Uncertainty in the Gravitational Wave Event Rates from the History of Star Formation in the Universe

Abstract

First proposed by Henry Poincaré in 1905 and subsequently predicted by Albert Einstein (1916), the study of gravitational waves has brought the field of astrophysics a new way of understanding the Universe and could, ultimately, help us answer some of the biggest questions in science. The aim of this thesis is to predict the gravitational wave transient event rates and find the relevant uncertainties that can contribute to the predictions. We constructed a model of the star formation history and first tested our model against the observed core-collapse supernovae versus redshift. We then used the BPASS v2.1 individual metallicity delay time distribution to estimate the rate of transient gravitational events assuming the Universe had various constant metallicities. The predicted rates mostly match those observed by LIGO/Virgo. We found that at low metallicities our models tend to produce more neutron star-neutron star and black hole - black hole merger events while at high metallicities the models produce the most neutron star - black hole mergers. We then refined our model by combining the star formation history with a model of the metallicity evolution. By allowing for evolution of the distribution of metallicities of stars in the Universe we were able to closely match the current gravitational wave event rates within the uncertainties. In addition, we allowed the initial binary population and the supernova kick model to vary, and then used the same method to combine BPASS models v2.2hobbs and v2.2bray. We found that changing the initial binary distribution has only a minor effect on all types of merger rates. Kick models, however, have a much greater effect on our predictions, with the rate for black hole - black hole mergers increasing fourfold compared with kick model v2.1. Future direction of gravitational wave research following this study would include the study of the star formation history of host galaxies of the gravitational wave events and constraints on the binary progenitors.