NEW YORK — 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 percent discrepancy in the value of a long-sought number called the Hubble constant, which describes how fast the universe is expanding. But in a measure of how precise cosmologists think their science has become, this small mismatch has fostered a debate about just how well we know the cosmos.
“If it is real, we will learn new physics,” said Wendy Freedman of the University of Chicago, who has spent most of her career charting the size and growth of the universe.
Under some theories, the universe will expand over several trillion years to the point at which it will be too cold to support life.
In an expanding universe, the farther something is away from you, the faster it is receding. Hubble’s constant tells by how much.
But the constant, named after Edwin Hubble, the Mount Wilson and Carnegie Observatories astronomer who discovered that the universe is expanding, has always given astronomers fits.
Measuring it requires divining the distances of lights in the sky — stars and even whole galaxies that scientists can never visit or recreate in the lab. 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.
But the calibrators themselves need to be calibrated, which has led to a rickety chain of assumptions and measurements in which small errors and disagreements — about, say, how much dust is interfering with observations — can build up to cosmic proportions.
Only three decades ago, renowned astronomers could not agree on whether the universe was 10 billion or 20 billion years old. Now everybody has settled on its age as about 13.8 billion years.
Using a new generation of instruments like the Hubble Space Telescope, astronomers have steadily whittled down the uncertainty in the Hubble constant.
In 2001, a team led by Freedman reported a value of 72 kilometers per second per megaparsec (about 3.3 million light years), 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 kilometers a second faster.
Hubble’s original estimate was much higher, at 500 in the same units of measurement.
Freedman’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. Those were derived from studies of microwaves emitted and still lingering in the sky from the primordial Big Bang fireball.
As a result, in recent years, astronomers have settled on a recipe for the universe that is as black and as decadent as a double dark chocolate chunk brownie. The universe consists of roughly 5 percent atomic matter by weight, 27 percent mysterious dark matter, and 68 percent of the even more mysterious dark energy that is speeding up the cosmic expansion.
Never mind that we do not know exactly what all this dark stuff is. Astronomers have a good theory about how it behaves, and that has allowed them to tell a plausible story about how the universe evolved from when it was a trillionth of a second old until today.
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, using the Hubble Space Telescope and the giant Keck Telescope on Mauna Kea in Hawaii and supernova explosions as the ultimate distance markers, got a value of 73 plus or minus only 2.4 percent 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 by the European Planck spacecraft of relic radiation from the Big Bang.
The Planck mission observations that show the universe when it was only 380,000 years old are considered the gold standard of cosmology.
Whether the standard cosmic recipe might now need to be modified — for example, to account for a new species of subatomic particles streaming through space from the Big Bang — depends on whom you talk to.