You spin me round: Measuring precession in the black hole population

Student thesis: Doctoral Thesis


Black holes: cosmic bodies of extreme gravity that nothing, not even light, can
escape. Einstein first told the world about the existence of these enigmatic objects in his groundbreaking theory of General Relativity. It was not until 2015 that black holes were probed directly through the first observation of gravitational waves. The treasure trove of knowledge gleaned from this observation revolutionised our understanding of the cosmos. Since this Nobel prize winning observation, the LIGO Scientific, Virgo and KAGRA collaborations have observed fifty gravitational waves.
The aim of this thesis has been to take advantage of the growing population
of gravitational wave sources to answer the following fundamental question: do
binary black holes undergo spin-induced orbital precession? This has significant
implications on our understanding of how binary black holes form in nature.
To answer this question, I first introduced a brand new formalisation for modelling a gravitational wave that originated from a precessing system. I then introduced the “precession signal-to-noise ratio” which naturally followed from this unique description. This novel tool quantified, for the first time, the significance of precession in an observed gravitational wave. This elegant new formalisation is presented in Chapter 2 and verified in Chapter 3.
In the subsequent four chapters, I used the “precession signal-to-noise ratio” to
answer the aforementioned question. In Chapter 4 I demonstrated that there is no evidence for precession in any of the binary black hole candidates from the first gravitational wave catalog. I then described how this lack of precession allows us to constrain the properties of black holes. In Chapter 5 I presented the properties of potentially the first neutron star-black hole binary observed – a system which is most likely to have measurable precession as a result of the asymmetric component masses. In Chapter 6 I calculated the “precession signal-to-noise ratio” for all gravitational wave candidates observed in the first half of the third gravitational wave observing run, and demonstrated that three observed gravitational waves could have originated from precessing systems. In Chapter 7, I used the gravitational wave data from the second gravitational wave catalog to determine the most likely spin distribution of black holes. By doing so, I was able to determine whether the population of binary black holes are likely to undergo spin-induced orbital precession.
Finally, in Chapter 8, I presented a new and innovative software package to analyse, display and combine posterior samples. This package has become one of the major workhorses of the LIGO Scientific, Virgo and KAGRA collaborations
and is widely distributed through the gravitational wave data analysis computing
Date of Award1 Jul 2022
Original languageEnglish
Awarding Institution
  • Cardiff University
SupervisorStephen Fairhurst (Supervisor)

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