Towards Robust Measurements of H0 with Dark Standard Candles

对宇宙膨胀率--哈勃常数--的测量在历史上一直是不确定的，但在过去的二十年里已经收敛到和谐值。然而，宇宙学最近的一个主要发展是两个主要的宇宙学探测器对哈勃常数的测定之间存在分歧。也就是说，通过宇宙微波背景各向异性来精确(间接)测量哈勃常数，给出的膨胀率比直接从附近星系和Ia型超新星获得的值要低得多。这两个衡量标准在“五西格玛”水平上相互不一致，对应的置信度约为99.9999%。寻找这种所谓的哈勃张力的解释已经成为宇宙学的一项核心努力。密歇根大学和宾夕法尼亚大学的科学家们提议大幅提高另一种测量哈勃常数的方法的可靠性，这种方法可以带来新的曙光，或者彻底解决哈勃的张力。这种方法利用了所谓的“暗标准警报器”引力波事件。作为该项目的一部分，该小组还将参与一些外联活动，包括为高中生举办一年一度的暑期课程，组织一年两次的宇宙学暑期学校(在密歇根)；以及在Astrofest期间组织讲座，并在宾夕法尼亚州立大学举办关于天体物理学和宇宙学的邻里研讨会。标准警报器是合并后产生的观测到的引力波信号，其波形提供了与这些事件的距离。暗警报器是那些没有发现电磁对应物(以及相应的红移没有测量)的事件，使随后确定哈勃常数的工作变得复杂。该团队将研究如何利用星系和引力波暗警报器之间的相互关联来测量哈勃常数。这种互相关法应该可以更清晰地将宇宙学信号与各种系统误差引起的偏差分开，但也可以纯粹是统计学上的。该团队将针对系统误差彻底测试这种方法，然后将其应用于当前数据，并用未来数据确定哈勃常数约束的前景。最后，该团队还将调查天体物理系统学对恢复哈勃常数能力的影响。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Measurements of the expansion rate of the universe -- the Hubble constant -- have historically been uncertain, but have in the past two decades converged to the concordance value. However, a major recent development in cosmology has been the disagreement between the Hubble constant determinations by two major cosmological probes. Namely, the precision (indirect) measurement of the Hubble constant by the cosmic microwave background anisotropies gives a much lower value for the expansion rate than the direct determination from nearby galaxies and type Ia supernovae. The two measurements are mutually inconsistent at the "five sigma" level, corresponding to confidence of about 99.9999%. Search for the explanation of this so-called Hubble tension has become a central effort in cosmology. Scientists at the University of Michigan and the University of Pennsylvania propose to dramatically improve the reliability of an alternative method to measure the Hubble constant, which has the power to shed new light or outright resolve the Hubble tension. This method utilizes the so-called ``dark standard siren'' gravitational-wave events. As part of this project, the team will also engage in a number of outreach activities, including running an annual summer course for high-school students and organizing the biannual Cosmology Summer School (at Michigan); as well as organizing lectures during AstroFest and running the Neighborhood Workshop on Astrophysics and Cosmology (at Penn State).Standard sirens are mergers that result in observed gravitational-wave signatures whose wave-forms provide distances to these events. The dark sirens are those events in which the electromagnetic counterpart is not discovered (and the corresponding redshift not measured), complicating the consequent determination of the Hubble constant. The team will study how the cross-correlation between galaxies and gravitational-wave dark sirens can be used to measure the Hubble constant. This cross-correlation approach should allow a much cleaner separation of the cosmological signal from biases that are due to a variety of systematic errors, but can also be purely statistical. The team will thoroughly test this method against the systematic errors, then apply it to current data, and identify prospects for Hubble-constant constraints with future data. Finally, the team will also investigate the effect of astrophysical systematics on the ability to recover the Hubble constant.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.