The Thermal and Gravitational Energy Densities in the Large-scale Structure of the Universe

Abstract

Abstract As cosmic structures form, matter density fluctuations collapse gravitationally and baryonic matter is shock-heated and thermalized. We therefore expect a connection between the mean gravitational potential energy density of collapsed halos, , and the mean thermal energy density of baryons, Ωth. These quantities can be obtained using two fundamentally different estimates: we compute using the theoretical framework of the halo model, which is driven by dark matter statistics, and measure Ωth using the Sunyaev–Zeldovich (SZ) effect, which probes the mean thermal pressure of baryons. First, we derive that, at the present time, about 90% of originates from massive halos with M > 1013 M ⊙. Then, using our measurements of the SZ background, we find that Ωth accounts for about 80% of the kinetic energy of the baryons available for pressure in halos at z ≲ 0.5. This constrains the amount of nonthermal pressure, e.g., due to bulk and turbulent gas motion sourced by mass accretion, to be about Ωnon‐th ≃ 0.4 × 10−8 at z = 0.