To measure cosmological parameters in our nearby universe, one must understand the motions of our galaxy and the galaxies around us. Arguably the best way to measure these bulk motions as well as the local expansion rate of the universe is with Type Ia supernovae (SNe Ia), a premiere cosmic measuring stick. Currently, there is tension between the locally measured value of the universe's expansion rate (parameterized by the Hubble constant H0) and the inferred value from the early-universe - exciting potential evidence for new physics. To measure H0, we must correct for the large-scale motions and the net effect of this corrected motion is now 50% or higher of the total statistical uncertainty on local H0. Unfortunately, while current supernova samples are now reaching the multiple thousands in size, there are ~0 SNe close to the giant superclusters responsible for our bulk motions.
Here we proposed a three-tiered project:
1. Measure anisotropy (signs of giant superclusters and voids) from our upcoming Pantheon+ cosmology results, which will be the largest sample of SNe Ia analyzed. We already see curious residual signals from our soon-to-be published Pantheon+ analysis that helps motivate the remaining tiers of our program.
2. Utilize already-rewarded time with the Dark Energy Camera to accumulate hundreds of SNe Ia in the southern hemisphere, prioritizing if possible those in the direction of the CMB dipole. While this sample will be limited in depth because it is a follow-up survey, it will still provide valuable constraints by more than doubling the largest low redshift SN Ia sample.
3. Conduct a new SN Ia survey that focuses on the largest superclusters in the local universe. This survey will be first of its kind as a discovery survey that targets SNe around superclusters and voids. From this, we can produce the leading constraints on bulk motions in the nearby universe, constraints that will likely remain unrivaled for some time.
