So this rumor over the detection of primordial B-modes has got me thinking. Gravitational waves are caused by perturbances in spacetime by asymmetrical objects. So for them to travel, they have to be propagated by something: spacetime. The primordial B-modes are supposedly the result of cosmic inflation. So two things don't make sense to me. Wouldn't spacetime had to have existed pre-Big Bang as to have B-modes produced from the inflation? Assuming spacetime is a result of the Big Bang, inflation would have been the expansion of spacetime itself, meaning no gravitational waves could be generated by inflation as they couldn't be propagated in the same spatial and temporal dimensions as their medium. Also, since "objects" must be asymmetrical to create gravitational waves, how could cosmic inflation produce them, since inflation is considered to be isotropic, and therefore, in a sense, symmetrical? I know the majority of what I'm asking is purely theoretical, but please explain as best as possible. And feel free to correct me anywhere, as I am no where near being a cosmological expert. Thanks.