static sphere should exist in all the no-horizon Bardeen and ABG solutions that can be surrounded by a Keplerian accretion disc. However, contrary to the RN naked singularity spacetimes, the ABG no-horizon spacetimes with parameter g/m > 2 can contain also an additional inner Keplerian disc hidden under the static antigravity sphere. Properties of the geodesic structure are reflected by simple observationally relevant optical phenomena. We give silhouette of the regular black-hole and no-horizon spacetimes, and profiled spectral lines generated by Keplerian rings radiating at a fixed frequency and located in strong gravity region at or nearby the marginally stable circular geodesics. We demonstrate that the profiled spectral lines related to the regular black-holes are qualitatively similar to those of the Schwarzschild black-holes, giving only small quantitative differences. On the other hand, the regular no-horizon spacetimes give clear qualitative signatures of their presence while compared to the Schwarschild spacetimes. Moreover, it is possible to distinguish the Bardeen and ABG no-horizon spacetimes, if the inclination angle to the observer is known.