From One to Many: Statistical Gravitational-Wave Astrophysics and Cosmology

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The 2015 discovery of gravitational waves, ripples in the fabric of space and time, electrified the scientific community and was awarded the Nobel Prize. Scientists are now routinely detecting gravitational waves from the collisions of neutron stars and/or black holes. A catalog of 50 such mergers has been assembled, with hundreds more expected in the coming years. This project will analyze this growing catalog, and determine the distribution of properties such as the mass and spin of the black holes as well as their distances. By analyzing all of this information, it is possible to infer important insights about how the black holes were formed in the first place. And the formation of black holes is particularly fascinating because it relates to the deaths of stars, since black holes are thought to be formed from dying stars. Even more interesting, the recent detections of big black holes, some of them well over 50 times the mass of our Sun, are related to the deaths of very big stars—these are specifically thought to be some of the earliest and most pristine stars. In short, by studying black hole collisions, this study will learn about the first stars in the Universe, and therefore the process by which the Universe makes stars, galaxies, the elements, and eventually, the Earth and everything on it. In addition to producing important and novel science, this proposal will also convey some of the beauty and excitement of the field to the broader public, ranging from primary school students through to senior citizens.The burgeoning catalog of events enables an entirely new approach to elucidating science from gravitational-wave sources: statistical inference from the detected population of gravitational events. This project will advance the field of gravitational-wave population astrophysics by building the tools and analysis pipelines to answer some of the most exciting astrophysical questions related to LIGO sources. Specific research projects will include characterizing mass gaps, spin and mass ratio distributions, and the redshift evolution of the detected population. These results will inform constraints on the astrophysical formation channels of binary systems. For small samples, every event is in some sense an “outlier”; as the population grows, the PI will explore the critical astrophysical relevance of outliers and sub-populations. In addition, the PI will explore population constraints on the neutron star equation of state, and further develop a novel probe of binary formation physics and astrophysics through the binary-host connection. The study will continue to develop standard sirens as uniquely clean and powerful cosmological probes, including extending counterpart and statistical standard sirens constraints, and refining pipelines and preparing for the possibility of exciting standard siren sources in the LIGO/Virgo O4 observational science run. Work will also include further development of novel approaches to standard siren science, including the exploration of mass-gap cosmology. The project will use the upper and lower edges of the mass distribution as observational “features” to provide independent constraints of the expansion history of the universe to very high redshift. As the number of gravitational-wave detections continues to grow by leaps and bounds, statistical characterization of the events will provide important insights, and this work will help pave the way for population gravitational-wave astrophysics and cosmology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.