The ortho:para H2 ratio in the primordial gas

We have solved the time-dependent equations for the populations of the rovibrational levels of H-2, from redshift z = 5000 to the present time. Population transfer, including ortho-para interconversion, is determined principally by collisions with H+. Simultaneously, we solved the chemical rate equations and for the temperatures of the radiation field and matter, distinguishing between electrons, ions and neutrals. The ortho:para H-2 ratio is found to decrease from the statistical value of 3 at z approximate to 100 until it freezes at a value of 0.25 for z < 20. The significance of proton collisions for the rates of heating and cooling of the gas by H-2 is investigated. Collisions with protons are found to be increasingly important with decreasing temperature. At z = 20, the net rate of heating of the gas, including proton collisions, exceeds the net heating rate neglecting proton collisions by approximately three orders of magnitude.