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Aug 6, 2022
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Introduce my personal research interests
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Sep 23, 2024 12:50 PM
No-Hair Theorem and Hairy Black Holes
Testing gravities by Extreme Mass Ratio Inspirals
Acoustic Black Holes in Curved Spacetime
For theoretical physicists, black holes are the fundamental particles in General Relativity. The study of black hole physics is the study of interaction between particles and particles or particles and the spacetime. Of course, black holes are far more informative or colorful than particles. They are the most special class of objects in the universe predicted by Einstein’s general relativity, whose event horizon play important roles in various aspects of gravitational physics. the vicinity of the black holes event horizon are highly-dynamical, strong-curvature regions, where not only the relevant properties and characteristics of black holes can be revealed, but also the fundamental theories and new physics are embedded.
Also, they are thermodynamic systems in the universe. The discussion related to Hawking radiation and black hole entropy led the era of black hole thermodynamics. The most wonderful thing is that thermodynamics and dynamics - two things that were once completely unrelated - are wonderfully coupled here in black holes.
In my short research life, my research interests are focused on the near-horizon properties of black holes and their detectable effects. Below is a short overview of my research and I will write posts about various aspects of my research as I go along.
Testing Gravities by Extreme Mass Ratio Inspirals
This would be my primary focus of research. Extreme Mass Ratio Inspirals(EMRI) describe a secondary body like stellar-mass black hole or other compact object orbiting into a central supermassive black hole which will radiate tens to hundreds of thousands of gravitational wave cycles. They are prime target sources of detection for space-based gravitational wave detectors such as LISA, Taiji and TianQin in the near furture.
The accumulated signal of EMRIs provides a very helpful tool to probe the near-horizon environment of black holes which inspires us to further study EMRIs in modified gravity theories. It remains the probe of chance to test the modified gravity theories and constrain the dynamic parameters beyond GR. Therefore, we can study the extra dimensions, dark matter/energy or even test the quantum gravity and black hole extra hair such as spontaneous scalarization.
In recent work, we have completed our work to detect the existence of scalar fields in the Kerr spacetime by EMRIs. The secondary body which brings the scalar charge describing the interaction with scalar field spirals into the central supermassive black hole adiabatically by the quasi-circular orbital evolution on the equatorial plane. We calculated the energy fluxes and dephasing with different parameters, the results show that the presence of additional scalar emission leads to a more significant rate of overall energy loss, which, in turn, decreases the total number of orbital cycles before the small object plunges into the central black hole. Moreover, for a central black hole with a higher spin, the imprints of the scalar charge on the resultant gravitational radiations are found to be more significant, which indicates the possibility of detecting the scalar charge.
No-Hair Theorem and Hairy Black Holes
One of the fascinating questions in General Relativity is the black hole’s uniqueness theorem, also referred to as the no-hair conjecture. It states that the black hole solutions are entirely characterized by three quantities, namely, the mass, electric charge, and angular momentum.
In these years, there has been a surge of interest in black hole solutions in the presence of a nonlinear field. In particular, gravitational theories beyond GR or even GR with specific matter sources were shown to evade the no-hair conjecture and instead develop hairy black hole solutions featured by more than three quantities.
Recently an attempt to challenge this theorem has been achieved by considering a quadratic scalar field coupling with the Gauss-Bonnet (GB) or Maxwell invariant. At linear level such coupling can trigger tachyonic instability, which spontaneously scalarizes a bald BH through a second order phase transition.
In this work, we relate such instability to possible holographic scalarization and construct the corresponding hairy black hole solutions in the presence of a negative cosmological constant in the Einstein-scalar-Gauss-Bonnet gravity theory. Employing the holographic principle, such bulk scalarization corresponds to the boundary description is that the scalar vacuum expectation value condensates and non-trival related holographic entanglement entropy emerges above a critical coupling value this indicates the phase transition should be a quantum type as without breaking any symmetry. We showed that the hairy black holes caused by spontaneous scalarization are not the same as the hairy black holes caused by holographic superconductivity. In fact, considering holographic scalarization, the gravitational attraction from the Gauss-Bonnet high curvature term becomes stronger and thus destroys the black hole scalarized hair when the coupling is larger than the critical value ; while when the coupling is smaller than the critical value , the holographic superconducting condensation participates and we have the combined stronger effects on the formation of hairy black holes.
Acoustic Black Holes in Curved Spacetime
Since real black holes are far away, some of its effects like Hawking radiation or other quantum effects like entanglement, cannot be directed probed by observations. However, analog systems in the lab utilizing optics, Bose-Einstein condensates, or even water and other fluid, have been constructed to mimic such effects.
Thanks to those significant realizations of astrophysical phenomena in the laboratory, acoustic black holes nowadays attract more and more attention. Most of the aforementioned studies were based on the acoustic models constructed in the real Minkowski spacetime.
In the four dimensional Schwarzschild spacetime, we firstly created an acoustic black hole which could be one of the simplest analogue black hole in curved spacetime. This is an encouraging attempt because astrophysical black holes in the universe could be in the bath of some kind of superfluid or just the cosmological microwave, and acoustic black holes could also emerge from black-D3 brane based on holographic approach. In the Schwarzschild acoustic black hole model, we studied the QNMs, grey-body factor, analogue Hawking radiation and as a first attempt, the acoustic black hole shadow. The conclusions show that the acoustic black hole is stable under the scalar perturbation and the QNMs is weaker than that of the astrophysical Schwarzschild black hole. The acoustic shadow which is a dumb region for listeners is enlarged by the tuning parameter, we hope these observations could be detected experimentally in the near future. Take it a step further, we also first analyze the horizon structure of the acoustic charged black hole in curved spacetime and then study its acoustic shadow as well as the near-horizon characteristics including the QNMs and analogue Hawking radiation.
