Cosmic Distance Scale
The determination of accurate and precise distances to other galaxies is a key component of modern cosmology. White dwarf supernovae can be used to obtain very robust relative distances between galaxies, a technique that led to the discovery of dark energy in the late 1990s. Cepheid variables provide absolute distances to nearby galaxies, serving as the "first rung" in the "cosmic distance ladder". Together, Cepheids and white dwarf supernovae help to narrow down the possible scenarios for the properties of dark energy and complement other techniques such as observations of the cosmic microwave background and "ripples" in the spatial distribution of galaxies (the so-called baryon acoustic oscillation peak).
Macri is a founding member of the SH0ES project (PI: Nobel Laurate Adam Riess) which has been using the Hubble Space Telescope over the past decade to steadily improve our knowledge of the local expansion rate of the Universe (Hubble constant, or H0). The team aims to measure H0 to better than 2% by the end of 2015, and aims to obtain a 1% measurement by the end of the decade using new facilities and missions such as the James Webb Space Telescope and the Gaia satellite.
Suntzeff was one of founding members of the Calan-Tololo Supernova Survey that established the use of white dwarf supernovae as excellent distance indicators. He also founded (along with Nobel Laureate Brian Schmidt) the High-Z Supernova Search Team that ultimately discovered the accelerating expansion of the Universe due to the effects of dark energy. He is currently a member of the Carnegie Supernova Project.