Quasi-normal modes of rotating black holes and neutron stars: gravitational waves and AdS/CFT correspondence
Astrophysics and Astronomy
Final Report Abstract
A significant number of goals of the project have been achieved. We have systematically investigated gravitational waves that are emitted from a large set of black holes and neutron stars and investigated the stability of these compact objects. Going beyond Einstein gravity, we have considered alternative theories of gravity with a scalar degree of freedom. For black holes we have mainly focused on the modes of black holes, where an instability with respect to the growth of a scalar field arises on the general relativistic black holes, either because of the presence of a curvature source term or of an electromagnetic source term. In both cases, we have studied the sets of solutions of the scalarized black holes, their stability and their spectrum. In the curvature case, the choice of the coupling function for the scalar field has turned out to be crucial in order to obtain mode stable scalarized black holes. We have shown, that for an appropriate coupling function a part of the fundamental scalarized branch of black holes can be stable. However, we have also found that the typical form of the wave equation for the perturbations is lost at a certain coupling strength and the formalism breaks down. Concerning the electromagnetic case, we have observed that the intriguing phenomenon of a split of the spacetime into two totally distinct parts can occur in these theories, leading to a scalarized black hole on the one hand and an extremal general relativistic black hole on the other hand. Furthermore we have studied the modes of Dirac fields, describing particles with half-integer spin, on rotating Kerr black holes and on rotating higher-dimensional black holes in general relativity, finding in both cases long-lived modes. We have also made significant progress with respect to the study of the modes of neutron stars in alternative gravity theories. Here we have for the first time obtained the sets of coupled equations for the general set of modes in scalar-tensor theories. Solving these mode equations we have obtained the spectrum of neutron stars with, in particular, the radial, dipole and quadrupole modes. Performing a systematic study for different classes of equations of state for the matter of the stars, we have been able to extract universal relations for the mode frequencies and the decay rates. Such relations are very valuable for comparison with observations as long as the true equation of state of neutron stars is not yet known. We have also found, that monopole and dipole radiation can be very long-lived in such scalar-tensor theories. As an interesting byproduct of our investigations we have found a new type of wormhole, that is supported by Dirac fields. With our work performed in this project, we have been able to obtain results, that are highly valuable for the analysis of gravitational waves observed by future generations of detectors, and we have succeeded to develop new approaches and techniques to obtain further predictions, that will lead to new bounds on the parameter space of alternative theories of gravity and thus scrutinize their viability.
Publications
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Axial quasinormal modes of neutron stars in R2 gravityty. Physical Review D, 98(10).
Blázquez-Salcedo, Jose Luis; Doneva, Daniela D.; Kunz, Jutta; Staykov, Kalin V. & Yazadjiev, Stoytcho S.
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Radial perturbations of the scalarized Einstein-Gauss-Bonnet black holes. Physical Review D, 98(8).
Blázquez-Salcedo, Jose Luis; Doneva, Daniela D.; Kunz, Jutta & Yazadjiev, Stoytcho S.
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Polar quasinormal modes of the scalarized Einstein-Gauss-Bonnet black holes. Physical Review D, 102(2).
Blázquez-Salcedo, Jose Luis; Doneva, Daniela D.; Kahlen, Sarah; Kunz, Jutta; Nedkova, Petya & Yazadjiev, Stoytcho S.
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Ultra-long-lived quasi-normal modes of neutron stars in massive scalar-tensor gravity. EPL (Europhysics Letters), 130(5), 50002.
Luis, Blázquez-Salcedo Jose; Scen, Khoo Fech & Kunz, Jutta
