Ever since Hubble first discovered the relationship between a galaxy’s distance and its motion away from us, astrophysicists have raced to measure exactly how fast the Universe is expanding. As time moves forward, the fabric of space itself stretches and the distances between gravitationally unbound objects increases, which means everyone should see the Universe expanding at the same rate. What that rate is, however, is the subject of a great debate raging in cosmology today. If you measure that rate from the Big Bang’s afterglow, you get one value for Hubble’s constant: 67 km/s/Mpc. If you measure it from individual stars, galaxies, and supernovae, you get a different value: 74 km/s/Mpc. Who’s right, and who’s in error? It’s one of the biggest controversies in science today.