January 31, 2023
Recently, in a series of three papers (Paper I, Paper II, Paper III), KICP members Yuuki Omori and Chihway Chang, and KICP alumnus Eric Baxter (currently faculty at the University of Hawaii), led an analysis combining and analyzing large data sets from the Dark Energy Survey and the South Pole Telescope. Each data set can be used independently to set powerful constraints on cosmology, and each data set is potentially subject to contamination that could bias the cosmological constraints. By comparing and combining the two data sets, Omori, Chang and Baxter seek to place stronger constraints on cosmology, and even more importantly to test for biases that are present in either data set alone but not in the combination, resulting in a significantly more robust cosmological model.
The Dark Energy Survey (DES) is an optical imaging survey that observed approximately 10% of the full sky between 2012 and 2019. The survey measured the positions and shapes of hundreds of millions of galaxies, from which statistical inferences were drawn to shed light on the evolution and composition of the Universe. The South Pole Telescope (SPT) is a millimeter/submillimeter-wave telescope designed to measure the oldest light in the Universe, the cosmic microwave background (CMB), distortions of which from gravitational lensing can be studied to characterize the mass distribution in the Universe. By design, the observing regions of DES and SPT overlap significantly, enabling joint analyses such as the one described here.
In this analysis, Omori, Chang and Baxter combined a measurement of the mass distribution in the Universe using CMB lensing data from SPT with the recent cosmology results from DES. This so-called “6x2pt” analysis used all the possible correlation combinations that can be measured from three observables: galaxy positions, galaxy lensing, and CMB lensing. The addition of CMB lensing data to the galaxy-only analysis can reveal biases in the individual data sets, and when combined, results in constraints that are extremely robust to these biases. This is especially important given the recent hints of discrepancies in and between galaxy and CMB data, as shown in other works in the literature. One of the main advances of this work is a new CMB lensing map constructed by Omori that is optimized for cross-correlation studies – that is, a lensing map that has minimal contamination from astrophysical effects, which could bias our results.
A number of crucial findings were presented in the three papers. The most important result is that the preferred cosmology from the full 6x2 cross-correlation analysis is completely consistent with the favored cosmology from the galaxy-only constraints from DES. The 6x2pt analysis also results in more powerful cosmological constraints compared to the galaxy-only constraints. This strengthens the results from DES and showcases how combining independent data sets can produce both more robust and more powerful constraints on our cosmological model.