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

该奖项全部或部分根据2021年美国救援计划法案（公法117-2）资助。在几乎一个世纪的时间里，爱因斯坦在广义相对论中的一个预言--引力波（GW）的产生--一直没有被发现。然而，我们现在生活在一个地面仪器相当经常地探测到GW的世界。然而，这些仪器只能看到全球引力波全谱的一小部分。相比之下，脉冲星定时阵列（PTA）实验在比地面仪器低得多的GW频率下运行，并使用我们银河系中毫秒级脉冲星的精确长期监测来梳理GW引起的定时延迟的微妙印记。NANOGrav和其他合作的最新结果暗示，PTA GW检测的时代即将到来。随着PTA数据集的数据量和监测的卫星数量越来越大，搜索和科学分析将变得越来越困难。该奖项中的项目将为下一代PTA数据分析开辟道路，并在此过程中打破领域专业知识的壁垒，使其他科学界和新学生能够利用PTA发现的令人兴奋的未来。PI、受资助的学生和合作者将开发新的PTA GW搜索方法，压缩时序数据，并使当前管道的速度提高几个数量级。该团队将探索何时PTA能够在这些频率下表征GW的天体物理和宇宙学景观，绘制纳赫兹GW天空，并限制引力偏离广义相对论。该奖项的综合教育方面将部署这些新的研究突破，以提高代表性不足的少数民族学生在PTA科学的参与，促进田纳西州中部的GW研究足迹，并利用现有的菲斯克-范德比尔特桥计划推进纳赫兹GW教学法。 该奖项的研究部分分为PTA数据分析变革性进展的两个关键领域：（i）GW背景信号的光谱表征，以揭示二元超大质量黑洞系统的动力学，并梳理出次主导宇宙学背景;以及（ii）探测脉冲星定时互相关的模式，以映射GW功率在纳赫兹频率下的角分布，并约束GW。编码偏离广义相对论在这两个领域，PI，学生和合作者将开发面向未来的技术，通过将时序数据压缩为足够的统计数据来克服数据量增加的挑战。对于频谱表征，这对应于将定时数据的低频行为表示为贝叶斯周期图和傅立叶系数的后验分布。对于互相关分析，这对应于作为GW频率函数的成对脉冲星相关性的测量。通过利用这些技术的模块化和速度，该团队将探索PTA何时能够表征GW应变谱中编码的超大质量二元黑洞系统的动力学。除此之外，研究小组还将评估将测得的GW背景分量分离为主导天体物理信号和较弱宇宙学信号的前景。同样，该团队将开发最大似然统计数据，挖掘PTA互相关的模式，以映射不同频率的纳赫兹GW天空，并限制超过GR GW极化特征的存在。与这些研究突破交织在一起的将是每年一次的REU机会，以扩大参与PTA的发现，从田纳西州中部地区的人才财富中汲取。此外，PI还将启动一个PTA数据分析暑期研究所，由此将产生一个全年数据俱乐部，该俱乐部将采用通过该奖项开发的研究技术，以增强学生的学习和发现能力。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.