Coupled Quintessence and the Halo Mass Function

A sufficiently light scalar field slowly evolving in a potential can account for the dark energy density that presently dominates the universe. This quintessence field is expected to couple directly to matter components, unless some symmetry of a more fundamental theory protects or suppresses it. Such a coupling would leave distinctive signatures in the background expansion history of the universe and on cosmic structure formation, particularly at galaxy cluster scales. With the arrival of greatly improved high redshift cluster surveys such as the Dark Energy Survey (DES) and Planck, the mass distribution of the most massive objects in the Universe are a powerful tool which may be used to distinguish between different models of dark energy.

Using semi–analytic expressions for the CDM halo mass function, I will make predictions for halo abundance in models where the quintessence scalar field is coupled to cold dark matter, for a variety of quintessence potentials. How the quintessence field behaves in the highly non-linear regimes of halo collapse has yet to be understood and properly modelled. I will present two different methods for calculating the threshold of collapse parameter and show that the assumptions made in both calculations strongly affects halo abundance in the high mass tail of the mass function. In general, depending on the form of the quintessence potential and the strength of the coupling, the predicted number of haloes at a given epoch of a given mass, can lie above or below that of Lambda-CDM.