arXiv subjects

Ka-Wai Chung

Publications and source records attributed to Ka-Wai Chung.

4 recordsLinked to original sources

arXiv2021

Lensing of Gravitational Waves as a Novel Probe of Graviton Mass

The diffraction patterns of lensed gravitational waves encode information about their propagation speeds. If gravitons have mass, the dispersion relation and speed of gravitational waves will be affected in a frequency-dependent manner, which would leave traces in the diffraction pattern if the waves are lensed. In this paper, we study how the alternative dispersion relation induced by massive gravitons affects gravitational waves lensed by point-mass lenses, such as intermediate-mass black holes. We find that the waveform morphology of lensed dispersive gravitational waves depends on the graviton mass more sensitively than their unlensed counterpart. Together with lensing amplification, the waveform-morphology modifications due to lensing can improve the measurement accuracy of the graviton mass. A single lensed gravitational-wave signal enables us to measure the graviton mass with an accuracy comparable with the combined measurement across $\mathcal{O}(10^3)$ unlensed signals. Our method allows us to incorporate lensed gravitational-wave signals into existing graviton-mass measurements. Our method can also be generalized to other lens types, gravitational-wave sources, and detector networks, preparing ourselves for thoroughly understanding the nature of gravitational waves in the era of lensed gravitational-wave astronomy.

gr-qc↗

arXiv2021

Ringdown Spectroscopy of Rotating Black Holes Pierced by Cosmic Strings

Multiple gauge theories predict the presence of cosmic strings with different mass densities $Gμ/c^2$. We derive an equation governing the perturbations of a rotating black hole pierced by a straight, infinitely long cosmic string along its axis of rotation and calculate the quasinormal-mode frequencies of such a black hole. We then carry out parameter estimation on the first detected gravitational-wave event, GW150914, by hypothesizing that there is a string piercing through the remnant, yielding a constraint of $Gμ/c^2 <3.8\times 10^{-3}$ at the 90\% confidence interval with a comparable Bayes factor with an analysis for a Kerr black hole without a string. In contrast to existing studies which focus on the mutual intersection of cosmic strings, or the cosmic string network, our work focuses on the intersection of a cosmic string with a black hole, with characteristics which can be identified in binary coalescence signals.

gr-qc↗

arXiv2020

Probing the Purely Ingoing Nature of the Black-hole Event Horizon

One of the most fundamental results of general relativity is that the event horizon of black hole is purely ingoing. On the other hand, semiclassical-gravity effects, such as particle creation and the quantization of black-hole area, suggest that black holes can emit energy. Since a black hole is characterized by the presence of the event horizon, the emitted energy must be extracted from the black hole through its horizon. These considerations provide a motivation to test the validity of the purely ingoing nature of black-hole horizon. In this paper, we propose a novel test of the purely ingoing nature of black-hole horizon through gravitational-wave detection. We study the effects of hypothetical out-going gravitational waves to a perturbed black hole by supplementing the boundary condition of gravitational waves at the horizon with a phenomenological outgoing part. We show that this leads to extra excitation of the usual quasinormal modes of a perturbed black hole and continuous emission of gravitational waves. These additional signatures enable us to test the boundary condition(s) of the black-hole event horizon through gravitational-wave detection. Reanalysing the merger remnant of GW150914, we constrain the intensity of the outgoing gravitational-horizon flux to be $ < 10^{40} \rm W$, which is roughly $ 10^{-9} $ of peak luminosity of GW150914.

gr-qc↗

arXiv2019

Phenomenological Inclusion of Alternative Dispersion Relations to the Teukolsky Equation and its Application to Bounding the Graviton Mass with Gravitational-wave Measurements

Existing constraints on the graviton mass from gravitational-wave detections rely on the phase difference developed between different frequencies during the propagation. Effects on the quasinormal-mode frequencies of the black-hole ringdown due to the graviton mass are often ignored. While perturbation theories of black holes have been well developed in the context of general relativity, this is not the case for modified gravity theories. We propose a phenomenological modification to the Teukolsky equation of perturbed black holes to include the dispersion relation due to a gravitational field of nonzero mass. Solving this modified Teukolsky equation by logarithmic perturbation theory, we compute the shift of the quasinormal-mode frequencies due to the presence of a graviton mass. This hypothetical shift can be used to constrain the graviton mass with ringdown signals, either standalone or in conjunction with the phase difference accumulated due to the wave propagation. We estimate that constraints on the graviton mass of $m_g \lesssim 10^{-15}\,\textrm{eV}$ can be put with a detection of the ringdown signal alone by second generation gravitational-wave detectors.

gr-qc↗