The universe doesn’t look right. It suddenly looks . . . out of whack.
That is the strange message coming from astronomers and physicists, who are wondering whether they need to revise cosmic history.
The universe is unimaginably big, and it keeps getting bigger. But astronomers cannot agree on how quickly it is growing — and the more they study the problem, the more they disagree. Some scientists call this a “crisis” in cosmology. A less dramatic term in circulation is “the Hubble Constant tension.”
Nine decades ago, the astronomer Edwin Hubble showed that the universe is orders of magnitude vaster than previously imagined — and the whole kit and kaboodle is expanding. The rate of that expansion is a number called the Hubble Constant.
It’s a slippery number, however. Measurements using different techniques have produced different results, and the numbers show no sign of converging even as researchers refine their observations.
No one is panicking. To the contrary, the theorists are intrigued. They hope the Hubble Constant confusion is the harbinger of a potential major discovery — some "new physics."
“Any time there’s a discrepancy, some kind of anomaly, we all get very excited,” said Katherine Mack, a physicist at North Carolina State University who co-wrote a recent paper examining the issue.
The Hubble Constant is a central feature of any theory about the evolution and ultimate fate of the universe. This number may have zero effect on daily human existence, but there’s a lot at stake cosmologically.
Where’s it all going to go? How’s it all going to end? That’s a big question,” Mack said.
One widely supported estimate of the cosmic expansion uses the background radiation that permeates space — light emitted when the universe was young. That gives a Hubble Constant of about 67 kilometers per second per megaparsec. (A parsec is a distance of a bit more than three light-years. According to this estimate, a galaxy one million parsecs from Earth is receding at 67 kilometers, or about 42 miles, per second, and a galaxy twice as distant is receding at 134 kilometers per second.)
But another carefully calibrated measurement, based on light emitted from exploding stars — supernovae — has come up with a Hubble Constant of 73.