Chaplygin gas please help!!! Hello every one! How many of you have learnt about the unification of dark energy and dark matter? I'm doing my research regarding the Chaplygin gas and i've come across this research paper. arxiv.org/pdf/grqc/0202064.pdf (pls add https:// part) At the end, they solve the equation 36 (3/2*ab′′ + (1 − w)ab′ − (1 + w)(1 − 3αw)b = 0 , and only use one boundary which is b′(aeq) = 0. Don't we need another boundary condition since this is a second order DE? Any knowledge will be much appreciated!!!