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Mapping gravitational waves from collisions of black holes

Periodic Reporting for period 4 - BlackHoleMaps (Mapping gravitational waves from collisions of black holes)

Reporting period: 2020-04-01 to 2022-03-31

This project was conceived in 2014 to make it possible to extract as much scientific information as possible from upcoming gravitational-wave observations of merger of black holes. Breakthrough work had already lead to the first model of the signal from mergers of generic configurations of black-hole binaries, but the model was based on several approximations, and was not tuned to full numerical simulations through the crucial merger and ringdown phases. Making that next leap in understanding of binary mergers was the goal of this project, through a large-scale targeted campaign of numerical simulations, and progress in analytic modelling techniques. Accurate models are necessary to correctly and precisely measure the parameters of black holes, and this information in turn could revolutionise our understanding of how black holes form, how many of them are in the universe, their masses and spins, and in turn the formation of stars and galaxies, and the overall composition of our universe.

Since the first gravitational-wave observation in 2015 there have been roughly 90 total observations, all from mergers of black holes or neutron stars. These observations have created the new field of gravtational-wave astronomy. Every one of these observations was analysed using one of the models developed as part of this project, and as such the work of this project has been necessary to measure the properties of every gravitational-wave observation to date, and has been instrumental in all of the astrophysical results that have been obtained in this field since 2015. The goal of this project -- in which it has been successful -- is to produce successfully more accurate models that keep pace with the growing sensitivity of the detectors, to make it possible to extract the maximum physical information from each observation.
The beginning of the project coincided with the first direct detection of GWs in 2015. The preliminary model, intended as a prototype for a precise tuned model, was the only generic model available to rapidly estimate the parameters of the signal. The first year of the project was consumed with testing the accuracy of that model (it was fortunately far more accurate than we expected!) and verifying that the measurements of that first discovery signal were not biassed by systematic errors. Since then the numerical simulation campaign has continued, and full generic modelling is underway. In addition, to meet the needs of binary black hole observations that came much earlier than anticipated at the beginning of the project, we have also developed the first inspiral, merger and ringdown model to include higher waveform harmonics, which will be essential if binaries are observed with mass ratios higher that 1:2.

Final period: Over the last years of the project, the main goal was completed: the generation of a set of numerical-relativity binary-black-hole merger waveforms that allowed us to produce the first generic binary model tuned to precession effects through the merger and ringdown. In the meantime a number of intermediate models were constructed, which were used to analyse the data from the first three observing runs of the LIGO-Virgo-Kagra detectors. These models included a more accurate treatment precession through the inspiral (the produce the PhenomPv3 model), and combination of this work with the higher-multipole work described above (to produce the PhenomPv3HM model). The first tuned precession model (PhenomPNR) was completed in 2021, and is now being extended to also include higher multipoles and multipole-asymmetry effects, and will be used in the fourth LIGO-Virgo-Kagra observing run. This will constitute the model complete and accurate full inspiral-merger-ringdown model to date.
The first higher multipole model has dramatically extended the state of the art, and will play an important role not only in potential observations, but also in assessing the science potential of future detectors. A tuned higher-multipole model is in development, along with the tuned generic model that is the ultimate goal this project.

Final period: the construction of the first generic model with precession effects tuned to full general relativistic waveforms has uncovered a number of physical features to these signals that were not understood previously. The insights gained throughout this project have been incorporated into other modelling efforts in the community, for example the complementary "PhenomX" models (which are now being extended using the work completed in the last stage of the project), and are also likely to play a role in future "EOBNR" and surrogate models. The models constructed in this project, through their use in analysing all of the LVK events, have also played a role in extending the state of our understanding of astrophysics and fundamental physics.
Comparison of precession-model waveforms to a model with no NR precession tuning