 Moreover, recent work has pointed out the importance of considering non-simply connected topologies. Unlike what happens for simply connected topologies, curvature does not immediately determine the finiteness or infinity of the space. Indeed, there are spaces of null or negative curvature that can be finite or infinite depending on the non-connected topology associated to them. This leads into cosmic topology, which investigates the global shape of space and how it can be determined experimentally. If space has positive curvature, then the universe is finite independently of the specific topology associated with it. But if the curvature is negative or null, whether the universe is finite or not will depend on the topology. Thus, determining whether the universe is finite or infinite requires not only determining the mean density of matter (which determines the curvature of space) but also the topology of space. Two major techniques that are employed experimentally to determine the topology of space are cosmic crystallography and the circles in the sky method (based on the cosmic microwave background).