Sample Variance in Cosmological Observations with a Narrow Field of View

Abstract Surveys with a narrow field of view can play an important role in probing cosmology, but inferences from these surveys suffer large sample variance, arising random fluctuations around the cosmic mean. The standard method for computing variance is based on two key approximations: treating perturbations linearly and survey geometry as box. We demonstrate that it lead to significant underestimate surveys. present new accurately apply our recent observations warm–hot intergalactic medium (WHIM) spectroscopic measurements blazars. find variances are significantly larger than quoted measurement errors; example, mean baryon density contained WHIM could be lower by 54% at 1 σ fluctuation estimated one observation. Accurately quantifying essential deriving correct interpretations small view.