Best-yet Value for Universe's Expansion

Big news! A team led by Adam Riess of the Space Telescope Science Institute and Johns Hopkins University upheld the theory of dark energy against a competing theory. How?

The team managed to measure the expansion rate of the universe with an impressively small margin of error through a complicated process involving supernovae, variable stars, and the new Wide Field Camera 3 on the Hubble Space Telescope. Click here to see the news release.

Observations show that the universe is expanding at an ever-increasing rate. So far, attempts to explain the accelerating expansion have mainly focused around a poorly understood repulsion force known as dark energy. Some astronomers, however, believe that the observed accelerating expansion of the universe could arise from our galaxy being in the center of a vast void; in this scenario, objects in the void, where there would be relatively little mass, would be gravitationally attracted to the edges of the void.

In order to measure the expansion of the universe — and in so doing, learn more about its possible causes — astronomers need two key ingredients: speed and distance. Speed is relatively easy; measuring the redshift of galaxies gives the rate of expansion to better than 1% accuracy.

"But the distances have been a source of pain and misery for astronomers since Hubble's time," says Robert Kirshner, a professor at the Harvard-Smithsonian Center for Astrophysics who uses supernovae to measure the universe's expansion rate.

Astronomers have long used a special kind of supernova (Type 1a) as a standard candle, a kind of cosmological ruler. Astronomers think that the brightness of this kind of supernova is always roughly the same. And if they know how intrinsically bright a certain kind of object is — i.e., how bright this type of supernova would be if you were unfortunate enough to be right next to it — then they can figure out how far away that supernova is by measuring how much dimmer it is compared to its intrinsic brightness.

Then comes a chicken and egg problem: in order to figure out how intrinsically bright an object is, at some point you need to know exactly how near or far that object is. But you can't know one without knowing the other.

That is, unless you use some other nearby, predictable object to calculate the distance to the supernova, which is exactly what Riess and his team did. They scoured the list of Type 1a supernovae, and found eight cases where the Hubble Space Telescope was able to observe Cepheid variables in the galaxy that hosted the supernova. Using the Cepheids, they were able to calculate the distances to these eight supernovae with unprecedented accuracy.

Cepheid variables are the best-studied standard candles of all; they’re what Edwin Hubble first used to measure the distance to the nearby Andromeda Galaxy in 1925. Unfortunately, they’re too faint to be seen in distant galaxies, even by the Hubble Space Telescope. Type 1a supernovae can be detected nearly to the edge of the known universe, but their intrinsic brightness is hard to determine. Riess’s study ties these two distance scales together better than ever before, significantly improving our ability to measure the distances of remote galaxies.

Having measured both recession speed and distance with good accuracy, Riess and his team came up with a figure for the expansion rate of the universe with an uncertainty of only 3.3%. (Their value is 73.8 ± 2.2 km per second per megaparsec.) By reducing the margin of error by 30% over the Hubble Space Telescope’s previous best measurement in 2009, his study made it clear that the void theory of accelerating expansion, already unlikely, is now virtually a statistical impossibility: the expansion rate calculated in this study is significantly larger than those that would be observed in the most plausible "void" scenarios, and the margin of error is too small to allow for any wiggle room.

This is how Kirshner puts it: "The Riess paper claims that the models that avoid dark energy by positing a big void are 5-sigma from the measured expansion rate. That's the polite way to say 'wrong.'"