Entrance in Princeton University for Post doct. in cosmology

Post-doctoral positions are positions that you're hired into. You don't qualify for them by having a certain GPA, or passing a particular exam. You compete for them against all the other applicants for that position, just like any other job. Things that can help are if your PhD work happens to be in the same area or on a similar problem to the one the post-doctoral work is proposing to work on, if you have particular skills that are needed to perform the work, or if you've had some kind of previous communication or collaboration with the group.