The universe doesn’t seem to agree with itself — or better said, with our perception of the universe. For decades, astronomers have tried to measure how fast the universe is expanding—a value known as the Hubble constant—and they’ve come up with different answers. Now, the James Webb Space Telescope (JWST) has thrown its weight behind one side of this disagreement, deepening a mystery that may point to something fundamentally new about the cosmos.
The latest findings, published in The Astrophysical Journal on December 9, offer a high-precision measurement of the Hubble constant. And these numbers, as precise as they are, just don’t line up with predictions based on the universe’s earliest light.
Adam Riess, a Nobel laureate and professor of physics and astronomy at Johns Hopkins University, who led the new study, said, “The more work we do the more it is apparent that the cause is something much more interesting than a telescope flaw. Rather it appears to be a feature in the universe.”
This “Hubble tension,” as scientists call it, might sound like an obscure footnote in astronomy. But it’s becoming an ever-louder riddle that could change our understanding of physics itself.
A Persistent Cosmic Discrepancy
Astronomers measure the Hubble constant in two main ways. The first method relies on the ancient glow of the Big Bang—called the cosmic microwave background (CMB)—captured by satellites like Planck. This gives a Hubble constant of about 67 kilometers per second per megaparsec. In other words, for every 3.26 million light-years you go out, galaxies are speeding away by an extra 67 kilometers per second.
The second approach measures the brightness of stars in the more recent universe. Cepheid variable stars, which pulsate with predictable rhythms, help astronomers map distances like rungs on a cosmic ladder. Hubble Space Telescope data from 2019 suggested a Hubble constant of 73 km/s/Mpc—a figure that refuses to align with the CMB measurements.
JWST’s new data now confirms the accuracy of these closer measurements. In a study covering thousands of Cepheid stars in distant galaxies, Webb improved upon Hubble’s precision significantly, narrowing uncertainties to just 2%. “Cross-checking Hubble might sound prosaic,” Riess told Ars Technica, “but the Hubble results demonstrate a profound tension in the Universe.”
The JWST result? A Hubble constant of 72.6 km/s/Mpc—almost exactly matching Hubble’s earlier measurements.
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A Window to New Physics?
With these findings, the idea that measurement errors might be causing the discrepancy is fading. Instead, scientists are left with an unsettling possibility: the universe might be expanding in ways our current physics can’t fully explain.
Marc Kamionkowski, a cosmologist at Johns Hopkins University who wasn’t involved in the study, is among those intrigued. “One possible explanation for the Hubble tension would be if there was something missing in our understanding of the early Universe, such as a new component of matter—early dark energy—that gave the universe an unexpected kick after the Big Bang,” he said in a press release.
Other ideas include strange properties of dark matter, particles yet to be discovered, or even changes in the fundamental constants of nature. “Theorists have license to get pretty creative,” Kamionkowski added.
These so-called creative ideas could reshape everything from the Big Bang to the fate of the cosmos. If early dark energy did exist, it might mean that the universe received an extra burst of acceleration billions of years ago. Or perhaps dark matter, the invisible scaffolding holding galaxies together, has quirks we’re only now beginning to detect.
But for now, no single explanation has emerged as the clear answer. “I wish I knew,” Riess told Live Science.
The Hunt Continues
This mystery underscores just how much remains unknown about the universe we inhabit. Scientists are planning more observations, new techniques, and sharper analyses. JWST, with its ability to pierce through cosmic dust and see farther than ever before, will continue playing a key role.
In the meantime, the universe keeps expanding—and the debate over its rate keeps widening. The Hubble tension is no longer just a nagging anomaly. It’s a potential signpost toward the next big breakthrough in our understanding of reality.
