The universe is expanding, but how fast?

Cosmos controversy: Recent measurements of the distances & velocities of faraway galaxies don't agree with a cosmos model...

There is a crisis brewing in the cosmos, or perhaps in the community of cosmologists. The universe seems to be expanding too fast, some astronomers say. Recent measurements of the distances and velocities of faraway galaxies do not agree with a hard-won “standard model” of the cosmos that has prevailed for the past two decades. The latest result shows a 9% discrepancy in the value of a long-sought number called the Hubble constant, which describes how fast the universe is expanding.

“If it is real, we will learn new physics,” said Wendy Freedman of the University of Chicago, USA, who has spent most of her career charting the size and growth of the universe. The Hubble constant, named after Edwin Hubble, the astronomer who discovered that the universe is expanding, has ever given astronomers fits. In an expanding universe, the farther something is away from you, the faster it is receding. Hubble’s constant tells by how much.

Getting closer

But measuring it requires divining the distances of lights in the sky. The strategy since Hubble’s day has been to find so-called standard candles, stars or whole galaxies whose distances can be calculated by how bright they look from Earth. Using a new generation of instruments, astronomers have steadily whittled down the uncertainty in the Hubble constant.

In 2001, a team led by Wendy reported a value of 72 km per second per megaparsec, in the galumphing units astronomers prefer. It meant that for every 3.3 million light years a galaxy was farther away from us, it was moving 72 km a second faster. Hubble’s original estimate was much higher, at 500 in the same units of measurement. Wendy’s result had an error margin that left it happily consistent with other more indirect calculations, that had gotten a slightly slower and lower value of 67 for the Hubble constant.

As a result, in recent years, astronomers have settled on a recipe for the universe that is as black and decadent as a double dark chocolate chunk brownie. The universe consists of roughly 5% atomic matter by weight, 27% mysterious dark matter and 68% of the even more mysterious dark energy. Astronomers have a good theory about how it behaves, and that has allowed them to tell a plausible story about how the universe evolved.

But now the Hubble precision has gotten seemingly better, and the universe might be in trouble again. Last summer a team led by Adam Riess of Johns Hopkins
University and the Space Telescope Science Institute, USA, using the Hubble Space
Telescope and the giant Keck Telescope on Mauna Kea in Hawaii, USA and supernova explosions as the ultimate distance markers, got a value of 73 plus or minus only 2.4% for the elusive constant. That made waves because it meant that, if true, the Hubble constant as observed today was now clearly incompatible with a result of the lower slower value of 67 inferred from data obtained in 2013.

Whether the standard cosmic recipe might now need to be modified depends on whom you talk to. Some say it is too soon to get excited about new physics sneaking through such a small discrepancy in a field noted for controversy. With more data and better understanding of statistical uncertainties, the discrepancy might disappear, they say. “No explanation I know of is less ugly than the problem,” Lawrence M Krauss, a theorist at Arizona State University, USA, said. Others say this could be the beginning of something big.

Adam and his colleague Stefano Casertano got roughly the same answer of 73 later last summer, strengthening the claim for a mismatch of Hubble constants. They used early data from the spacecraft GAIA, which is measuring the distances of more than one billion stars by triangulation. They calculated that the odds of this mismatch being a statistical fluke were less than one part in 100.

What comes next?

There is wiggle room, Adam and others say, for both the modern and the primordial results to be right. Other parameters could be tweaked. That is where new physics might come in. The most likely candidates to fill the gap, Adam said, might be a new form of the ghostly particles called neutrinos, already known to be abundant in the cosmos. They come in three types that can change into one another as they traverse space.

Their discovery could unlock new realms in particle physics and perhaps shed light on the quest to understand the dark matter that suffuses space. Another possibility is that the most popular version of dark energy — known as the cosmological constant — might have to be replaced in the cosmological model by a more virulent and controversial form known as phantom energy. This could cause the universe to eventually expand so fast that even atoms would be torn apart in a Big Rip billions of years from now. “This is a very interesting tension,” Adam said. “This is why we play the game. We look for something not fitting.”