Lensed Gravitational Wave Ringdowns as a Probe of General Relativity

Abstract

[eng] The first detection of a gravitational wave (GW) signal in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors opened a new exciting line of research in astrophysics, which allows us to obtain information about the universe through non-electromagnetic signals. The current network of ground-based observatories is formed by the two LIGO observatories in the United States (Hanford and Livingstone), one in Italy (Virgo) and one in Japan (Kagra), which have joined efforts to form the LIGO-Virgo-Kagra (LVK) scientific collaboration. To date, the LVK has detected about ninety GW events, a number that is expected to increase significantly in the coming years. Its noise limitations will also improve with the implementation of new third-generation detectors. Gravitational lensing is a phenomenon predicted by general relativity (GR) which causes electromagnetic (EM) and GWs to deflect when passing near a massive object. Although EM gravitational lensing is widely used in astronomy, GW lensing has not yet been observed. The type of lensing studied in this work is strong lensing. This effect is caused by massive astronomical objects such as galaxies or galaxy clusters, allowing us to observe the signal as multiple time-delayed copies of the initial GW, which we refer to as images. This thesis will analyse the last stage of merging Kerr black holes (BH), where the newly formed BH settles producing vibrations, known as the ringdown (RD) phase. This phase is very useful for studying the frequencies of quasi-normal modes and probing GR through GR tests. Combining lensing and RD in third-generation detectors will give us a new view into GR. The project aims to probe GR through lensed GW RD and compare its advantages against non-lensed events, in particular, for the third-generation detector network. In this thesis, we simulated ten years of binary BH (BBH) merger data, including both lensed and unlensed events, as observed by the third-generation detectors Cosmic Explorer (CE) and Einstein Telescope (ET). The simulations were performed using the LeR Python package. From here, we were able to compute the RD signal-to-noise ratio (SNR) of these events and compare both lensed and unlensed cases. The unlensed scenario presented a 1.06% of RD-dominated events, while the lensed case presented a 0.73%. These RD-dominant events are significant because of their association with larger mass systems and allow us to extract meaningful physics. We conducted individual RD tests to assess the impact of lensing. Comparing a single unlensed image with multiple lensed images demonstrated an improvement of (1-3) orders of magnitude when using multiple lensed images. Additionally, when comparing two stacked unlensed images with a pair of lensed images, the resulting posteriors were tightened by (2-3) orders of magnitude when lensed. The final goal of the project was to perform parameter estimation (PE) on both lensed and unlensed events focusing on 330 higher-order modes (HOMs). We selected 50 events with the highest SNR and recovered the δf330 and δτ330 modes successfully. The results indicated that lensing breaks degeneracies in our GW models by a factor of 11.4 and 6.17 respectively. Lensing is a not yet observed GW effect, however, current expectations promise an exciting future for these observations. This work in a novel way and for the first time contributes to the readiness of the tests of GR also accounting for lensing