Constraining Cosmological Parameters Using the Splashback Radius of Galaxy Clusters

Cosmological parameters such as Ω_M and σ ₈ can be measured indirectly using various methods, including galaxy cluster abundance and cosmic shear. These measurements constrain the composite parameter S ₈, leading to degeneracy between Ω_M and σ ₈. However, some structural properties of galaxy clusters also correlate with cosmological parameters, due to their dependence on a cluster's accretion history. In this work, we focus on the splashback radius, an observable cluster feature that represents a boundary between a cluster and the surrounding Universe. Using a suite of cosmological simulations with a range of values for Ω_M and σ ₈, we show that the position of the splashback radius around cluster-mass halos is greater in cosmologies with smaller values of Ω_M or larger values of σ ₈. This variation breaks the degeneracy between Ω_M and σ ₈ that comes from measurements of the S ₈ parameter. We also show that this variation is, in principle, measurable in observations. As the splashback radius can be determined from the same weak lensing analysis already used to estimate S ₈, this new approach can tighten low-redshift constraints on cosmological parameters, either using existing data, or using upcoming data such as that from Euclid and LSST.