Four-dimensional, asymptotically flat spacetimes with an ergoregion but no horizon have been shown to be linearly unstable against a superradiant-triggered mechanism. This result has wide implications in the search for astrophysically viable alternatives to black holes, but also in the understanding of black holes and Hawking evaporation. Here we investigate this instability in detail for a particular setup which can be realized in the laboratory: the {\it hydrodynamic vortex}, an effective geometry for sound waves, with ergoregion and without an event horizon.Comment: 10 pages, 8 figures, 2 table
We analyze the tidal forces produced in the spacetime of Reissner-Nordström black holes. We point out that the radial component of the tidal force changes sign just outside the event horizon if the charge-to-mass ratio is close to 1 unlike in Schwarzschild spacetime of uncharged black holes, and that the angular component changes sign between the outer and inner horizons. We solve the geodesic deviation equations for radially falling bodies towards the charged black hole. We find, for example, that the radial component of the geodesic deviation vector starts decreasing inside the event horizon unlike in the Schwarzschild case.
Under certain conditions, sound waves in a fluid may be governed by a Klein-Gordon equation on an `effective spacetime' determined by the background flow properties. Here we consider the draining bathtub: a circulating, draining flow whose effective spacetime shares key features with the rotating black hole (Kerr) spacetime. We present a complete investigation of the role of quasinormal (QN) mode and Regge pole (RP) resonances of this system. First, we simulate a perturbation in the time domain by applying a finite-difference method, to demonstrate the ubiquity of `QN ringing'. Next, we solve the wave equation in the frequency domain with the continued-fraction method, to compute QN and RP spectra numerically. We then explore the geometric link between (prograde and retrograde) null geodesic orbits on the spacetime, and the properties of the QN/RP spectra. We develop a `geodesic expansion' method which leads to asymptotic expressions (in inverse powers of mode number $m$) for the spectra, the radial functions and the residues. Next, the role of the Regge poles in scattering and absorption processes is revealed through application of the complex angular momentum method. We elucidate the link between the Regge poles and oscillations in the absorption cross section. Finally, we show that Regge poles provide a neat explanation for `orbiting' oscillations seen in the scattering cross section.Comment: 18 pages, 9 figures, 1 table. Added a new section on Regge Pole residues and `orbiting' oscillations in scattering cross section
Analogue systems are a powerful instrument to investigate and understand in a controlled setting many general-relativistic effects. Here, we focus on superradiant-triggered instabilities and quasi-normal modes. We consider a compressible hydrodynamic vortex characterized by a polytropic equation of state, the {\it polytropic hydrodynamic vortex}, a purely circulating system with an ergoregion but no event horizon. We compute the quasinormal modes of this system numerically with different methods, finding excellent agreement between them. When the fluid velocity is larger than the speed of sound, an ergoregion appears in the effective spacetime, triggering an "ergoregion instability." We study the details of the instability for the polytropic vortex, and in particular find analytic expressions for the marginally-stable configuration.Comment: 11 pages, 3 figures, 5 table
We compute the quasinormal mode frequencies and Regge poles of the canonical acoustic hole (a black hole analogue), using three methods. First, we show how damped oscillations arise by evolving generic perturbations in the time domain using a simple finite-difference scheme. We use our results to estimate the fundamental QN frequencies of the low multipolar modes l = 1, 2, . . .. Next, we apply an asymptotic method to obtain an expansion for the frequency in inverse powers of l + 1/2 for low overtones. We test the expansion by comparing against our time-domain results, and (existing) WKB results. The expansion method is then extended to locate the Regge poles. Finally, to check the expansion of Regge poles we compute the spectrum numerically by direct integration in the frequency domain. We give a geometric interpretation of our results and comment on experimental verification.
With the advance in hardware integration, system-on-a-chip (SoC) test activities using only automatic test equipments (ATEs) result in an expensive option. Hardware-based test may reduce the ATE dependency. However, hardware-based test imposes some constraints like area overhead and processing speed degradation. The main objective of this work is to investigate and evaluate a less intrusive test approach called software-based test. Software-based test uses an embedded processor as source and sink of the test, sending the test patterns and reading the responses. A new integrated design and test environment has been developed to automatically synthesize test programs to test non-programmable cores of SoCs. Some benchmarks ISCAS85 and ISCAS89 are used to evaluate the proposed methodology.
We investigate the radiation to infinity of a massless scalar field from a source falling radially towards a Schwarzschild black hole using the framework of the quantum field theory at tree level. When the source falls from infinity, the monopole radiation is dominant for low initial velocities. Higher multipoles become dominant at high initial velocities. It is found that, as in the electromagnetic and gravitational cases, at high initial velocities the energy spectrum for each multipole with l ≥ 1 approximately is constant up to the fundamental quasinormal frequency and then drops to zero. We also investigate the case where the source falls from rest at a finite distance from the black hole. It is found that the monopole and dipole contributions in this case are dominant. This case needs to be carefully distinguished from the unphysical process where the source abruptly appears at rest and starts falling, which would result in radiation of an infinite amount of energy. We also investigate the radiation of a massless scalar field to the horizon of the black hole, finding some features similar to the gravitational case.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.