The ortho:para H2 ratio in the primordial gas

Abstract

We have solved the time-dependent equations for the populations of the rovibrational levels of H2, 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 H2 ratio is found to decrease from the statistical value of 3 at z ≈ 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 H2 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.