Towards Robust Measurements of H0 with Dark Standard Candles

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.