Collaborative Research: Toward a 1% measurement of the Hubble Constant with gravitational time delays

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In recent years, high precision measurements have allowed astronomers to realize that the current expansion velocity of the Universe does not agree with the velocity predicted, on the basis of the standard cosmological model, by observing phenomena that have taken place in the first few hundred thousand years of the Universe. If the conflict between the prediction and the measurements is confirmed beyond any reasonable doubt, then the standard cosmological model would be incorrect, which would therefore lead to the discovery of new physical phenomena and thus to a true revolution in the way we understand the natural world. In order to achieve the most precise measurement of the current expansion velocity of the Universe independently of any other method, this project will use a technique called "gravitational time delays," which the collaborating teams have developed over the past twenty years. The measurement will determine whether the conflict is real and whether new physics is required. The broader impact of the program will be achieved through a comprehensive educational/public outreach component, aimed at communicating both the content of the research and the importance and significance of the scientific method. The program includes teaching the foundations of scientific research at the high-school, undergraduate, and graduate levels, as well as mentoring of undergraduate students in the context of the NSF-funded CALBRIDGE program. The 8% discrepancy between cosmic microwave background measurements and the local determination of the expansion rate of the Universe H0 hints that the standard cosmological model might not be entirely accurate. If confirmed, this discrepancy could lead to the discovery of new physics such as dark energy that is not the cosmological constant, or additional families of relativistic species. Our ambitious plan based on gravitational time delays aims to achieve the most precise absolute distance measurement to date, independent of the local distant ladder. A three-step program will be carried out. First, time delays will be used to measure absolute cosmic distances and thus cosmological parameters. With state-of-the art techniques developed by the PIs' teams, it has been shown that just four lenses are sufficient to measure H0 with 3% precision. Second, the teams will carry out follow-up (including high resolution imaging, deep spectroscopy and monitoring with dedicated telescope access) and modeling of lenses that they have recently discovered to further improve the precision. Third, the program will lay the foundations for time delay cosmology in the Large Synoptic Survey Telescope era by developing tools and carrying out simulations and data challenges with the ultimate goal of reaching sub-percent precision and accuracy.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.