Gravity heats the Universe
  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.
