The Hubble Space Telescope has been used to measure the phenomenon first observed by the astronomer after which it was named: the expansion of the universe. It is the most precise measurement of the rate of expansion yet made, and it confirms something that's been bothering astronomers for a while. The universe appears to be expanding faster than our measurements of the cosmic background radiation indicate it should be, suggesting either one of the measurements is wrong, or the expansion is being influenced by an unknown force.
As the universe expands, galaxies get further apart. Investigations into the way this is happening have been central to some of the most important discoveries in cosmology over the last century, but it seems we're not done yet.
We can precisely measure the rate at which a galaxy is moving away from us from how much the wavelength of its light has stretched (known as red shift). Measuring the distance is harder, relying on the brightness of specific stars and supernovas known as “standard candles”. Comparing their brightnesses can indicate distance.
Using Hubble's more exact measurements of certain standard candles, a paper to be published in The Astrophysical Journal reports the Hubble constant, a measure of the expansion of the universe, as 67 (42 miles) kilometers per second per megaparsec. This means that for every additional million parsecs (3.3 million light-years) away from us a galaxy is, its speed increases by 67 kilometers (42 miles) per second.
The problem is this figure is 9 percent larger than the value we calculate using observations of the situation shortly (378,000 years being "shortly" to cosmologists) after the Big Bang. This is based on the Planck satellite's measurements of the cosmic background radiation created at that time, processed using models of how this should've changed as the universe evolved.
Previous estimates of the current Hubble constant had also been higher than that derived from the Planck data, but there was uncertainty in the measurements. Using new methods to calibrate the brightness of Cepheid variables, the standard candles used to measure relatively nearby galaxies, Hubble decreased its uncertainty range to 2.3 percent. Getting the Hubble constant accurate to within 10 percent was one of Hubble's original goals, so this dramatically exceeds that. With this newfound precision, the chance of the current and early universe expansions aligning drops to one in 5,000, which seems a bit low to rely on.