Universe Expanding Faster than Expected
Measurements of the Hubble constant have varied hugely since Edwin Hubble first observed that the universe appears to be retreating in all directions. By observing the fading glow of the Big Bang, researchers with the Planck mission uncovered on number for the Hubble constant. But in general, Hubble constant queries rely on knowing an object’s distance combined with how quickly it’s receding. And distance in the universe is surprisingly tricky to pin down.
For the nearby universe, Cepheid variable stars (shown in the image above: Cepheid variable stars are in the red circles) can reveal distances. They change brightness at regular time intervals, and the length of this period is directly related to their intrinsic brightness. For the less local universe, astronomers use type Ia supernovae, which all light up with the same brightness. In both cases by measuring how bright the objects appear, astronomers can calculate the intervening distance between them and Earth.
Now, astronomers led by Nobel laureate Adam Riess have used the Hubble and Keck observatories to combine these Cepheid and supernova distances into one highly accurate number.
They’ve done the world’s best job of decreasing the uncertainty in the measured rate of universe expansion and of accurately assessing the size of this uncertainty, yet they found that their measured rate of expansion is probably incompatible with the rate expected from observations of the young universe, suggesting that there’s something important missing in our physical understanding of the universe.
 See Korey Haynes, “Universe Expanding Faster than Expected,” Astronomy (44, 10, October 2016, p.11).