  We compute the tensor CMB anisotropy power spectrum for singular and
non-singular instantons describing the beginning of an open universe according
to the Euclidean no boundary proposal. Singular instantons occur generically,
whereas non-singular instantons require more contrived scalar field potentials.
For the latter, we consider potentials in which a sharp feature, either
negative or positive, is added to a gently sloping potential. In the first case
one finds a nearly divergent contribution to the low multipole CMB anisotropy,
in conflict with the COBE observations. In the second case the divergence is
weaker, but matching the low multipoles forces the added feature to be large
and narrow. For singular instantons, there is a better match to the
observations, without any such contrivance. The distinction between singular
and nonsingular instantons disappears in the limit as the universe becomes
flat, but is still observable for densities as high as 0.7 of the critical
density.
