Sample Variance in Cosmological Observations with a Narrow Field of View

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

Automated summary

Surveys with a narrow field of view are important in cosmology, but suffer from large sample variance. The standard method for computing sample variance underestimates it in narrow surveys. A new method is presented to accurately compute sample variance and applied to recent WHIM observations. The sample variances are significantly larger than the measurement errors.