A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. But quantitatively knowing what the expansion rate is (and was) in the present (and past) is vital to understanding our cosmic history and future. Image credit: NASA / CXC / M. Weiss.

Scientists Still Don’t Know How Fast The Universe Is Expanding

A cosmic controversy is back, and at least one camp — perhaps both — is making an unidentified error.

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.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy fights against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. Image credit: E. Siegel / Beyond the Galaxy.

If the Universe is expanding today, that means it must have been more compact, denser, and even hotter in the distant past. The fact that things are getting farther apart, on a cosmic scale, implies that they…