4 Simulated circle matching
The topology of the universe describes how different regions are connected and could therefore leave its imprint on the cosmic microwave background. For example, if our physical space is smaller than the observable universe (as recent data suggest it is) then the horizon sphere wraps around the universe and intersects itself. As a result, duplicated images of the cosmic microwave background (in which the colours represent temperature fluctuations) will intersect along a circle and we would observe this circle on different sides of the sky.