Gravity heats the Universe

TL;DR

This paper presents a simplified model linking cosmic structure formation through gravity to the heating of intergalactic gas, predicting rapid heating around redshift 10 and a hot universe today.

Contribution

It introduces a toy model connecting gravitational structure growth with the thermal history of intergalactic gas, highlighting a rapid heating phase around redshift 10.

Findings

Gas heats rapidly from structures at z~10

Current gas temperature exceeds 10^6 K

Universe was cold from z~100 to 10

Abstract

Structure in the Universe grew through gravitational instability from very smooth initial conditions. Energy conservation requires that the growing negative potential energy of these structures is balanced by an increase in kinetic energy. A fraction of this is converted into heat in the collisional gas of the intergalactic medium. Using a toy model of gravitational heating we attempt to link the growth of structure in the Universe and the average temperature of this gas. We find that the gas is rapidly heated from collapsing structures at around $z {\sim} 10$, reaching a temperature ${>} 10^6$K today, depending on some assumptions of our simplified model. Before that there was a cold era from $z\sim100$ to $\sim10$ in which the matter temperature is below that of the Cosmic Microwave Background.