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 experiments and observations to date, but these are mostly limited to 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 the nonlinear nature of gravity. If there are any deviations from GR in such a strong-field regime, the signature would be imprinted on the observed signal. To see if the deviations exist, I propose appropriate waveform models and analyze the GW data. 

2. General Relativistic Simulation Based on Black Hole Perturbation Theory

Electromagnetic observations have revealed the presence of supermassive black holes (SMBHs). In the next decade, 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.