CAREER: Unveiling the Nanohertz GW Discovery Landscape by Broadening Participation In Multi-Messenger Astrophysics

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). For almost a century, one of Einstein’s predictions from his General Theory of Relativity went undetected- the production of gravitational waves (GWs). Yet we now live in a world where GW detections by ground-based instruments are fairly regular. However, these instruments see only a sliver of the full spectrum of GWs. By contrast, Pulsar Timing Array (PTA) experiments operate at much lower GW frequencies than ground-based instruments, and use the precision long-term monitoring of millisecond pulsars in our galaxy to tease out the subtle imprint of GW-induced timing delays. Recent results from NANOGrav and other collaborations hint at the era of PTA GW detection looming tantalizingly close. As PTA datasets grow ever larger in data volume and the number of monitored pulsars, searches and scientific analyses will become increasingly difficult. The programs in this award will forge a path to the next-generation of PTA data analysis, and in so doing break down the walls of domain expertise to empower other scientific communities and new students to capitalize on the exciting future of PTA discovery. The PI, funded students, and collaborators, will develop novel approaches to PTA GW searches that compress the timing data and lead to speed-ups of several orders of magnitude over current pipelines. The team will explore when PTAs will be able to characterize the astrophysical and cosmological landscape of GWs at these frequencies, map the nanohertz GW sky, and constrain departures of gravity from General Relativity. The integrated educational aspects of this award will deploy these new research breakthroughs to improve participation in PTA science of underrepresented minority students, boost the GW-research footprint of Middle Tennessee, and leverage the existing Fisk-Vanderbilt Bridge Program to advance nanohertz GW pedagogy. The research component of this award is split into two key areas of transformative advancement in PTA data analysis: (i) spectral characterization of a GW background signal to unveil the dynamics of binary supermassive black-hole systems, and tease out sub-dominant cosmological backgrounds; and (ii) probing the pattern of pulsar-timing cross-correlations to map the angular distribution of GW power at nanohertz frequencies and constrain GW-encoded departures from General Relativity. In both areas, the PI, students, and collaborators will develop future-proof techniques that overcome challenges of increasing data volume by compressing the timing data into sufficient statistics. For spectral characterization this corresponds to representing the low-frequency behavior of the timing data as Bayesian periodograms and posterior distributions of Fourier coefficients. For cross-correlation analyses this corresponds to measurements of pairwise pulsar correlations as a function of GW frequency. By harnessing the modularity and speed of these techniques, the team will explore when PTAs will be able to characterize the dynamics of supermassive binary black-hole systems that are encoded in the GW strain spectrum. Beyond this, the team will assess the prospects for component separation of a measured GW background into a dominant astrophysical signal and a weaker cosmological signal. Likewise, the team will develop maximum-likelihood statistics that mine the pattern of PTA cross-correlations to map the nanohertz GW sky at different frequencies, and constrain the presence of beyond-GR GW polarization signatures. Interwoven with these research breakthroughs will be an annual REU opportunity to broaden participation in PTA discoveries, drawing from the wealth of talent in the Middle Tennessee area. Additionally, the PI will launch a summer institute for PTA data analysis, out of which will spawn a year-round data club that will employ the research techniques developed through this award to empower student learning and discovery.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.