Circles in the sky : finding topology with the microwave background radiation

@article{Cornish1997CirclesIT,
  title={Circles in the sky : finding topology with the microwave background radiation},
  author={Neil J. Cornish and David N. Spergel and Glenn D Starkman},
  journal={Classical and Quantum Gravity},
  year={1997},
  volume={15},
  pages={2657-2670}
}
If the universe is finite and smaller than the distance to the surface of last scatter, then the signature of the topology of the universe is writ large on the microwave background sky. We show that the microwave background will be identified at the intersections of the surface of last scattering as seen by different `copies' of the observer. Since the surface of last scattering is a 2-sphere, these intersections will be circles, regardless of the background geometry or topology. We therefore… 
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