[Carpet] Which mpi communicator?

[Yosef Zlochower]

Hi,

  I have  routines scheduled in global mode that use mpi collective calls.
Which communicator should I use, *GetMPICommUniverse(cctkGH),
or *GetMPICommWorld(cctkGH)? Are they the same when there is only
1 model used? Can these communicators change during the evolution?

Thanks

Yosef