Measuring the topology of the universe.

Observations of microwave background fluctuations can yield information not only about the geometry of the universe but potentially about the topology of the universe. If the universe is negatively curved, then the characteristic scale for the topology of the universe is the curvature radius. Thus, if we are seeing the effects of the geometry of the universe, we can hope to soon see signatures of the topology of the universe. The cleanest signature of the topology of the universe is written on the microwave sky: There should be thousands of pairs of matched circles. These circles can be used to determine the precise topology and volume of the universe. Because we see hundreds of slices through the fundamental domain of the universe, we can use the microwave observations to reconstruct the initial conditions of the entire universe on the scale of a few megaparsecs.