Transcript: General relativity relates the dynamics of the expanding universe to the curvature of space. Thus space curvature is one of the most important things to measure about our universe, but it’s an extremely difficult measurement because the curvature is so subtle. In principle it would be possible to measure space curvature if there were objects of a fixed linear size in space. We could then measure how their apparent size varied with distance or redshift. But galaxies do not have fixed sides or edges, and their size actually evolves with cosmic time. So the size of galaxies cannot be used for this test. It also might be possible to measure the number of objects in equal volumes with distance or redshift to see what the space curvature was. In flat space, the number of objects per unit volume goes as the cube of the distance. In positively curved space, since the volume of the universe is smaller, the number of objects goes as a lower power than the third of radius, and in negatively curved universes where the volume is larger, it goes as a higher than the third power of distance. But this is a difficult measurement too because the objects we are counting, galaxies, both merge and change their properties with cosmic time.