Signs of a non-zero equation-of-state for Dark Matter

Abstract

We demonstrate how a changing and negative equation-of-state (EoS) for dark matter can alleviate cosmic tensions and explain the integrated Sachs-Wolfe (ISW) void anomaly. We discuss the effect of the model on the cosmic expansion history, growth of structure and the ISW. We show that a negative EoS at late times is able to produce a larger Hubble constant and smaller $\sigma_{8}$, which can explain both cosmological tensions. Furthermore, the model uniquely predicts larger ISW at low redshift, a prediction which is in agreement with observations of larger ISW from voids. The preference for a negative EoS for dark matter at late times is indicative of a unified dark sector and degenerate with models of dark matter and dark energy interaction. Future measurements of the ISW from cosmic voids can provide a unique test for this solution to tensions in cosmology, should they continue to persist.