Photo: LIGO Livingston, Louisiana, US

1. Gravitational Wave Data Analysis for Testing General Relativity

General Relativity (GR) has been accepted as the standard theory of gravity due to its success in explaining the observations so far, but these are mostly limited in the weak field where the nonlinearity of gravity is not so significant. From a theoretical perspective, GR should be considered as an effective theory on the low energy scale and is believed to be taken by another quantum theory on the high energy scale. 

Gravitational waves (GW) from compact binary coalescences, the primary targets of the LIGO-Virgo-KAGRA collaboration, contain much information about gravity's nonlinear nature. If there are any deviations from GR in such a strong gravitational regime, the signature must be imprinted on the observed signal. To see if the deviations exist, I create appropriate waveforms and analyze the GW data with them. 

2. General Relativistic Simulation Based on Black Hole Perturbation Theory

Electromagnetic observations have revealed the presence of supermassive black holes (SMBHs). In a few decades, GWs from such a huge BH will be observable with the space-based GW interferometers. Toward future detection, I am trying to discover interesting general relativistic phenomena that might amplify the beyond-GR effects by performing general relativistic numerical simulations based on the black hole perturbation theory. 

Another interesting background is that both observation and theory of astrophysics prefer highly rotating SMBHs whose masses are suitable for the Laser Interferometer Space Antenna (LISA).  With this in mind, I am working with colleagues on observational forecasts of how precisely we can infer the nature of the SMBH, calculation of the GWs emitted by rapidly spinning SMBHs, and so on.

Recently, I found that the intermediate mass ratio (IMR) mergers involving highly spinning supermassive black holes excite the higher harmonic quasi-normal modes very efficiently. This suggests that detection of IMR mergers with LISA will provide unprecedented opportunities to infer the nature of SMBH and to test GR, both with excellent precision.