CIRCLES IN THE SKY - FINDING TOPOLOGY WITH THE MICROWAVE BACKGROUND-RADIATION

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 i s writ large on the microwave background sky. We show that the microwa ve background will be identified at the intersections of the surface o f last scattering as seen by different 'copies' of the observer. Since the surface of last scattering is a 2-sphere, these intersections wil l be circles, regardless of the background geometry or topology. We th erefore propose a statistic that is sensitive to all small, locally ho mogeneous topologies. Here, small means that the distance to the surfa ce of last scatter is smaller than the 'topology scale' of the univers e.