Gravitational-Wave Paleontology: A New Frontier to Probe Massive Binary Stars Across Cosmic History

Abstract

Pairs of stellar–mass black holes and neutron stars across our vast universe occasionally collide, unleashing bursts of gravitational waves that can now be detected on Earth since the first observation of a binary black hole merger in 2015. The detectable properties of these double compact object mergers, like their masses, carry valuable information about the physics of black holes and neutron stars and probe the massive stars that once formed them. These detections open a new frontier in astronomy that is the center of this thesis called Gravitational–Wave Paleontology: studying massive stars from their ‘remnants’ as compact object coalescences, with the goal to answer some of the key questions in gravitational–wave astronomy today: How do these gravitational–wave sources form? What can we learn from them about the formation, lives, and explosive deaths of massive stars across cosmic time? How do these sources help to enrich the universe with heavy metals? This is particularly exciting as gravitational waves probe massive stars over cosmic scales. Although massive stars evolve into black holes and neutron stars within just a few million years, the time it takes the double compact object binary to merge can take billions of years. This means that the observed population of mergers, even the current “local” detections, probes a mixture of progenitor stars that could have formed throughout the vast cosmic history. At the same time, this mixture also makes it challenging to unravel the formation histories of gravitational-wave sources as their detections do not directly measure the properties of their progenitor stars. Making the most of these gravitational-wave observations thus requires comparing the observed properties of the black hole and neutron star mergers, such as their rates, masses, and spins, to theoretical models of their formation pathways. In this thesis, we address, however, that at present this endeavor is limited by the so-called progenitor Uncertainty Challenge: uncertainties within the theoretical models are so large, and the models so computationally expensive, that learning about the underlying fundamental physical processes in massive star evolution from gravitational-wave observations is challenging. This thesis identifies and addresses the Uncertainty Challenge. We present work that helps quantify and understand key uncertainties in theoretical models of the formation of gravitational-wave sources with a focus on the formation from isolated massive binary stars. Our work highlights the need to transform the field from focusing on single-model predictions to quantifying model uncertainties with the goal to enable learning about the formation, lives, and deaths of massive binary stars across cosmic time from gravitational-wave observations. The results in this thesis present important steps forward toward tackling the Uncertainty Challenge, which combined with the rapidly growing number of gravitational-wave observations in the next decades will help make unprecedented discoveries in the new gravitational-wave paleontology frontier.