How fast is the universe really expanding? The mystery deepens.

ByCharles Q. Choi

Published September 20, 2023

• 5 min read

New data from NASA’s James Webb Space Telescope have deepened a mystery about how fast the universe is expanding. The discovery hints that unknown physics may be needed to help explain this cosmic enigma.

Ever since the universe was born about 13.8 billion years ago, it has continued expanding in every direction. By analyzing the current rate of cosmic expansion, known as the Hubble constant, researchers can estimate the age of the universe and possible details of its fate, such as whether it will expand forever, collapse upon itself, or even rip apart.

There are two primary strategies researchers use to measure the Hubble constant. One monitors nearby objects whose properties scientists know well, such as supernovae and pulsating stars called Cepheid variables, to estimate their distances and how rapidly they are moving away from us. The other method examines the cosmic microwave background, the leftover radiation from the big bang, to study the initial conditions of the universe and estimate how quickly it has expanded since that early time.

Unexpectedly, in the past decade, this pair of methods has produced two conflicting results. Observations of the cosmic microwave background from the European Planck space observatory suggest that the universe is expanding at the rate of about 67.4 kilometers per second per megaparsec (a distance equivalent to 3.26 million light-years). In contrast, data from nearby supernovas and Cepheid stars suggests a faster rate of about 73 km per second per megaparsec.

Resolving this crisis, known as the Hubble tension, could shed light on the evolution and fate of the cosmos. One possibility, simply, is that one or more of the methods to estimate this critical number is flawed.

“There was this hope that this discrepancy would just go away, that it was maybe just a measurement error,” says Adam Riess, an astrophysicist at the Space Telescope Science Institute in Baltimore, who won the Nobel Prize in Physics in 2011 for helping discover that the expansion of the universe was accelerating.

In the new study, Riess and colleagues relied on Webb’s sharp resolution. They analyzed more than 320 Cepheids in two galaxies—NGC 4258 about 23 million light-years away, and NGC 5584 about 100 million light-years away.

The researchers found Webb displayed a roughly threefold improvement in precision compared with the Hubble Space Telescope. “I would’ve been happy with 20 percent, so a factor of three is really outstanding,” Riess says.

Nevertheless, the new observations largely agreed with Hubble’s previous distance estimates. “The earlier results pass the JWST test,” says John Blakeslee, an astronomer at the National Optical-Infrared Astronomy Research Laboratory in Tucson, Arizona, who did not participate in this study.

“At some point, you have to say that it’s not a measurement error, and if so, it’s saying something very interesting about the universe,” Riess says. “It’s really a deepening enigma, but a good one.”

Unknown workings of the cosmos

These new findings suggest the Hubble tension may be due to something more fundamental than imprecision. If both values are right, then astronomers are missing some piece of information about how the universe has grown.

The data from nearby supernovae and Cepheid stars indicates the expansion is accelerating faster than expected based on the conditions when the universe was young, which are reflected in the cosmic microwave background. This enhanced acceleration is even greater than researchers can currently explain with dark energy, the mysterious force that theorists believe drives the accelerating expansion of the universe.

“We have a clear divergence between the observations and the dominant model of universe,” says Pierre Kervella, an astronomer at the Paris Observatory, who did not participate in this study. “It becomes more probable now that the problem is in the model of universe rather than in the observations, which are quite solid.”

One possible explanation is “there could be a problem with the theory of gravitation we are using—general relativity,” Kervella says. The Hubble constant value derived the cosmic microwave background depends on a model based on general relativity, Kervella explains.

Another possibility is that a previously unsuspected form of dark energy may have existed in the early universe, Riess notes. Or dark energy might have changed in nature over time, from when the universe was very young and compact to when it grew older and larger.

“There are a ton of ideas, and they all have pros and cons,” Riess says. “At present, none is fitting like Cinderella’s foot in the shoe.”

Additional clues

Recently scientists developed another technique for measuring the Hubble constant that may help shed light on this mystery. It relies on gravitational waves, ripples in the fabric of space and time produced when mass accelerates.

In 2017 scientists detected gravitational waves from colliding neutron stars. These ripples can theoretically be used to pinpoint the distance of the crashes from Earth, while light from the impacts can reveal the speed at which they are moving relative to Earth. Researchers can use both these sets of data to calculate the Hubble constant.

Preliminary findings using this method suggest a Hubble constant value of nearly 70 kilometers per second per megaparsec—right in between the other two methods. Analyzing crashes between about 50 pairs of neutron stars in the next five to 10 years may yield enough data for more conclusive results.

In the meantime, JWST will measure the Cepheid star distances for another dozen galaxies, Blakeslee notes. That will constitute an even stronger test of the measurement in the nearby universe.

But until someone can find the missing piece of this cosmological puzzle, the Hubble tension looks like it is here to stay.

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