Heretical Idea Faces Its Sternest Test

摘要

When Vera Rubin began mapping massive hydrogen gas clouds that swarm around spiral galaxies like sluggish electrons around a nucleus, the astronomer at the Carnegie Institution in Washington, D.C., had a good idea of what she should find. Moving out from the galactic center, the gravitational pull of the ever-larger amount of mass within a cloud's orbit should make the cloud wheel faster and faster around the hub. But very distant clouds, orbiting beyond a certain radius that encompasses virtually all of the galaxy's mass, should chug more slowly, as the galaxy's gravitational pull diminishes with distance. Although Rubin did see the predicted rise in cloud velocity away from the center, the farthest clouds—to her astonishment—never did slow down.