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Cosmic variance is the intrinsic scatter in the number density of galaxiesdue to fluctuations in the large-scale dark matter density field. In this work,we present a simple analytic model of cosmic variance in the high redshiftUniverse ($z\sim5-15$). We assume that galaxies grow according to the evolutionof the halo mass function, which we allow to vary with large-scale environment.Our model produces a reasonable match to the observed ultraviolet luminosityfunctions in this era by regulating star formation through stellar feedback andassuming that the UV luminosity function is dominated by recent star formation.We find that cosmic variance in the UVLF is dominated by the variance in theunderlying dark matter halo population, and not by differences in haloaccretion or the specifics of our stellar feedback model. We also find thatcosmic variance dominates over Poisson noise for future high-$z$ surveys exceptfor the brightest sources or at very high redshifts ($z \gtrsim 12$). Weprovide a linear approximation of cosmic variance for a variety of redshifts,magnitudes, and survey areas through the public Python package galcv. Finally,we introduce a new method for incorporating priors on cosmic variance intoestimates of the galaxy luminosity function and demonstrate that itsignificantly improves constraints on that important observable.

A Flexible Analytic Model of Cosmic Variance in the First Billion Years