Constraints on the topology of the Universe derived from the 7-year WMAP
  data
  We impose constraints on the topology of the Universe determined from a
search for matched circles in the temperature anisotropy patterns of the 7-year
WMAP data. We pay special attention to the sensitivity of the method to
residual foreground contamination of the sky maps, and show that for a full sky
estimate of the CMB signal (the ILC map) such residuals introduce a
non-negligible effect on the statistics of matched circles. In order to reduce
this effect, we perform the analysis on maps for which the most contaminated
regions have been removed. A search for pairs of matched back-to-back circles
in the higher resolution WMAP W-band map allows tighter constraints to be
imposed on topology. Our results rule out universes with topologies that
predict pairs of such circles with radii larger than \alpha_min \approx 10
degrees. This places a lower bound on the size of the fundamental domain for a
flat universe of about 27.9 Gpc. This bound is close to the upper limit on the
size of Universe possible to detect by the method of matched circles, i.e. the
diameter of the observable Universe is 28.3 Gpc.
