Parameter Estimation with Future Gravitational Wave Detectors

The first detection of a gravitational-wave signal of a coalescence of two black holes marked the beginning of the era of gravitational-wave astronomy, which opens exciting new possibilities in the fields of astronomy, astrophysics and cosmology. Driven forward by the wealth of new information provided by detections of the currently operating detectors, the field of gravitational wave science is developing quickly - next-generation detectors that will lift our understanding to a new level are already being planned. In this thesis, I investigate the possibilities for parameter estimation using the Fisher-matrix formalism with combined information from present and future detectors in different frequency bands. The scientific questions I set out to answer are: • How many sources can these future detectors detect, and how well can an individual detector reconstruct the source parameters? • How does parameter estimation benefit from combining information from several detectors, in particular from detectors in different frequency bands (multiband approach)? • How do different choices for the detector (detector geometries for ground-based detectors, orbits for space-borne detectors) influence science goals? The detectors considered are the LIGO/Virgo detectors, the Einstein Telescope (ET), Cosmic Explorer (CE), the Laser Interferometer Space Antenna (LISA), and the first stage of the Deci-Hertz Interferometer Gravitational wave Observatory (B-DECIGO). The simulation results show that future detectors will detect many more sources (ET), and also new classes of sources (e.g. intermediate-mass binary black holes in ET, B-DECIGO and LISA). However, it became clear that LISA is unlikely to detect any stellar-mass black hole binaries that would merge within a few years, as required for multiband observations. With respect to the parameter estimation capabilities of the detectors, it became clear that future detectors will provide parameter estimates orders of magnitude better than the estimates of the currently operating detectors. Long observation times and the movement of the detectors during the observation time help to break parameter degeneracies and to improve the estimates of the sky localization angles in particular. ET, for example, will be able to determine the sky localization of neutron star binaries, due to the long observation time of these binaries in ET. With regard to multiband observations, the finding is that the mass parameters in particular benefit from combining information from ground-based and space-borne detectors. The Einstein Telescope is currently envisioned in a triangular configuration consisting of 6 interferometers in total, which, at least in thought experiments, gives flexibility to how these instruments are best combined. I investigate the impact of different configuration scenarios on detection rate and on compact-binary parameter-estimation errors, in particular on sky localization estimates. The scenarios considered are triangular versus L-shape, two sites versus single site, and separation of the low- and high-frequency interferometers forming the xylophone detector of the baseline configuration of the Einstein Telescope. It emerged that the triangular design, while being more complex, offers significant advantages for parameter estimation - it allows for the determination of the sky localization for a few select binaries, and of the inclination angle. I also find that the largest contribution to the Signal-to-noise ratio comes from ET’s low frequency sensitivity - the vast majority of detections can be made with the low-frequency interferometers alone. For multimessenger observations, binary neutron stars are the most interesting sources. With a state-of-the-art astrophysical model, I studied detection rates of binary neutron stars in triangular ET, and how well it can reconstruct the sky localization, also as a network with the currently operating 2G detectors and CE. I found that ET will detect a vast number of sources, and it will be able to determine their sky position, in particular when working as a network with the 2G detectors and CE. These capabilities are extremely promising for finding answers to many important questions in astrophysics and cosmology. In conclusion, the results presented in this thesis draw a comprehensive picture of the potential scientific gains of future ground-based and space-borne detectors, and it sheds light on the ramifications of design choices like the choice of detector geometries and orbits.