Characteristic microwave-background distortions from collapsing spherical domain walls

We analyze the magnitude and angular pattern of distortions of the microwave background by collapsing spherical domain walls. We find a characteristic pattern of red-shift distortions of red or blue spikes surrounded by blue disks. The width and height of a spike is related to the diameter and magnitude of the disk. A measurement of the relations between these quantities thus can serve as an unambiguous indicator for a collapsing spherical domain wall. From the red-shift distortion in the blue disks we find an upper bound on the surface energy density of the walls σ≲8 ${\mathrm{MeV}}^3$.