Big bounce cosmic models, as a research topic, shouldn't be confused with cyclic models where there is a recurrent bounce. The earlier thread by mykamakiri got a bit distracted because of that kind of confusion so I thought I would split off a separate topic just to focus on the question in the case of a one-time bounce. There's a legitimate question here! It's not a hard one to answer, but it's reasonable to ask and it can get lost in an unfocused broader discussion. Actually, in the earlier thread by myKK (BTW kamakiri means praying mantis) the question was already answered by Wabbit who pointed out that the Friedmann equation is time reversible----a cosmos with the same physics and same physical constants as ours would contract along the same curve (reversed) as ours is now expanding. But let's look at that at a basic level and get some intuition for the role played in the collapse by the cosmological constant Lambda which we assume to be the same in the collapsing phase as it is in the expanding phase we are now in----just as we do with all the other physical constants.