Back Reaction from Walls
  We study the distance-redshift relation in a universe filled with 'walls' of
pressure-less dust separated by under dense regions. We show that as long as
the density contrast of the walls is small, or the diameter of the under dense
regions is much smaller than the Hubble scale, the distance-redshift relation
remains close to what is obtained in a Friedmann universe. However, when
arbitrary density contrasts are allowed, every prescribed distance-redshift
relation can be reproduced with such models.
