Constraining cosmological parameters using the splashback radius of galaxy clusters

Cosmological parameters such as ΩM and σ₈ can be measured indirectly using various methods, including galaxy cluster abundance and cosmic shear. These measurements constrain the composite parameter S₈, leading to degeneracy between ΩM and σ₈. 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 ΩM and σ₈, we show that the position of the splashback radius around cluster-mass halos is greater in cosmologies with smaller values of ΩM or larger values of σ₈. This variation breaks the degeneracy between ΩM and σ₈ that comes from measurements of the S₈ 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 S₈, 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.