What do cluster counts really tell us about the Universe?

Understand

We study the covariance matrix of the cluster mass function in cosmology.

• We adopt a two-line attack: firstly, we employ the counts-in-cells framework to derive an analytic expression for the covariance of the mass function.
• Secondly, we use a large ensemble of N-body simulations in the LCDM framework to test this.
• Our theoretical results show that the covariance can be written as the sum of two terms: a Poisson term, which dominates in the limit of rare clusters; and a sample variance term, which dominates for more abundant clusters.